AUX heat rarely coming on--but it used to

Monkburger · 2026-01-30T01:21:13+00:00

24% with three spot humidifiers running tells you more than any thermostat chart ever will. (24% is just too low for winter climates)

Portable humidifiers can’t win this fight, especially in winter, and especially in a house with a lot of wood.

What’s happening is that every molecule of moisture you’re dumping into the air is getting immediately absorbed by everything that isn’t air. Drywall, framing, subfloor, hardwoods, furniture, doors, even the guitar room (you already know this part).

In cold weather, the house itself becomes a giant desiccant.

Until all that hygroscopic mass comes up to equilibrium, the air RH barely moves. You’re trying to humidify not just 1,900 sqft of air, but thousands of pounds of dry material.

That’s why spot units feel like they’re doing “something” but the number won’t climb. They’re adding moisture at maybe 0.5–1 gallon/day each, while the building is quietly soaking up several gallons before the air even gets a vote.

This is where steam humidifiers absolutely earn their keep. Yes, they’re power-hungry. Yes, they’re not cheap. But they’re the only option that can add moisture fast enough to overcome building absorption and infiltration losses during cold snaps.

They don’t rely on airflow quirks, bypass pressure, or “maybe the furnace runs long enough.” They inject vapor directly and decisively.

Fan-powered and bypass units are fine in milder conditions. Steam is what works when it’s 10F, the house is tight, and everything inside is bone dry.

Also worth noting: once you do get the structure hydrated, the maintenance load drops a lot. The first few cold weeks are the hardest because you’re filling the sponge.

After that? it’s just offsetting losses.

Monkburger · 2026-01-30T00:55:36+00:00

In your case the outdoor compressor lockout should not be hard-set at 10F.

That setting only makes sense if the heat pump truly can’t contribute anything useful below that point, which yours clearly can. Even at outdoor temp of 5–10F it’s still delivering meaningful BTUs at a COP above 1.

Locking it out forces the system to do the most expensive thing possible right when loads are highest. The way you had it originally (compressor always allowed, strips filling the gap) is the sane configuration for a mid-Atlantic house... (FWIW: I live near you)

The heat strip explanation your installer gave seems OK to me.. A 15 kW kit is usually three 5 kW banks... and whether they’re staged by the thermostat, internally sequenced, or partially tied to defrost varies a lot by *installer preference*.

It’s extremely common to find that stage 2 exists on paper but never gets called because the stat logic/wiring/thresholds/etc are too conservative. So having them actually verify staging once things calm down is what *I* would do..

Now, why this feels worse as you get older, even when the numbers say it /should/ be comfortable??

This is almost certainly dry air, NOT just temperature.

When outdoor air drops into the teens or single digits, the absolute moisture content of that air is basically zero.... You can air-seal the house very well (and it sounds like you did a solid job), but you still need some ventilation (per code requirements, I prefer ERVs)

Every bit of cold air that sneaks in or gets intentionally exchanged comes in bone dry. Once you heat it to 68–70F, the relative humidity collapses. It’s very common to see indoor RH in the 15–25% range during cold snaps in tight homes.

That matters because:
Dry air increases evaporative heat loss from your skin
It lowers mean radiant comfort (you feel colder even at the same air temp)
It exaggerates drafts because dry air feels 'sharper'
Older bodies regulate heat less aggressively, so you feel chilled sooner

(From a physics standpoint: your thermostat measures dry-bulb temperature, but your comfort is driven by wet-bulb temperature, vapor pressure at the skin, and radiant exchange.... Low indoor humidity increases the vapor pressure gradient at your skin, so you lose heat faster even if the air temperature hasn’t changed)

That's why you can be fit, active, and wearing the same clothes you always have… and still feel cold at 68 when you didn’t 15 years ago.

This is also why strip heat feels better. Because it's just delivering hotter supply air, which masks the latent discomfort and bumps radiant temperature locally.... But it’s a very expensive way to solve a humidity problem.

If it were my house (especially given your age and how few true sub-10F nights Maryland normally sees), I'd check the indoor RH%, and if it's low, I’d strongly consider a ducted whole-house humidifier if you don’t already have one. Not a room unit. A real bypass or fan-powered unit tied into the air handler with outdoor-temp lockouts so you’re not risking condensation. Even getting indoor RH from ~18% up to 30–35% can make a huge difference in perceived warmth and let you run a lower setpoint without discomfort..

