how many amps of HVAC electric-resistance backup heat can I support ?

  • Active since 1995, Hearth.com is THE place on the internet for free information and advice about wood stoves, pellet stoves and other energy saving equipment.

    We strive to provide opinions, articles, discussions and history related to Hearth Products and in a more general sense, energy issues.

    We promote the EFFICIENT, RESPONSIBLE, CLEAN and SAFE use of all fuels, whether renewable or fossil.

RustyShackleford

Minister of Fire
Hearth Supporter
Jan 6, 2009
1,352
NC
I need to replace my HVAC system soon (it's 26 years old now, and only still running through the selfless and heroic efforts of the moonlighting cryogenic tech who installed it, and the fact I heat almost entirely with wood). It's a dual-fuel system, heat pump with propane backup, and I'd like to ditch the propane. It seems that some of the newer heat pumps might suffice without backup (the cold-weather design temp here is 20 degrees) but I'd feel more comfortable with some electric resistance backup (aka. heat strips). So I'm trying to figure out how much electrical reserve I have, in my 200amp service. (This is a spinoff from a discussion about my heat pump frying contactors).

Rather than do the Section 220 calculation, I've decided to do measurements. Rather than try to max out the house consumption and measure everything, I tried to measure each big load independently (except the EVSE, which is well-defined) Here's what I've got so far:

Screen Shot 2023-08-31 at 2.10.38 PM.png

This accounts for all my 240vac loads, except for the shipping container workshop. Note that it's a pretty pessimistic situation. The EV is charging. Both water heaters are kicking in after peak electric hours have ended (I'm on a time-of-day usage billing plan). My main HVAC plus a minisplit in an addition are running. Both ovens in the range are pre-heating. The clothes-dryer is running on max heat. And the well pump is going.

But there are some other potentially significant loads. Refrigerator might be auto-defrosting (only 100 watts or so otherwise). Dishwasher pre-heating water. Washing machine. Central vacuum. Small compressor in the workshop. Misc kitchen loads (electric kettle, microwave). The Section 220 allocation of 3VA/ft^2 for lighting gives me another 20 amps or so, but that's wildly pessimistic in the age of LED lights.

So what do you think ? How many amps of heat strips would you feel comfortable with ?
 
I think you're fine. These are all intermittent loads. We have a 60 amp feed to the resistance coils backup in our heat pump system, 200 amp service with electric everything except the stove cooktop (EV charger included). The coils put out a lot of BTUs so it only runs for short cycles. It's never been an issue though we have only used it occasionally when both are away from the house and didn't want the house sitter using the stove.

With a modern high-efficiency heat pump and totally sealed and insulated ductwork, the coils probably will never come on unless the compressor is down for some reason. Our system is 17 years old and I am contemplating upgrading, but the ROI is not there yet unless well subsidized. If I get a high-end Mitsubishi Hi-Heat or Daikin system I may forgo the heat coils if a new plenum is needed.
 
Last edited:
Can you do a heat load calculation?

Based on my historic furnace run time (80,000 btu for 8 hrs per day) I'd easily get away with 40amps at 240 volts.
 
Does your heatpump allow compressor and strips on at the same time?
 
Point of reference. When we moved in we had an in law apartment in the basement. So two water heaters, oven/ranges and dryers, all with 10 kw strips on a 60 A with the compressor/air handler on a 40 A.

Now the system is a single 80 gallon HPWH, 9kw steam generator on 60 amp. And a single oven and dryer with a 20A EV charger set to only pull 13 A. Same compressor and heat strips.

Really you need to follow the heatpump recommendation on sizing the strips. What capacity of heatpump are you installing? So your hvac load is for cooling what about heating? The same?

I am assuming your main panel has a 200 A breaker?

Get an accurate heating and cooling load calc. Pick your equipment follow guidelines for strips. As long as it’s less than 10 kw and I had a main breaker I would be comfortable.

I was trying to envision the worst usage case. Cold holiday lots of hungry in-laws show up with an almost dead EV they road tripped for 900 miles to your door. I’d probably be found in the shipping container.

The economics on HPWH are great. That’s saves 25 A. drop the charging rate to 20 amps. Now you have saved 55A less than the heat strips consume.
 
Does your heatpump allow compressor and strips on at the same time?
Probably not. Ours switches from primary to secondary heat mode via the thermostat preset temp.
 
