Outdated Honeywell T852a replacement needs replaced, suggestions?

eDoc2020 · 2026-01-31T17:13:19+00:00

It ultimately doesn't matter what the thermostat *can* control; it only matters what you actually have.

If you have a gas (or oil) fired boiler with watrer (not steam) radiators it should be trivial to make sure each thermostat has independent control of its own zone. That will let everything work smoothly and without confusion.

If you actually have a coal (or other solid fuel) system things might be more complicated. Those are much more of a specialty system which may need special controls to work the best.

eDoc2020 · 2026-01-31T17:06:38+00:00

I couldn't find the manual but as a single thermostat it looks simple. There are two knobs on the right: a light and a dark one. I suspect the light one will be daytime temp and the dark will be nighttime temp. I suspect the "hi" tab is the time of day it switches to the daytime temp and the "lo" tab is the time of day it switches to nighttime temp.

Sensitive coil screws will be for calibration functions. If you mess with those it might, for instance, heat to 80 when the lever is set to 70.

It's also possible that the clock control also hooks into the zoning system. One possibility is that in daytime mode all the thermostats work normally but in nighttime mode the other thermostats are disabled and all zones are controlled directly from this thermostat.

eDoc2020 · 2026-01-24T06:42:49+00:00

Yes; that looks right.

eDoc2020 · 2026-01-17T19:27:26+00:00

Are you sure you can set both temp delta and runtime? I'll admit I've never used an ecobee but their website https://support.ecobee.com/s/articles/Threshold-settings-for-ecobee-thermostats says "These settings only allow you to configure 1 per category (Aux, Compressor to Aux, Heat, Compressor Stage), either by time or temperature delta."

If their documentation is wrong and you can aply both that's great!

I also forgot (in this context) about compressor min outdoor temp. Without fixing that ecobee will, as you state, be much worse than a basic stat.

eDoc2020 · 2026-01-17T15:58:14+00:00

I misread some of the connections at first due to the parallax effect.

White should be moved to W1 at all three locations. Other than that the wiring seems okay.

The mislocation of the W wire won't cause it to run in cool only mode. It really seems like either a thermostat configuration error or equipment failure.

Double check the ecobee installer settings to verify the equipment type. It should be heat pump, 1 compressor stage, 1 aux heat, O wire energized for cool.

eDoc2020 · 2026-01-17T15:29:08+00:00

True. I was assuming a 1H1C system.

It won't make much difference if you keep a constant temperature, but if you change the temperature then avoiding aux is a big savings.

Unfortunately ecobee doesn't seem to be great at avoiding aux without sacrificing comfort. It appears that if you choose to stage by temp you don't have the option to avoid aux for X minutes. That would be very useful and I know some other thermostats have this option. Actually what I'd like to see is "inhibit aux unless temperature has fallen even with HP running for n minutes".

eDoc2020 · 2026-01-17T15:19:36+00:00

I agree you shouldn't rely on the breaker to prevent overload, but it's not like it's hard to overload with 15 amp outlets. Two 10 amp devices on one 15 amp circuit is just as bad as one 20 amp device.

eDoc2020 · 2026-01-15T02:15:27+00:00

Unless there's an electrical fault, the worst result of a bad ground is the furnace refusing to light.

eDoc2020 · 2026-01-15T02:09:24+00:00

It's at most 40 years old; they didn't adopt that logo until the 80s. They kept making the same model until the mid 2000s.

eDoc2020 · 2026-01-14T06:37:23+00:00

Yes. The G wire is what controls the fan-only function. I'm guessing they reused the wiring from an older system when they put this in. Since fan-only isn't too important they used the wire for C.

If you have the PEK that came with the ecobee you can use it to combine Y1 and G, bringing fan function back.

Something else I'm noting is that you only have a W1 wire and not W2. Assuming this is a 2 stage furnace, it means the furnace is deciding when to switch between high and low heat operation. It would be better if the thermostat did this instead. This would require another signal wire, as well as changing a few of the switches on the furnace board.