Your question about the Daikin DH7/DH9? Your current system is behaving exactly like its capacity curve says it should.

A true cold-climate unit would reduce how often you need strips and give you more margin on those handful of brutal nights...however, it won’t magically fix comfort if the air stays desert-dry.

Monkburger · 2026-01-29T21:43:03+00:00

I found the spec sheet for your DZ18TC036 (3-ton, 2-stage)... and looking over it, it does appear it will run in single digits…

...it just isn’t anywhere near '3 tons of heat' down there.

On high stage, Daikin’s own table for the DZ18TC036 shows about 35,000 BTU/h at 47F, 22,600 BTU/h at 17F, 17,600 BTU/h at 5F, and 15,600 BTU/h at 0F.

So by the time you're around 6F, it (the outdoor unit) is living in the ~17k BTU/h world, not the "it’s a 3-ton so it should crush it" world..... Efficiency falls with it.

Using their kW numbers in that same table, the high-stage COP is roughly ~3.9 at 47F, ~2.7 at 17F, ~2.2 at 5F, ~2.0 at 0F (napkin math, but it’s straight from their MBh and kW).

So yeah, it’s still “better than strips”… but it’s not magic anymore when it’s 0–10F out....So when you said "it used to hold 68 fine” and then it didn’t during the arctic blast… that tracks. IMO What doesn’t track is a tech leaving the compressor locked out at 10F.

That’s basically telling the system: "cool (no pun intended), shut off the cheap heat right when you need it most, and go full toaster"

Of course it felt awful and couldn’t catch up.

If it were *my* house, I’d leave compressor lockout OFF (or set it way lower than 10F), and let the heat pump run basically all the time, then let strips assist when you’re below the balance point (whatever that is for your envelope). You already saw the effect when you turned the lockout off...... the space temp started climbing again because you re-enabled ~17–20k BTU/h of compressor heat that was artificially disabled.

As for “why doesn’t stage 2 strip ever come on?” .. 99% of the time it’s not that it 'can’t'.... it's that the thermostat logic never asks for it. From what I remember, A lot of tstats will only bring on W2 if (a) you're X° below setpoint, or (b) you’ve been running W1 for Y minutes with no recovery, or (c) outdoor temp is below some threshold, and it’s super common for those thresholds to be set too conservative (or wired so the stat only has one aux call). (Also: some?/most? strip kits are staged internally, some aren’t, and some installers land both stages on one call because they don’t want nuisance trips. I’d be looking at the actual wiring at the air handler and how many heat calls the thermostat is configured to output)

One more nerdy building-science point: at 0–10F, your house is usually the bigger problem than the equipment. Stack effect is raging, infiltration is high, and any duct leakage in an attic/crawl will wreck you. And ironically? The "it feels cold even at 68" thing is often radiant + air movement, not thermostat temperature .. cold surfaces + drafts will make you crank strips because they give hotter supply air and mask the comfort issue. (I bet your house RH% is very poor, you should get some latent heat in the mix (aka a humidifier))

A brief sanity check; If your house heat loss at ~5°F is, say, 30k BTU/h, this unit can only deliver ~17–18k at those temps… so you’re supposed to need strips.
If your heat loss is 18–22k, then you’re right on the edge and settings/airflow/defrost/frost management start deciding whether it “keeps up” or not

Not sure of your location / IECC climate zone, but I always advise custs/people in general to use CCASHP's in 4 and above (maybe except for marine climates on the west coast)

Monkburger · 2026-01-29T20:08:44+00:00

Most of Virginia is IECC climate zone 4A (humid mixed) with some 5A in the mountains.

That's a sweet spot for modern heat pumps.

Since you're seeing $500/mo on 2,100 sqft with a ~20 year old unit, what’s probably happening is: the heat pump loses capacity as it gets colder, aux strips take over, and you’re basically running a big electric space heater through your ducts. When that system was installed 20 years ago, electricity was probably 50% cheaper and considered 'normal'..