With a modern high-efficiency heat pump and totally sealed and insulated ductwork, the coils probably will never come on unless the compressor is down for some reason. Our system is 17 years old and I am contemplating upgrading, but the ROI is not there yet unless well subsidized. If I get a high-end Mitsubishi Hi-Heat or Daikin system I may forgo the heat coils if a new plenum is needed.
I am tempted to forego any backup as well. But heat strips are cheap, and your compressor is always going to need to defrost. If I feel like my electrical service can handle the load. FWIW, my crawlspace is encapsulated.
Can you do a heat load calculation?

Based on my historic furnace run time (80,000 btu for 8 hrs per day) I'd easily get away with 40amps at 240 volts.
Yeah, using this neat tool https://hvac.betterbuiltnw.com/ I'm coming up with about 40Kbtu/hr for both heating and cooling. Using 15 and 98 as design temps; nasty summers, probably getting worse, here in central NC. Winters mild and getting milder.

Confused on second sentence, because 40amps/240v is only about 33Kbtu/hr. I guess you mean in addition to the heat pump.
Does your heatpump allow compressor and strips on at the same time?
It's a future heat pump but ... My understanding is that allowing both at once is normal. Would ease the electrical system somewhat (if only one could run at the same time), but would force the heat strips to handle the entire heating load, and when it's the coldest.
Really you need to follow the heatpump recommendation on sizing the strips. What capacity of heatpump are you installing? So your hvac load is for cooling what about heating? The same?
Yeah, see above, both loads are about 40Kbtu/hr. No sure I totally trust the heating number. Last guy put in a 72Kbtu/hr propane furnace (as part of my elderly and frail dual-fuel system). Dunno if that was overkill, or not, since I always heat with wood. I'll try to figure it out this winter. (Next summer is the target date for the new system).

As far as heat strip sizing, there's also this cool site: https://ashp.neep.org/#!/product/51105/7/25000///0 ... that gives you really complete stats on performance of virtually any ASHP. So should be able to figure it out for myself. Just trying to figure out the ceiling, how much can my electrical system handle, right now.
I am assuming your main panel has a 200 A breaker?
Yes. It's also completely full (including space-saver breakers), so I'm going to have to bust some of the 120vac loads out into a subpanel. But hey, it keeps me off the street.
I was trying to envision the worst usage case. Cold holiday lots of hungry in-laws show up with an almost dead EV they road tripped for 900 miles to your door.
One option that could save 40 amps on my peak load, is to put in an Emporia EVSE. You can set the charge current anywhere from 6 to 48 amps. If you pair it with the Emporia VUE energy monitor, it'll actually turn down the charging current, if it seems the system is getting overloaded. I put the EVSE into other house, and it's great; haven't really looked into the VUE thing yet. The Emporia stuff is super-affordable too. I'd have to ditch the ClipperCreek I installed though.
 
Last edited:
  • Like
Reactions: EbS-P
Rusty, I think you will be fine installing 50-60 amps of resistance strips. All you got to do is not run the clothes dryer in really cold weather to have adequate headroom on the main breaker. Or your family can agree to not bake stuff in the ovens while they are doing laundry.

I left Chapel Hill in 2008 when I came to Alaska, and I was appalled by the weather forecasts this far west. I had a patient, probably 2010 or so, who was a meteorologist and I asked him why the weather forecasts for Alaska were so bad. He said, "Hmmm, you must be from back east. When you have a continent's worth of data to look at making a 3-5 day forecast is a piece of cake. This far west, with only satellite data over the Pacific, there is a fair amount of experience and educated guesswork involved in forecasting."

On the NC piedmont, you can see cold snaps (and hurricanes) coming from hundreds of miles away. Get the EV charged up, and look to see if you can't run reverse; that is run the house off the battery in the EV for a little while when the power is out. Get caught up on laundry (I know, I know, I have four kids and a wife), bring in some dry cordwood and find something else to worry about.

40k BTU/hr at +15dF feels like a lot to me for an ordinary home. The low hanging fruit for insulation envelope is air leaks. If you have air leaks, come up with a plan and fix the leaks that aren't in the plan.
 
For old housing stock that has seen changes I really don’t trust a load calc that doesn’t include a blower door number.

I paid $550 for a professional load calc. It said I needed 48k btus. 4 tons. I’ve cooled the whole house on 24k btus this summer. Maybe at 24 F I’d need that for heat???

