Anyone got scientific figuring out change over temp for dual fuel?

A few months ago I changed out my old AC unit and put in a heatpump and now I am running a dual fuel setup. I have a 93% 75K btu gas furnace and my heatpump has a hspf of 9.5 (means 9.5 btu per 1 kwh) but the COP even at -5f outside is still 147%. But obviously at some point even though the heatpump is more efficient … It cost more to run and even at 36000 btu I am not sure it could even keep up without my furnace cycling at one point.

Im going to build a HA soon, but my sense hasnt found stage 2 yet. Stage 1 is ~1719 watts with HVAC fan, compressor and outdoor fan (low 90’s temps from the vent). Stage 2 adds around another 1000 watts but the vent temp is mid 90s. With the gas furnace running the HVAC fan is ~527 watts but the temps are 130+… and Im buying gas

You’ll want to find a data sheet for the COP vs. outside temperature of your particular unit, if you can – sounds like you already have that if you know the COP is 1.47 at -5F? If you don’t have that, I’m not sure what the best way to approximate it would be… but maybe your HVAC installer will have the data sheet. In the data sheets I’ve seen, they helpfully account for the efficiency loss due to defrost cycles, and include the power consumed by the matching indoor unit.

Then, you can do the following, for the furnace: gas price / furnace efficiency gives you the gas cost per unit heat, and furnace elec. power * elec. price / furnace output capacity = electric cost per unit heat. An example, with all the unit conversion factors, for silly American units, using prices from around me:

\$1.50/thm (gas price) / .93 (furnace efficiency) = \$1.61/thm in gas costs from the furnace
\$0.20/kWh * 0.5 kW / (75,000 BTU/hr * 0.93 / 10^6 BTU/thm) = \$0.14/thm in electrical costs from the furnace
So, \$1.75/thm from the furnace.

In equivalent units, you can figure out the COP you’d need from the heat pump to match that price. The heat pump’s cost per unit heat would be \$0.20/kWh * 29.3 kWh/thm / COP, or \$5.86/COP per thm.

So, to match the furnace’s cost with these prices, the COP needs to be above around 5.86/1.75, or around 3.3.

Of course – the actual installed efficiency of these units is going to vary, so trying to fine tune it too much might prove a waste of time!

1 Like

Turns out I was looking at the incorrect size unit when I looked up the values. It’s 177% on low and 168% on high.

My tariff rate for gas is a flat rate of \$9.914 per MCF… which some how MCF stands for “one-thousand cubic feet”… and also it’s not really 1000, it’s 1,032 cubic feet. MCF is = to 1 million BTU’s. Seems like trading money with the bank.

Then my tariff rate for electric will be somewhere around ~12¢ per kWH. We actual have 2 tiers of bill which gets cheaper the more we use, then in Dec, Jan and Feb we have a 3rd tier. I normally hit the 2nd tier in the winter (over 500kwh) which ends up around 8.5¢… 3rd is over 1350kwh and is ~5.2¢

Furnace ….
Are you saying \$9.914/ .93 = \$10.66
Then the Electric \$0.085 x .5 kWh/ 75,000btu *.93/ ???

Then heatpump
\$0.085x (completely lost me here)

I read online people are doing the switch over in the low 30’s with the 80% eff furnaces and mid to high 30’s with a 90+ eff… Im guessing that’s too early for my setup and billing rates.

Ah, so, my units don’t match yours because my gas company bills in “therms” – which are a unit of energy, not of volume (they apply a conversion factor when they calculate the bill.) A therm is 100,000 BTU, or enough energy to heat 100,000 lbs of water by 1 degree Fahrenheit (see, these are all very silly US units…) I decided to stick with therms through the calculations, because it comes up with some nice reasonable dollar figures in the end, rather than having to deal with fractions of cents. A MCF of gas is about 10.37 therms of heat energy, so your gas rate in my units would be about \$0.96/therm. Electricity is also billed in units of energy, and one therm is 29.3 kWh. (So, an electric space heater would consume 29.3 kWh to put out one therm of heat.)

Going through my silly calculations, then (ignore the weird formatting, the forum software thinks this is code so it’s highlighting certain keywords):

For the furnace:

``````\$0.96/therm (gas rate)
---------------------- = \$1.03/therm (gas bill per unit heat)
0.93 (efficiency)

\$0.085/kWh (electric rate) * 0.5 kW (furnace fan, 500W)
------------------------------------------------------------------ =
75,000 BTU/h (furnace input) * 0.93 / 100,000 BTU/thm (conversion)

\$0.0425/hr (cost to run furnace fan)
----------------------------------- = \$0.061/therm (elec. bill per unit heat)
0.698 thm/hr (furnace heat output)

Total for furnace: \$1.09/therm
``````

For the heat pump

To match \$1.09/therm with your heat pump, here’s the calculation:

``````\$0.085/kWh * 29.3 kWh/therm (unit conversion)
--------------------------------------------- = \$1.09/therm (to match furnace)
COP

Rearranging it, we get:

COP =

\$.085/kWh (elec. rate) * 29.3 kWh/therm (conversion)
---------------------------------------------------- = 2.28
\$1.09/therm (furnace cost per unit heat)
``````

End result

So, looking at the data sheet, you could set your switchover point as low as 5-10 degrees! Whether the unit can actually provide enough heat for your home at that temperature is another question, but your thermostat should handle that for you. Given you have a 75,000 BTU/hr furnace, unless the furnace is drastically oversized, I suspect the heat pump wouldn’t be sufficient, producing only around 18,000 BTU/hr at those temps.

One thing you should make sure of is that the electric rate you quoted was inclusive of both generation and delivery charges – for whatever reason, at least in my state, the utility puts these two prices on very different parts of the website.

3 Likes

Thanks! I felt pretty surely that the change over temp would be much lower.

My thermostat does do a lot of cool things and I can change a lot of setting / set points / temps/ humidity / dead and temps or timers, fan speeds/ cycles/ defrosts and such. Even can run a local/open API and still work on their cloud. But I can’t make my “fossil fuel” work like stage 3 or 4. I have to set a lock out temp and a change over point.

I take that back about the my thermostat doesn’t work like that. If i have the setpoint at +6 degrees, the thermostat stays “Stage 4 heating” and actually kicks the gas in. I did know that it would do that. I use the Geofencing and the scheduling so this function will probably work pretty well and I am sure that I will tweak it since I can set it to kick in if the 2nd stage hasn’t done anything for (however many mins) and/or (However many degrees.). I can also set it to go back to stage 2 after it reaches what ever temp difference or finishes the cycle. Ive actually found a setting in here that I can use the fossil fuel as my stage 3… Then have an outside temp that the heatpump is disabled

This morning its 24deg outside and I moved the Dual Fuel to 20 deg, The duct supply has reached 90 and its moving up a degree every 15-20 mins or so. This heatpump works so much better than the one that I had in my last house. I am sure that thing wouldn’t keep up once it was below 36 or so.

Awesome! I was surprised when you said your thermostat didn’t support that, but glad to hear that it does!

I think the main issue with low duct temperature is the potential for discomfort with lukewarm air blowing on people, but that really depends on where vents (and people) are located. It’s not a direct indication of how much heat is being produced, because the system might be using a different fan speed for gas heating vs. heat pump heating.