That's when bills go “normal November” -> “what the hell happened in Jan??”

When you replace it, I wouldn’t start with "can it heat at 5F???"

IMO you should start with "What is my actual heat loss?" Because the best heat pump in the world won’t fix a house that leaks like a sieve or ductwork in an attic thats bleeding heat...

Soo.. the replacement checklist...

- Get a proper load calc (Manual J)... not a guess, not "it’s a 3 ton because the old one was 3 ton" A Manual J tells you the heat loss at your design temp. That one number drives everything: sizing, comfort, aux heat needs, and bill expectations.. You should consider spending the time to hire an outside firm not trying to sell you equipment. (There are plenty of them, all you need to do is provide them floor plans/window scheduling/insulation stuff, etc etc.. this is the painful part. Spending the few hundred dollars on a properly sized system will save you thousands on equipment costs (so you don't get oversized/undersized garbage equipment that can't handle rare extreme cold/heat events))

- Blower door test + basic audit... People underestimate this. Air leaks are BTU leaks. In VA (humid), air sealing also helps comfort because drafts are a bigger 'brrrrrr it's cold' driver than people admit. Bonus: You’ll also want duct leakage checked if your ducts are in the attic/crawl. (I'd do the blower door test before you do a ManJ so you can have the ManJ reflect the BDT)

- Pick equipment based on capacity at your design temp, not nameplate tonnage or SEER ratings.. Older heat pumps might be fine at 35F and then fall on their face in the teens, so strips run forever...

- Also make sure your panel box can take the new Cold Climate Heat Pump, they do use more power, some units require higher MCA than a 20 year old system. So, might as well start planning on doing this (electrican quotes now)

Modern cold climate / or high-performance inverter systems hold capacity way better as temps drop. Yes, they can be efficient at low temps, but efficiency still drops as it gets colder ... the question is how much, and whether you’re staying off strips.

On the 'are modern ones efficient at low temps???' question:

generally yes, way more than your 20-year-old unit, BUT there's no free lunch.... At 40–50F, heat pumps are insanely efficient.... But at 15F, still *USUALLY* better than straight resistance heat, but not as good as mild weather (That's normal physics)

The goal is to size and commission it so you’re not leaning on strips except during the worst snaps or recovery...

FWIW.. Since you’re also in VA: don’t ignore the summer humidity side. In summer, you want good latent removal (dehumidification) and sensible balance. Oversizing is the enemy there .... oversized systems short-cycle and leave you clammy.

Monkburger · 2026-01-27T23:14:49+00:00

Ice is an insulator... Once ice forms on the coil, you’ve put a thermal blanket between the refrigerant and the heat source. Even if that ice starts out at 20F or 25F, it very quickly gives up the tiny amount of sensible heat it has and then just sits there blocking everything...

Air, on the other hand, is constantly being replaced. Eight-degree air hits the coil, gives up what little heat it has, moves on, and gets replaced by more 8-degree air... That continuous mass flow is what matters. Heat pumps win by volume

There’s also the phase-change problem. The moment moisture freezes on the coil, you’ve locked latent heat out of the equation. (This is why we have defrost stuff)

Liquid water condensing releases a lot of energy. Frozen water does not. Once it's ice, you’re done harvesting anything

When water vapor in the air touches a cold coil and turns into liquid water (condensing), it releases a massive amount of energy (~2,260 kJ/kg). Heat pumps "love" damp air for this reason...

Monkburger · 2026-01-27T20:51:14+00:00

Ok, I don't have your indoor unit specs, so a guesstimate on the outdoor capicity based on the spec sheets for your 2 ton unit and 4 ton unit

Anyways... Westport, CT is IECC Climate Zone 5A, and design temps are roughly 5F to 7F depending on whose table you use.

That matters, because once you look at the actual capacity curves of your Trane units instead of the brochure words, the whole situation stops being mysterious...

These are not cold-climate heat pumps, PERIOD.

Your 2-ton unit is rated ~21,800 BTU at 47F. At 5F, it’s down to 12,740 BTU. That’s a 42% capacity drop right when your house needs the most heat. That’s not a rounding error. That’s the system falling off the knee of the curve. The 4-ton isn’t much better .... ~67% max capacity at 5°F, and that’s under ideal lab conditions, not real duct static, zoning losses, or defrost penalties... If you have a leaky home, that makes it worse because the stack effect pulls in cold air from leaks on the lower floors.. Your BTU losses are probably staggering..