Things I have found. Cooling load calc is just sensible and does not include latent heat. So if you have a leaky house and it’s hot and humid your actual cooling capacity of your equipment depends on the RH of the inside air. Understanding your latent load and the systems latent to sensible ratio is important, especially at partial load conditions. New variable speed equipment prioritizes efficiency over water removal. The industry really doesn’t have a good single heatpump/ dehumidifier combined system.

In terms of year round comfort my whole house dehumidifier has made more difference than adding heat strips. But I do remember a few really cold nights when I just turned the system completely off and plugged in space heaters. (We had 2-3 winters before I realized the heat strips were not working). Having the strips for is the way the ducted systems are designed for our climate. 8kw I think can fit on a 40A. But if your getting 4 ton system it will want 10kw or more.
 
Rusty, Lots to unpack in that posting.

Our winter climate is similar in temperature. Summer is when your heat pump system will get a real workout as compared to ours. Our crawlspace is also fully insulated. I have tested our resistance strips at 15º to see how well they handle the heating requirements in our old farmhouse. They did well. The system cycled more frequently due to heat loss with all of the excess glazing this house has, but the system could handle the load. I'd have to dig out the paperwork to find out the actual BTU output.

FWIW, my sister in NC is heating her place with Daikin mini-splits and no heat strips. She has a insert in the fireplace for supplemental heat and power outages.

Our heat pump is a 2 stage unit, the resistance coils are the stage 3 setting on thermostat. That said, it seems like it would be possible to have the resistance coils run in parallel if the thermostat permitted this. It could be that our system's Honeywell thermostat is set up to prevent the backup heat running if the backup was an oil or gas furnace. Running them simultaneously could damage the heatpump's indoor coil in that case. But the resistance coils are after the heatpump's indoor coil so that would be safe.

Is the 72Kbtu/hr the input or output rating of the propane furnace?
 
Last edited:
Lots to unpack in that super long posting.
Our winter climate is similar in temperature. Summer is where your heat pump system will get the real workout. Our crawlspace is also fully insulated. I have tested our resistance strips at 15º to see how well they handle the heating requirements in our old farmhouse. They did very well. The system cycled more frequently due to heat loss with all of the excess glazing this house has, but the system could handle the load. I'd have to dig out the paperwork to find out the actual BTU output.

FWIW, my sister in NC is heating her place with Daikin mini-splits and no heat strips. She has a insert in the fireplace for supplemental heat and power outages.

Our heat pump is a 2 stage unit, the resistance coils are the stage 3 setting on thermostat. That said, it seems like it would be possible to have the resistance coils run in parallel if the thermostat permitted this. It could be that our system's Honeywell thermostat is set up to prevent the backup heat running if the backup was an oil or gas furnace. Running them simultaneously could damage the heatpump's indoor coil in that case. But the resistance coils are after the heatpump's indoor coil so that would be safe.

Is the 72Kbtu/hr the input or output rating of the propane furnace?
I still think even with a COP of 1.5-2 at really cold temps it is more efficient to run the compressor. What I don’t know is how much cycling you will get. Some t stats allow for reverse staging meaning once the criteria are met to kick on strips say deviation of 2 degrees from set point, once the temp increases to the difference is less than 2 the strips kick off but keep the compressor running. There should be some setting to set fan speed when strips kick on. So now your 3 stage heat (2 compressor 1 strips) has 3 fan speeds.

None of these were changed from stock settings on my equipment install or the new t stat install. Get to the staging set is really important.
 
Our system was good back in 2006, but below 25º I think it is like frost up too frequently and may be below 1.5 COP at that temp? I will have to dig up the literature to see. The thermostat is also 2006 tech. The fan speed is DC variable. It does go up to full speed when the heat strips are on and IIRC I was getting a reading of around 120º F at the hallway floor register when it was something like 20º outside.

At this point, I am not tinkering with any settings. What I have to decide is whether and when it will be worth it to upgrade all to a modern high-efficiency Daikin or similar system. I have not run the numbers with the new tax credits and discounts to see if it makes sense or not. What I do know is that as we age, we will be relying more on the heat pump and less on wood so at some point that too factors in.
 
