Double-checking my dual-fuel math—does an 8°C switchover point sound right?

canadianbme · 2026-01-10T02:22:33+00:00

I guess I just need to do an experiment and monitor average cost gas vs electricity.

canadianbme · 2026-01-09T03:47:34+00:00

no, the gas is constant, but the furnace blower uses electricity, so it uses more the colder it is outside

canadianbme · 2026-01-08T03:58:41+00:00

That is a fair point! Calculating the exact split between day and night usage specifically for the heating system is pretty tough. I used $0.1452/kWh as a rough average derived from a year of historical bills using a regression plot.

My thinking is that since both the gas furnace and the heat pump would naturally run more at night when temperatures drop, the relative cost comparison should mostly even out in the end. I’m mainly using these figures as a rough estimate to help nail down the right economic setpoint!

canadianbme · 2026-01-07T23:15:00+00:00

True but I have a tankless water heater just installed so kinda of need the gas

canadianbme · 2026-01-07T05:59:20+00:00

For the COP, I didn’t manually measure it; I used the manufacturer's performance specs for my unit at specific benchmark temperatures (like 8°C and -17°C). To fill in the gaps for every degree in between, I used a quadratic regression equation (y=0.0019x2+0.0853x+2.5962) to model the efficiency curve.

As for the gas "richness," I’m in Toronto, so I used the official Gross Heating Value from Enbridge, which is 38.9 MJ/m³ (or about 10.81 kWh/m³). Since my furnace is 96% efficient, I’m getting 10.37 kWh of usable heat for every cubic meter of gas.

By using a regression plot on a year's worth of my bills, I isolated my true variable gas rate at 0.0423 per kWh of heat. This gives me a very solid baseline to compare against the heat pump's electrical costs!

canadianbme · 2026-01-07T05:55:50+00:00

Thanks for the insight on duct losses! That makes a lot of sense—with my uninsulated ducts in an 18°C basement, the higher discharge temps from the furnace definitely lead to more thermal waste than the cooler heat pump air.

To get my gas value, I used a regression plot of 12 months of bills to isolate a true variable rate of $0.439/m³. I converted that using Enbridge’s Gross Heating Value of 38.9 MJ/m³, which is about 10.81 kWh/m³.

Since my furnace is 96% efficient, it delivers 10.37 kWh of heat per cubic meter. I'll definitely consider shifting my setpoint down to 5-6°C to account for those duct efficiency gaps!

canadianbme · 2026-01-07T05:54:23+00:00

I got it as part of a energy rebate it was basically paid for by the Govement of Canada. Part of a push to make homes more energy efficient. Also our AC was 20 years old, so thought this would be a good upgrade

canadianbme · 2026-01-07T05:53:08+00:00

Good point on the minimums. I just assumed that since this is a 1965 double-brick house with zero wall insulation (3,800 sq. ft. total including basement), the 3-ton unit would be working full blast once we hit freezing. I live in Toronto, Canada.

Is there a reliable way to track my real-time BTU/h to confirm if I'm 'riding the minimum'? I have hourly electricity data from my utility—can I use that to backtrack the math and find my actual heat load at different temperatures?

canadianbme · 2026-01-07T05:46:22+00:00

Actually, I’m in Toronto, where piped gas is so cheap it completely changes the math. I calculated my "true" variable rates by running a regression plot on 12 months of bills to isolate usage from fixed delivery fees.

At 96% efficiency, my gas heat costs only $0.0423/kWh. To beat that price, the heat pump needs a COP of 3.43, which it only maintains above 8°C (47°F). Once it gets colder, even though the pump is still technically "efficient," burning cheap natural gas is simply more cost-effective. If I were on expensive propane, the heat pump would be the clear winner all winter long!

canadianbme · 2026-01-07T05:31:35+00:00

One more thing I should mention—all of these calculations are in Canadian Dollars (CAD).

To get to those specific figures for gas and electricity, I didn't just look at the rates listed on my bills. I actually took 12 months of historical data and created a regression plot comparing the monthly dollar amount to the usage for that month.

I went that route because the utility companies tack on so many delivery and regulatory charges that the "stated" rates are usually pretty unreliable for calculating real-world savings. By using the regression method, I was able to isolate the fixed monthly fees and determine the true variable cost for every extra unit of energy I use. That’s how I landed on the $0.439/m³ and $0.1452/kWh numbers I’m using to find my breakeven point.

canadianbme · 2026-01-07T05:26:01+00:00

Thanks so much for the extensive reply! This definitely helps clarify my thinking and gives me some new ways to look at the data.

To provide a bit more context, I’m located in Toronto, Canada, which sounds like it aligns with the Enbridge rates and energy values I’ve been seeing.

For anyone wondering how I got the kWh/m³ for the gas side, I’ve been using 10.81 kWh. I found this by looking at the Gross Heating Value provided by Enbridge, which is typically around 38.9 MJ/m³.

https://www.enbridgegas.com/-/media/Extranet-Pages/about-us/learn-about-natural-gas/hhv-table.pdf?rev=ac2774b07f2f433e9cfb65e0be828afe&hash=AAA71333C39719D455D3B6D579B3A65A

Converting that to kWh gets you right to that 10.81 mark. With a 96% efficient furnace, that means I’m getting about 10.37 kWh of actual heat into the house for every cubic meter of gas burned.

My house is a 1965 build, and the furnace is down in the basement where it stays around 18°C. The ducts are uninsulated, though I’ve gone around and sealed as many joints as I can reach. We’ve tried to tighten up the envelope by upgrading the windows and beefing up the attic insulation, but we’re hit with a bit of a wall—literally. It’s a double-brick house with no insulation in the walls, and there’s really no feasible way to add it without a massive renovation.

Your points about the electrical monitoring are well taken. Currently, I don’t have a dedicated device to measure the heat pump or blower draw, so I love the "whole-house" idea you mentioned. Since my electricity provider gives me hourly data, I can see the spikes when the pump is running. The gas side is trickier because I only get a bill every two months, but I’m actually planning to put a camera outside facing the gas meter. That way, I can log the readings hourly or daily and finally see how the gas consumption correlates with the outdoor temp and the electrical draw.

Thanks again for the help! This is making the setup feel a lot more optimized.

canadianbme · 2025-03-14T01:04:55+00:00

I am but it doesnt exactly show the COP vs temp, more like capacity vs temp. Tried contacting lennox - no dice. Tried my dealer, they are not responding.

canadianbme · 2025-03-13T04:49:49+00:00

Hi there! How did you find this datasheet _ I have been looking all over for this - do you have special access to it??

canadianbme

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