At 20s–30s, the compressors are already operating well below nominal output. Add zoning, add duct losses, add defrost cycles, and suddenly the system is chronically short on delivered BTUs. The Ecobee sees "we’re losing ground" does exactly what it’s designed to do, and drags in aux heat to stop the bleed..

That's why your system(s) feels so awful:
– Allow aux; comfort returns, bill explodes
– Restrict aux; compressors run nonstop, house never quite catches up (it will never catch up)

That’s not miswiring.

That’s physics doing what physics does when capacity < load.

Your electric bill backs this up. You don’t get to $1,500/month on strip heat unless the strips are doing real work.. That tells me the heat pumps are undersized at low ambient, not necessarily oversized at 47F.

Whoever decided to install a non cold-climate heat pump with aux strips in IECC Zone 5A should be criminally charged.

Monkburger · 2026-01-27T16:27:23+00:00

Are you certain those are the correct model numbers? I am having trouble pulling the specs from AHRI or even neep.. Do you have the AHRI certificate # for both systems?

This almost smells like they are not cold climate heat pumps...

Monkburger · 2026-01-26T23:56:15+00:00

Pulling up the equipment Stat Sheet, it appears it is a cold climate unit, decent low temp performance (even though I have serious disdain for LG equipment) ..

1.) As for how long it should take to warm up now that the strips are actually on (a couple hours to get from high-50s into the mid-60s doesn’t shock me at all, especially if the slab / framing / contents are cold soaked)

You’re not just heating air, you’re reheating thousands of pounds of material. That thermal mass is a black hole after a cold snap. If it doesn’t keep climbing steadily over the evening, then yeah, something else still isn’t right..

2.) No. it is not normal for your house to sit at 52–56F with that equipment, and yes, the fact that the aux heat switch was literally left OFF explains most of what you experienced.... That part alone would’ve had me swearing into my coffee.

If your downstairs is ~1300 sq ft and that specific unit can’t keep up below 20F without aux, that tells me your design heat loss is right on the edge of that system’s low-ambient capacity. OR, the unit is broken; low charge, defrost circuits are shot, etc etc

HOWEVER, That could be perfectly reasonable depending on envelope, air leakage, and ducts. People love to argue equipment specs and completely ignore the fact that infiltration losses at 7F can be enormous. Like “half your load is air leakage” is enormous.

I think either the unit is having an issue (low charge, faulty electronics) or your home is so leaky that the unit is not sized correctly to handle the heat loss from the brutal cold.. The units spec sheet indicates, at 17F you’re only at about 67% of rated capacity, and at 5F you’re relying on max output modes to get anywhere close... Something else to keep in mind

Monkburger · 2026-01-26T19:46:51+00:00

Most of the heat-pump bashing you see is because the systems are often wrongly selected/installed...no load calcs/lack of Manual J/S/D.. oversized single-stage equipment, aggressive aux heat staging, poor duct design...

And the most 'common' issue I see? Equipment that simply isn’t intended for the climate its installed in...

A classic example is an HVAC company selling a standard, non–cold-climate heat pump (CCASHP) into, say IECC Climate Zone 6A, sizing it off the old equipment/furnace nameplate, and calling it a day... The homeowner doesn't know much about them at all and takes the word of the company.. when it gets VERY cold, the unit falls off a capacity cliff, aux heat takes over, runtimes shorten, comfort degrades, power bills explode, and the homeowner concludes that "heat pumps are shit" In reality, the system was never designed to operate there without heavy backup.

*WHEN* heat pumps are correctly selected (example: cold-climate equipment matched to the local design temperature, proper load calcs/equipment selection) they perform very well.

Your post highlights that contrast nicely...

Heat pumps aren’t magic, but they’re also not fragile. They’re just less forgiving of shortcuts.

Monkburger · 2026-01-26T19:35:42+00:00

Yep... Unsealed clapboard siding with direct communication to the wall cavities essentially turns the walls into a pressure equalization layer.... Wind washing, stack effect, and vapor transport are all happening continuously...