Rusty, I think you will be fine installing 50-60 amps of resistance strips. All you got to do is not run the clothes dryer in really cold weather to have adequate headroom on the main breaker. Or your family can agree to not bake stuff in the ovens while they are doing laundry.
I hate to put those constraints, but I also think 40 amps (32 kbtu/hr) is all the heat strips I might possibly need.
I left Chapel Hill in 2008 when I came to Alaska, and I was appalled by the weather forecasts this far west. I had a patient, probably 2010 or so, who was a meteorologist and I asked him why the weather forecasts for Alaska were so bad. He said, "Hmmm, you must be from back east. When you have a continent's worth of data to look at making a 3-5 day forecast is a piece of cake. This far west, with only satellite data over the Pacific, there is a fair amount of experience and educated guesswork involved in forecasting."
Interesting.
Get the EV charged up, and look to see if you can't run reverse; that is run the house off the battery in the EV for a little while when the power is out. Get caught up on laundry (I know, I know, I have four kids and a wife), bring in some dry cordwood and find something else to worry about.
I already have our Nissan Leaf rigged with 1 kW inverter that runs off the 12vdc system, into a generator inlet. Both sides of the panel get the same hot (since the inverter is only 120vac output) but that's ok, since I don't have any MWBC (sharing neutrals driven off opposite phases). So the EV's 40kWh battery can run lights (LED), modem+router, A/V (without the tube amps), and fridge, for several days. Unless the fridge goes into auto-defrost, that kills the inverter; haven't figured out how to disable that yet - might have to hack a little relay into the power to the evaporator heater.
40k BTU/hr at +15dF feels like a lot to me for an ordinary home. The low hanging fruit for insulation envelope is air leaks. If you have air leaks, come up with a plan and fix the leaks that aren't in the plan.
House was built in 1988, with pretty good insulation standards. It's about 1600-1700 ft^2. You're saying you think the 40Kbtu/hr number is probably not an under-estimate ? Ah well, I'll run that 72kbtu/hr furnace this winter and try to figure it out.
 
For old housing stock that has seen changes I really don’t trust a load calc that doesn’t include a blower door number.

I paid $550 for a professional load calc. It said I needed 48k btus. 4 tons. I’ve cooled the whole house on 24k btus this summer. Maybe at 24 F I’d need that for heat???
So they did a poor job of the calc, sounds like. I think my best bet is testing heating with that furnace this winter. It probably won't get down to the design temp, but it's probably pretty linear, so I'll just see if can get the house 60 degrees warmer than outside, that should be good enough. Of course, the house will be at 90 degrees or so ... Now the current (to be replaced) system is 2.5 tons cooling, and it handles things just fine, even in its dotage, so I'm certainly comfortable with 3 tons there.
In terms of year round comfort my whole house dehumidifier has made more difference than adding heat strips.
I do have a nice dehumidifer in the encapsulated crawlspace.
 
Last edited:
Our winter climate is similar in temperature. Summer is when your heat pump system will get a real workout as compared to ours.
The current elderly system is 2.5 tons cooling, and does ok. So I'm comfortable with 3 tons.
FWIW, my sister in NC is heating her place with Daikin mini-splits and no heat strips. She has a insert in the fireplace for supplemental heat and power outages.

Our heat pump is a 2 stage unit, the resistance coils are the stage 3 setting on thermostat. That said, it seems like it would be possible to have the resistance coils run in parallel if the thermostat permitted this. It could be that our system's Honeywell thermostat is set up to prevent the backup heat running if the backup was an oil or gas furnace. Running them simultaneously could damage the heatpump's indoor coil in that case. But the resistance coils are after the heatpump's indoor coil so that would be safe.
Yeah, that's right, my propane furnace. won't run at the same time as the heat pump. But it's my understanding heat strips usually DO run simultaneously.
Is the 72Kbtu/hr the input or output rating of the propane furnace?
Input, I believe. And I think it's 80 or 90% efficiency.
 
What I do know is that as we age, we will be relying more on the heat pump and less on wood so at some point that too factors in.
Yeah, depressing to think, but I'm no spring chicken, and I want to make sure the new system can keep the place warm even with no fire (which is just something that's not an issue now).
 
Anyhow, the answer to my original question seems to be that my system afford at least 40 amps (aka. 32 kbtu/hr) of heat strips. Doubt I'd need that much. Here's a candidate heat pump for me, not a particularly stellar cold-weather performer: https://ashp.neep.org/#!/product/66923/7/25000///0 and it looks like even 30 amps (24 Kbtu/hr) of heat strips would give me 48Kbtu/hr, easily enough even if the 40Kbtu/hr calc is a significant underestimate.