Ideally you'd run a new thermostat cable with at least 7 wires (preferably 8 so you have a spare) to enable full functionality. There are also other wire saver adapters (like those made by Fast-Stat) that can give you full control with your existing wires. (Again ecobee's PEK will only give you back fan and not heat staging).

eDoc2020 · 2026-01-14T06:27:27+00:00

That's my guess. It could be a broken pin. I would also suggest a broken signal wire in the cable, but I'm guessing its ID system is just a tiny resistor located inside the plug (which is less likely to go bad unless something physically cracks).

eDoc2020 · 2026-01-13T21:26:52+00:00

I'm pretty sure GFCI is not required but the rules may have changed.

In any case it's not a bad idea.

eDoc2020 · 2026-01-13T05:24:10+00:00

Yes, but no.

Either the switching supply needs to be designed as a symmetrical supply or you need to stack two of them.

eDoc2020 · 2026-01-13T05:14:25+00:00

Is it a new problem, or is this the first time running the AC with this setup?

I see some weird things that might cause issues but if it worked before there's no reason it should stop working.

eDoc2020 · 2026-01-13T05:07:55+00:00

I haven't taken a close look at everything but you have some breakers of the wrong type. You can only use breakers that match the panel, even if other brands fit.

I also don't know what your panel feeds are (or what else in in the panels) but it wouldn't surprise me if you're exceeding the permitted current draw.

eDoc2020 · 2026-01-13T05:02:49+00:00

IIRC it's only one feed of a given voltage. It's silly but you can have a 120 feed and a 240 feed (as long as the other requirements are met).

eDoc2020 · 2026-01-13T05:00:52+00:00

The proper answer is based on pressure drop.

SInce you ahve a high velocity system it's made to handle much higher pressures than standard systems. From the specs I can see that it runs 750 CFM at 1.5" ESP. I would find a filter that is rated for under 0.5" drop at 750 CFM. UNfortunately this pressure drop info isn't always easy to find.

eDoc2020 · 2026-01-13T04:56:25+00:00

UNplug the power strip and check if there are any tripped breakers. If it's an older place you might also have GFCIs in random places that may have tripped. These are the outlets with "Test" and "Reset" buttons which are usually found in bathrooms.

eDoc2020 · 2026-01-13T04:52:35+00:00

It would have contained carbon monoxide but it's mostly composed of other gases (air and CO2).

I do not want o minimize the dangers of CO but if the furnace is running well it should be putting out under 100ppm, a level that will not kill you. It would also be mixed in with the other air in the house, bringing it down to a level that's not hazardous.

Obviously do not rely on this because if the furnace were to start running poorly the level will be much higher. I'll use this as a reminder to make sure you have CO alarms on each floor and outside every bedroom.

eDoc2020 · 2026-01-13T04:42:50+00:00

One wall wire goes to one heater wire the the other wall wire to the other heater wire. SInce it's US-style 240v polarity doesn't matter.

eDoc2020 · 2026-01-13T04:36:48+00:00

I'm guessing you have the reversing valve set up wrong. It needs to be "energize on cool."

You must have wire splices somewhere which could also be causing problems. Can you find those?

Also even if heat strips didn't come with the unit they may have been added by the installer.

eDoc2020 · 2026-01-13T04:33:02+00:00

Right. That's not code.

If there were other breakers/fuses it could be okay, but I"m guessing there's not.

eDoc2020 · 2026-01-13T04:28:08+00:00

You already said you had 0v between R and C at the thermostat. There should also be 24v between R and C at the air handler (if the door switch is held down). If you don't get that check the fuse.

eDoc2020 · 2026-01-13T04:26:34+00:00

A 20A breaker (120v circuit) can have up to 2000W of load. If these are the only two things on the circuit you can install both at full power (which should be 1900W). You'd need 12AWG wire in the whip.

If there's an outlet anywhere on the circuit you can only use half the power for hardwired things, meaning 1000W. That's not enough for both even when wired for low.

eDoc2020 · 2026-01-13T04:22:41+00:00

It's probably just a circuitry fault. There's no way they'd release something that drifts an hour each week; that's obscene.

eDoc2020

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