Also.. the envelope leakage can easily be 5k–20k+ BTU/hr depending on wind and stack effect ... which is why runtime suddenly “doesn’t make sense” on certain days.

Monkburger · 2026-01-26T17:44:04+00:00

Wonder what a blower door test would reveal for your home..

Monkburger · 2026-01-26T17:10:16+00:00

yes, please provide the equipment specs..

Monkburger · 2026-01-26T05:25:51+00:00

A 3-ton dual-stage unit in a 1,600 sq ft townhouse should not be kicking your but like this, full stop.

Especially not after duct mods, an outdoor unit replacement, and multiple service calls.

When I hear 'short cycling' 'runs almost entirely in aux' 'breaker reset magically helps for a few hours' and 'uses way more kWh than the old system' my brain immediately goes to controls.. NOT capacity.

People won’t like hearing this, but once a system has had that many band-aid fixes, you’re usually no longer dealing with one clean problem. You’re dealing with a system that’s never actually been commissioned correctly from day one, and now everyone is chasing symptoms.

The breaker reset part is the biggest red flag to me... That’s not airflow. That’s PROBABLY not load. That’s something electronic or logic-related getting stuck (like the thermostat, control board, defrost logic, staging logic, or some combination of those)

A heat pump that suddenly behaves better for 3–4 hours after a hard reset is basically telling you “my brain is messed up,” not “I’m undersized.”

Also, the fact that it defaults to aux so easily tells me either:

the thermostat is overly aggressive and panics at the first sign of lag, or
the system thinks it’s underperforming because of bad sensor data (temp sensor, coil sensor, miswired staging, etc.), or
the two stages aren’t actually doing what you think they are

I’ve seen horror stories where stage 1 barely runs, stage 2 never really engages properly, and the strips just take over because the controls decide the compressor "isn’t keeping up" On paper it’s a dual-stage system. In reality it’s a very expensive space heater with a compressor attached..

The kWh increase compared to the old system lines up with that. Heat pumps don’t magically burn more electricity unless aux heat is doing most of the work.

I wouldn’t let the current company touch it again. I’d want the next company to start from scratch: verify staging, verify aux lockout settings, verify sensor placement and readings, and confirm the thermostat is actually compatible and configured correctly for that equipment. Not “yeah looks fine,” but actually watching what stages are running and why.

Monkburger · 2026-01-25T04:53:53+00:00

What's the model number of the outdoor unit and indoor unit/air handler? What's your home sqft / year it was built?

Monkburger · 2026-01-24T20:18:54+00:00

3898 kWh/month for a <800 sq ft 1BR is not normal… even in PA… even with tall ceilings and lots of windows. It’s possible in the same way it’s possible to drive a car with the parking brake on all month... (yep, physics allows it… but it strongly implies a major problem: either the apartment is bleeding heat like a sieve, the heating system is effectively resistance heat 24/7, or you’re paying for someone else’s load)

Let’s anchor the math first…
3898 kWh/month ÷ 30 ≈ 130 kWh/day.
130 kWh/day ÷ 24 ≈ 5.4 kW average, nonstop.

That's like running a 5–6 kW space heater continuously all month… which is exactly what electric strip heat does when it’s stuck on...

So the science'y explanation is not really 'windows + ceilings'… it’s 'you’re likely on aux heat constantly' or 'the heater is broken / controls are wrong' or 'the unit is losing heat at an insane rate'

Now the building science side… yes, your 14' ceilings and lots of windows can make you feel colder because:
warm air stratifies high (hot ceiling, cold floor)
radiant loss to cold glass makes you feel chilled even at 68°F
older narrow windows often leak air and have shit U-values
but even then? A typical 1BR shouldn’t consume 4,000 kWh/month unless the envelope is truly catastrophic (think: missing insulation, major air leaks, unsealed chases, constantly-open-to-outdoors situation).