The other issue besides can my electrical service handle it (which we've decided it can), is getting the juice to the unit. Right now I have 6awg NM-B running to where the compressor will be. So good for 60amp breaker and 48amp continuous. The companion heat strip they sell (that's close to what I think I need) draws 33 amps. That heat pump wants a 30 amp breaker; but it actually draws a lot less than 30amps, more like 17 amps max (per the performance tables https://www.acwholesalers.com/manuals/c6532a38727cae1b80ee5f2517df363c.pdf), of continuous load (even though the RLA is 27 amps). So am I ok there ? I can't see how the continuous draw of the heat pump is anywhere near 27 amps.

But right now that 6awg is also supporting a minisplit. So I'd need to run a separate 12awg line for that.
 
Last edited:
It probably won't get down to the design temp, but it's probably pretty linear, so I'll just see if can get the house 60 degrees warmer than outside, that should be good enough.
It's in fact fairly linear. BTUs/deg is a thing. Getting it 60 deg warmer than outside is a bit false unless you heat it for a LONG time to get all your walls, tile, furniture, etc up to that temp. Otherwise it will appear lossier than it is.
Monitoring your furnace run time for a decently cold day and keeping the temp fairly stable will get you pretty close, assuming you don't have WILD temp swings inside or outside that day, especially if you do it on a colder day (Not when it's 55 outside, but say 20s or low 30s?)
Example: say it's a 92% efficient furnace, and 72K BTU input. Output is around 66k BTU.
If it (the burners) run 6.2 hours that day (24 hours), looking at 409.2K btu delivered. Average temp inside minus average temp outside...lets say it averaged 28 outside that day, and 70 inside....so 42 deg avg temp difference. (70-28)
409.2k btu/ 24 hours = 17,050 BTU/hr, temp difference was 42 deg, so your heating load would be 406 btu/deg.

Take that down to 15 deg....... 55 deg temp difference.... 55x406 = 22,330 btu/hr needed on your design temp day.

This is easier to do if you're on natural gas and have a gas meter.....you can look at heating degree days vs gas use and figure it out roughly.



That heat pump wants a 30 amp breaker; but it actually draws a lot less than 30amps, more like 17 amps max
Continuous loads (like electric heaters) are limited to 80% for wire sizing...... so a 20A wire (12ga) wouldn't be enough for a 17A load, it'd only be good for a 16A continuous load. So they probably would have to bump it up anyway to a 30A circuit.... RLA is locked rotor amps....so it would pull 27 amps instantaneously TRYING to start the motor....but not continuous ever. So yeah, I'd think you're good there.
 
  • Like
Reactions: EbS-P
Thanks for the ideas - about measuring how well the propane furnace does.

I'm talking about the 20amp circuit for my existing minisplit, so that the 6awg circuit no longer has to support it - so it can hopefully handle the heat pump and the heat strips. I put the minisplit in awhile back when I built an addition; I think I'm going to ignore it in sizing this new unit, so it can be emergency backup.

So how do I calculate the continuous load of the minisplit plus heat strips, the one I need to keep under 48 amps for my 6awg circuit ? Obviously the heat strips are just whatever they are. But what about the heat pump ? Would I use the RLA ? Or just the biggest number I see in any of the heating performance tables, which looks to be about 17 amps ?

I'm not in love with this Goodman unit, dunno if it's even considered a good brand. It's just a straw man for now.
 
House was built in 1988, with pretty good insulation standards. It's about 1600-1700 ft^2. You're saying you think the 40Kbtu/hr number is probably not an under-estimate ? Ah well, I'll run that 72kbtu/hr furnace this winter and try to figure it out.
40K BTU/ hour seems like a lot at +15dF to me.

The current Ashford 30 is rated for 35, almost 36KBTU/hour with 10 hour burns. The current Princess is rated 37-38 KBTU/ hr on ten hour burns.

Needing to run either of those two stoves at wide open throttle and not being able to keep up with heat load at 15F above in 1700sqft, would not be something I would do for two winters in a row up here.

One thing you could do is walk around the interior with some blue chalk in one hand. Anywhere you feel cold air coming through the exterior walls, make a chalk mark. Window frames, receptacles, light switches, all of it. Dryer vent. Come spring time, seal them up.

Degree loss per hour at my place is fairly predictable horizontal linear, until it gets really cold. My envelope has an exponential knee on the degree loss per hour curve. When the bottom drops out and degree loss per hour becomes vertically linear it is time to be throwing sapsicles into the wood stove.