Minimal heat demand - heat pump vs. resistive backup (big setback)

[SwampSurvivor]

One thing to note - there are heat pumps available that heat well down to -25f or so. I have multiple friends in Saskatchewan who use heat pumps and the heat strips are only needed when it gets colder than that.

It can get down to -40 or even a bit colder but….the actual amount of time it is that cold is minimal on an annual basis. There’s good equipment that will do the job in cold climates but it can take some research.

My state (NY) is pushing heat pumps pretty heavily. They can work down to lower temperatures for sure. We were debating on doing a heat pump for primary , but the cost to retrofit the electric resistive backup is where the costs went crazy. The house already has central air (or had, replacing it in the spring) so running the heat pump wasn't a big deal. Upgrading the electric to handle the resistive heating is where things really got crazy for costs.

The other issue is - while we're not rural, our power goes out a lot. I can run a furnace on a portable generator, my 7500w generator would struggle to run a condenser. As it is now , when the power goes out we have to fall back to a window unit.

Ended up going with converting to gas. Then in the spring we might do a supplemental heat pump when we install the central air. Not sure.

 

[DCDG]

My new apartment is all electric and has an 18k BTU heat pump (low-end Carrier split system, 14 SEER, newish), with 8k watts of backup resistive heat.

I like to sleep very cool and am gone for most of the day during the week. If I had a more conventional system like a gas furnace, I'd keep the apartment around 60F most of the time (that's also my ideal sleeping temperature), and program the thermostat to bump up to 70F when I'm home and awake (say, 5-10pm on weekdays and all day on weekends). Now, this runs exactly opposite the conventional wisdom of heat pump thermostat settings - keep them quite consistent around the clock.

For what it's worth, I'm not trying to be cheap, I just think it is an interesting question given the apartment can go most of the time with very little heat (it hasn't dropped below 66 yet this year inside, despite lows around freezing and days with highs in the 40s). That said, it gets pretty cold in Nebraska - think typical winter lows in the teens/20s and highs in the 30s with cold snaps colder than that.

Further complicating this, the apartment also faces south, so it seems to stay plenty warm even on cold days as long as the sun is out. I share most walls so the heat demand is quite low. I wouldn't be surprised if it never got much below 60F in here with the heat shut entirely off.

Any thoughts on the best option below?

A: have it programmed to 60F most of the time, and then use resistive backup heat (aux heat) to overcome the setback (the thermostat automatically jumps to aux heat if the delta is over one degree (which was smaller than I expected)). Due to that small delta, the aux heat will click on every time it comes out of setback. This is the "let the system handle it" approach, but the system's logic is limited to a cheap programmable thermostat which just automatically engages aux heat if the delta is over a degree or if it's taking too long to catch up.

B: keep it at 70 around the clock so that resistive backup (aux heat) is rarely needed, and rely only (mostly) on the heat pump.

C: keep it locked in EmHeat on the thermostat (locking out the heat pump entirely so it never runs) and program it to 60F night/away, and 70F when home.

My math suggests that aux heat comes out on top for costs if it runs 1/6 the time of the heat pump or so (given the heat pump likely pulls around 1200-1500 watts, and the heat strips are 8000 watts). That time split seems plausible, though, considering the air that the heat pump puts out is pretty lukewarm compared to the backup heat (think "is this even on?" vs. a steady stream of hot air - I'd guess 80 or 85F vs. 125F, based on a quick Google).

If I had a gas furnace and the heat pump, I think the furnace would be the clear winner here, but it's nice not having a gas bill and the associated fees for just an apartment (fees that would probably eclipse usage). My electric rate is 9 cents per kWh, for what that's worth.

1.5 ton unit with 8kw resistive heat going you’re looking at around 8800 watts with the blower motor running x your .9 per kWh

 

[Brons2]

Sorry to piggyback on this thread. How much resistive heating would you need in a warm climate like Texas? Could you possibly get away without it if you had a heat pump that was rated for extreme cold operation? It's pretty rare we get under 15F and many winters not under the mid-20s. Normal lows are around 40F in January.

I want an inverter powered HVAC/heat pump that makes a lot less noise than my current builder supplied AC unit. Also I find the furnace function is extremely inefficient on my setup, although the AC has been efficient. I have my gas on level billing and I'm over $110 year round, it's ridiculous. 9 months of the year we don't go over 30 ccf, yet these bills.

 

[KrisZ]

Sorry to piggyback on this thread. How much resistive heating would you need in a warm climate like Texas? Could you possibly get away without it if you had a heat pump that was rated for extreme cold operation? It's pretty rare we get under 15F and many winters not under the mid-20s. Normal lows are around 40F in January.

I want an inverter powered HVAC/heat pump that makes a lot less noise than my current builder supplied AC unit. Also I find the furnace function is extremely inefficient on my setup, although the AC has been efficient. I have my gas on level billing and I'm over $110 year round, it's ridiculous. 9 months of the year we don't go over 30 ccf, yet these bills.