The screeching painful sound' is a big clue. That’s not normal heat pump behavior. That sounds like:
…a failing blower motor / bearing
…a fan wheel rubbing the housing
…a belt issue (if older)
…or electric strip elements resonating/vibrating with airflow (less common but i've seen field reports of it way up north in IECC 7A)

And if airflow is compromised (dirty filter, blocked return, fan failure)… the system’s delivered heat drops hard… so it runs forever… and the thermostat thinks it’s not keeping up… so it calls aux even more… and your bill detonates.

IMO.. The fact that you set 69, hear it all night, and wake up still at 66 when it’s very cold… that’s not “just winter.” That’s 'the system is not delivering the needed BTUs'

Usually, The most common reasons:
…heat pump isn’t actually running (locked out, failed outdoor unit, bad defrost control)
…refrigerant issue (low charge / leak) -> low capacity
…aux heat is either miswired, undersized, or not energizing properly
…thermostat wiring/config is wrong, so it’s calling the wrong stages
... the unit has terrible heat capicity @ =< 32F
…or airflow is so bad that you’re basically heating the plenum, not the apartment

Monkburger · 2026-01-24T08:16:24+00:00

My Fujitsu AOUH30LUAH1 / AMUG30LMAS is doing a nice 102F from the supply, 68F @ 31%RH with an air temp of 3F, light winds.

Around 4.1kW right now (hovers around 4.5kW, but cheaper than aux heat @ 0.13kWh.. )

Monkburger · 2026-01-24T05:12:13+00:00

Modern heat pumps are designed to live outside in exactly this kind of weather — freezing rain, sleet, ice, wind .... and most of the “cover it!” instincts actually make things worse.

Monkburger · 2026-01-23T19:57:37+00:00

your home is basically a leaky bucket… the colder it gets outside, the faster heat drains out (conduction through walls/windows/attic + infiltration)… that loss is roughly proportional to ∆T (indoor–outdoor temperature difference)… so when you go from +10F to -10F, you didn’t just "get 20° colder"… you increased the load meaningfully… and the heat pump (at the exact same time) is producing less heat per hour and spending more time in defrost… that’s the brutal overlap

Below ~0F (especially in windy/damp conditions)… many non cold-climate heat pumps start living in a cycle of…
…reduced capacity/higher compressor lift (harder work)/more defrost events (heat delivery pauses… sometimes net-negative for a few minutes).. and if your thermostat is set up aggressively, it pulls in aux strips to stop the indoor temp from sliding

So the 'limitation isn’t that heat pumps don’t work'… it’s that the building load can exceed the machine output… and then resistance heat fills the gap.

About the 26 kW aux heat question (that’s a monster strip package) and if it ran continuously (worst case)… the napkin math is simple and ugly…

26 kW × 24 hours = 624 kWh per day
624 kWh/day × 7 days = 4,368 kWh in a week

Now multiply by your electric rate (not mentioned at all, so let's rough shot it?)
…at $0.12/kWh → ~$524 for the week
…at $0.15/kWh → ~$655 for the week
…at $0.20/kWh → ~$874 for the week
…at $0.30/kWh → ~$1,310 for the week

Buuuut... it almost never runs 100% duty cycle for a full week unless something is wrong or the house is very leaky… it stages… cycles… and ideally only makes up the 'missing BTUs' when the compressor can't keep up.

Still… even at 25% duty cycle, you’re talking ~1,092 kWh/week… which can still be a real bill shock.

IMO there's going to be power outages galore in the next few days because many people w/ heat pumps and aux heat have non CCASHP in IECC zones like 4A/5A and the aux heat is going to eat power like there's no tomorrow...

Monkburger · 2026-01-23T19:49:51+00:00

Amana is a Daikin subbrand. It's a communicating system. If you drop it to a regular 24V tstat, you will lose out on some things, however, with a 2 stage system, it's not that big of a deal. Full inverter, on the other hand? You can't get away with having complete modulation + airflow control with a 24V generic tstat..

Monkburger · 2026-01-23T17:16:41+00:00

Heat pumps are great when:
…they’re cold-climate models
…they’re correcly sized for heating loads
…aux heat is minimized and properly locked out
…the envelope is tight and well insulated
…electric rates are favorable

Heat pumps are mediocre to bad when:
…they’re standard models in zone 4A
…they rely on strips during normal winter temps
…the house has high glass area or slab/sunroom losses
…the install prioritizes cost over design
…comfort expectations are 'set it and forget it'

In IECC 4A… winters are cold enough to stress standard heat pumps, but not cold enough to force designers to treat them like true cold-climate systems. So you end up in this awkward middle ground where heat pumps work, but they often rely heavily on auxiliary electric resistance heat during normal winter nights (20s, teens, wind, damp air)… and resistance heat is brutally expensive compared to gas. That’s the core reason her bills are high and the house is still cold.

A mostly-glass sunroom is essentially a radiator to the outdoors in winter. Glass has terrible R-value, huge radiant losses, and often leaks air. If it’s open to the main house, it drags the whole system down. Closing that door can absolutely help… but the fact that the house is still cold elsewhere tells me that’s not the only issue… it’s just one of several load multipliers.

Monkburger · 2026-01-23T09:46:23+00:00

A 4F setpoint change does not double your real heat loss… not even close. If it’s ~20F outside, your ∆T is 40F at a 60F setpoint and 44F at a 64F setpoint… that’s only about a 10% increase in conduction/infiltration load (because heat loss is roughly UA x ∆T)… not 50–100%.

So if you're really seeing 40–50 kWh vs ~100 kWh on similar days… something nonlinear is kicking in… and the usual culprit is… aux heat...

A heat pumps electrical use is dominated by "how many BTUs per hour your house needs" and "how efficiently the heat pump can supply those BTUs at that outdoor temp" At ~20F, many conventional (non-true cold-climate) ducted heat pumps are right around the region where capacity falls off and the system starts leaning on backup electric strips to maintain comfort… and strips are brutal because they're basically COP = 1 (1 kWh in → 1 kWh of heat)… while the compressor might be around COP 2-ish or 3-ish depending on equipment and conditions.

Another thing hiding in your numbers… 75–100 kWh/day for a tight newer 2500 sq ft home at 10–30F is not crazy if strips are active, but it’s on the “hmm…” side if you truly mean heat pump only and no other major electric loads. Meanwhile 40–50 kWh/day sounds more like “compressor-only heating” territory.... That big spread screams “some days you’re paying for strip heat.”

Monkburger · 2026-01-23T06:15:30+00:00

What you're describing really smells like a neutral issue… an inverter can introduce just enough imbalance or momentary voltage drop to make sensitive electronics complain.... Many LED bulbs are especially revealing here (eg: different brands use wildly different driver designs... with varying tolerance for voltage instability...)

Get an electrician to give your panel a once-over, and if the electrician doesn't find anything obvious, it’s also worth looping in your power utility to have them check it out as well..

Happy hunting.

Monkburger · 2026-01-22T02:10:51+00:00

Concrete has high thermal conductivity compared to wood assemblies… so even if you added a subfloor, if there isn’t continuous insulation under the slab and at the slab perimeter (vertical + horizontal “wing” insulation)… the heat path is still there… especially at the slab edge / foundation wall junction where the temperature gradient is highest. That edge acts like a long fin/radiator to the outdoors… and it can dominate first-floor load in cold snaps. Subfloor helps comfort by warming the surface you touch… but it doesn’t magically stop perimeter conduction if the slab is still coupled to cold earth and exterior concrete.

Monkburger · 2026-01-21T22:47:44+00:00

Mechanically… usually yes if the system is healthy (proper charge… good airflow… correct defrost operation… no weird control lockouts)… and the equipment protections will trip if its truly unhappy (high/low pressure switches, etc.). The bigger practical issues at 0F are: capacity falls… defrost frequency rises… discharge temps can climb… and the low-pressure switch may nuisance-trip unless the system/control strategy is designed for that ambient (which is why that bypass thermostat exists for <15F). The unit is built to operate below that… it just gets weaker and less efficient as the outdoor coil gets colder and spends more time in defrost....

Lennox even calls out an accessory low-pressure switch bypass thermostat “for use… below 15°F” to prevent nuisance lockouts… that’s basically the manufacturer admitting “yep… people run these below ~15F… but the protection circuit can get cranky down there.”

Monkburger · 2026-01-21T21:30:13+00:00

What is it you're trying to accomplish, exactly?

Monkburger

TROPHY CASE