I believe the aux heat is chosen based on the equipment size, not necessarily the demand as the demand was already accounted for in the whole system size.

In my current house I had the aux heat come on twice this winter and that was only because two zones came on at once and the heat pump was taking too long, so the thermostat used aux heat.

In reality I could’ve gone all winter just on the heat pump alone. But you also have to think about those rare winters when it does dip well below the normal temps, and have enough heat for those events.

 

[Brons2]

I believe the aux heat is chosen based on the equipment size, not necessarily the demand as the demand was already accounted for in the whole system size.

In my current house I had the aux heat come on twice this winter and that was only because two zones came on at once and the heat pump was taking too long, so the thermostat used aux heat.

In reality I could’ve gone all winter just on the heat pump alone. But you also have to think about those rare winters when it does dip well below the normal temps, and have enough heat for those events.

That's interesting because of the systems I've looked at, you had your option of several different SKUs/BTU sizes for the resistive heating elements. But maybe that's based on region of install, i.e., what the manufacturer thinks you will need for the climate you're in.

 

[KrisZ]

That's interesting because of the systems I've looked at, you had your option of several different SKUs/BTU sizes for the resistive heating elements. But maybe that's based on region of install, i.e., what the manufacturer thinks you will need for the climate you're in.

How wide was the BTU range? I would go with something in the middle, if you don’t want max BTU, to be on the safe side.

 

[Brons2]

How wide was the BTU range? I would go with something in the middle, if you don’t want max BTU, to be on the safe side.

Everything from 3000 to 24000 BTU, for a 4 ton heat pump. I haven't seriously looked into moving forward, because we're thinking of moving back into closer into town. Since we're going to private school next year and beyond, it kind of frees us from having to live in a particular area because of the public school it feeds into.

 

[KrisZ]

Everything from 3000 to 24000 BTU, for a 4 ton heat pump. I haven't seriously looked into moving forward, because we're thinking of moving back into closer into town. Since we're going to private school next year and beyond, it kind of frees us from having to live in a particular area because of the public school it feeds into.

Well 3000 btus is nothing, it’s like an average BBQ output. So you would definitely need something above 10k btu range. The temps don’t go low enough in Texas to where a heat pump is totally useless, so aux heat doesn’t need to be big. I will check what I have once I’m back from vacation next week.

 

[Al]

One thing is (and this is probably irrelevant). Contractors always install way to much A/C and of course installed heat. Its bad in the case of A/C bc you don't get the reduction in humidity is summer ....just cool to the setpoint and then off and cool back to the setpoint....etc

 

[DCDG]

One thing is (and this is probably irrelevant). Contractors always install way to much A/C and of course installed heat. Its bad in the case of A/C bc you don't get the reduction in humidity is summer ....just cool to the setpoint and then off and cool back to the setpoint....etc

Not necessarily true I’m in South Carolina and general consensus is I’m coming across these new subdivisions going up having undersized units. The contractors say the units should run longer to better dehumidify vs meeting set points fast also sounds counterintuitive but it’s harder on the units to cycle on/off repeatedly they’re more efficient running longer with fewer cycles.

 

[Rand]

Well 3000 btus is nothing, it’s like an average BBQ output. So you would definitely need something above 10k btu range. The temps don’t go low enough in Texas to where a heat pump is totally useless, so aux heat doesn’t need to be big. I will check what I have once I’m back from vacation next week.

a small stove burner is 5000 btu.. 3000 is beyond tiny.

my 2 burner grill is 30k btu.
my furnace is 60k btu, but I'd rather a 40k btu (or 2 stage furnace) because it runs for 5-10 min and shuts off... small house.

Typical is 17k-51k btu in resistive strips (5-10-15 kw)

 

[DCDG]

a 3 ton heat pump unit I recently seen installed had a 10kw strip heater kit ordered for the air handler. which provides around 34,000 btu's which closely matches a 3 ton units of 36,000 btu's. this unit is 208v so output will be reduced a bit

 

[Al]

Not necessarily true I’m in South Carolina and general consensus is I’m coming across these new subdivisions going up having undersized units. The contractors say the units should run longer to better dehumidify vs meeting set points fast also sounds counterintuitive but it’s harder on the units to cycle on/off repeatedly they’re more efficient running longer with fewer cycles.

I understand..that is smart. But I am also telling you in PA, going back many decades. Contractors over installed bc they did not wand callbacks when during very hot days the customer was dissatisfied when it did not instantly cool down to 68F

 

[DCDG]

oh I totally get that and people are still like that today. to meet requirements nowadays as long as the unit can maintain a 75 degree interior temp with 95 degree outdoor temps it complies. and realistically if a unit is sized correctly functioning as intended in perfect conditions between the outdoor temp and indoor set point there will be a 20 or so degree difference.