I know this borders on a number of variables, but in general, for a 2000 sq foot home, how many KW should an aux heat coil provide to heat the home with Emergency Heat/Backup completely?  Would 14 KW be enough?  Home is located in Pennsylvania.  Assume a not necessarily tight home or particularly drafty.  I know it's very variable, but could you all post your approx sq footage, location, and aux heat capability, and also if you house is particularly tight/drafty/normal?

14 kw will give you approximately 45000 BTU. That should work for a 2000 sf average home as a back up heat source. May not hold 70 at design temp but it will keep the pipes from freezing.

Do you have a Manual J building load for the home? It will give btuh needed on a design day to maintain a selected indoor temperature. Divide the btuh value by 3.413 to get required watts for use of resistance heat as a sole source.

If you want the aux heat coil to be able to completely heat your house to a nice warm 68-70 degrees right down to the minimum design temp (balance point), you'll need it to be able to generate about as many btu's as the heat load your house requires at that balance point. That likely means about as many BTUs as your heat pump can deliver, if your system was sized to meet 100% of load at the design temp. Google tells me that kw * 3414 = btu, and since electric heat is considered to be very close to 100% efficient in turning kw into btu, geotek is absolutely right that 14kw should deliver something in the 45,000 to 47,000 BTU range.

Bottom line: if you've got a 4 ton unit sized for 100% of your load, 14kw should be just about right.

(FWIW, I have two 3 ton units capable of about 34,000 btu each in a house with a conservative 80,000 BTU heat loss requirement at a -26 deg F design temp. We've had no need for aux heat despite brutally cold overnight lows getting very close to that design temp, and we're still far from having the heat pumps running flat out. Each unit has a 10kw heat strip (really 9,600kw - so about 32,000 btu each). That'll keep the house above freezing in any weather, and fairly warm in all but the most extreme conditions. We also have a modern, high-efficiency 60,000 btu/hr wood insert and a few cords of nice dry birch to provide enough extra heat to keep us nice and toasty warm if we have a heat pump failure during a cold snap.)

(Edit: I see engineer was faster than me on the response, and did a much better job in providing a direct answer)

We went away on vacation so I set the thermos down to about 60 and enabled aux heat in case. When we got back, the house took hours and hours and hours to get back up to 68...with aux heat enabled...14.4 KW in addition to the regular geo compressors running. Aux heat was "on" according to the thermostat, but in my mind, the house should have warmed up faster with a 4 ton unit running PLUS 14.4 KW of electric heat on. I think it took 5 hours.

The thermal mass of the house and its contents affects recovery time. Another factor would be the outside temp during recovery - if it was cold out during recovery, or windy or both, recovery will take longer yet. I'm not suprised at 5 hours even with aux

jml, -26* design temp?! where do you live, Antarctica? Is that maybe your Bin low (i.e. Lansing MI design temp is 1* with an -8* low in the Bin Data)? Your numbers don't jive (i.e. your 3 ton units likely aren't capable of 34,000btu when you are in negative double digits).

Stuart, as the others said load determines auxiliary size. Emergency heat is when you want the coil to cover 100% of load, and that can be dicey as power requirements could exceed house capacity in some cases.

Good Luck,
Joe

Well, it *was* particularly cold, and windy....weather forecast was "gales" at about 10 degrees F...maybe that's expected then. Hmmm.

Our Climatemaster thermostat *supposedly* has a smart vacation response mechanism that will turn on the system before you get home to allow enough time for it to gradually come to temperature without resorting to Aux heat.   It does and should take a long time to come back up, our installers warned us of this before we installed the system. 

I say *supposedly* because this is our first winter with geo, and we haven't used this feature yet.  Am going to try this weekend and will report back on efficacy.

How much are you planning to set back and then restore to? Our Envision units can lock out if the return temp is 55f. Not sure if your system could lockout due to this. In my non-pro opinion, if I were to use a vacation set back, I wouldn't go below 60f or 5 degrees less than your current low setting (whichever is higher).

If there is potential for pipes freezing if your geothermal system were to lock out, I would make sure your aux breakers are on, and disable any aux lockout you may have to let emergency heat come on if there is a system problem and there is a call for aux or emergency heat.

Now that I think of it, I probably wouldn't go more than 2-3 degrees below my normal set point.  More than this may invite aux to go on when recovering.  I'd rather leave the heat strips fully enabled, and set back less, than take a chance on a lockout without full heat strip functionality.

Joe:

Not Antarctica, but it sure feels close. I'm way up in Northern Ontario - north of Minnesota. Duluth and International Falls would be the closest US locations for weather data. My professionally-done heat calc report shows a -32 degree celsius (-26f) design temp @ just over 80,000 btu. I don't have the report in front of me, but I seem to recall that it was based on a 99.6% annual heating design condition. I don't know what software was used to generate this, but yes, it sure seems quite, um, conservative to me.

We've actually seen lows of -29 celsius (-20f) on a few occasions in the past month. There will be a few nights in Jan and Feb where the lows will get even closer to that -26f point, and every few years we'll get a real cold snap where we can briefly get down to a low of -40f/-40c. Most traditional oil or gas heating systems up here seem to be designed (well, over-designed) to easily meet that -40 temperature. Our old furnaces actually had over 150,000 BTU - kinda funny how even in the middle of winter they'd usually only run for a few minutes at a time before the thermostat would get back to 68 degrees. The idea that we really only needed 80,000 BTU was initially tough to understand. It seemed even stranger at first to understand the idea of designing a system around only 85% of peak load (68,000 BTU) plus the occasional touch of electric heat on a few super-cold nights.  It took a while until I was comfortable with that.

The key bit of info in making my 34,000 btu per 3.2 ton unit number jive is that I'm on an open loop with a very high-flow artesian well and very steady year-round water temps.  From my air and water temp readings, water pressure readings, and flow rates, it seems that my TTV038's are currently producing heat a little better than the middle of the range in the IOM book specs. 34,000 BTU would obviously be rather optimistic with these units for a closed loop system with colder EWT, especially as the winter progressed.

Climatemaster's specs actually claim 34,800 btu in second stage for 9 gpm water flow, 1250 CFM air flow, 40deg F EWT, and 70deg F EAT. My water temp is a little warmer than that, and I'm running 10 gpm on stage two on each heat pump (yep, up to 20gpm total, plus domestic water use too). I've spent some time making sure that even with the washing machine and shower and dishwasher all running, the flow to the heat pumps stays high enough to keep the LWT reasonable. (No FP1 lockouts yet, unless I do something really stupid to trigger one, and no, I haven't cut the jumper to disable the warning).

To try and bring this back on topic.... the peak power requirements for aux heat reminds me of how we looked at this issue. We have a 200 amp service, and the two AGL-10 aux coils could use 80 amps of power (9,600w/240v * 2 units), plus 22 amps for each heat pump's compressor and fan, plus another 20 amps for the hot water heater. Those few items alone add up to 144amps on a 200 amp service.

We had a few opinions from electricians about this. One said that we'd need to upgrade to a 400amp service, because it was good practice to make sure that the total load should never exceed 80% of the service rating (which would be 160amp). Another did detail electrical load calcs that indicated that 200 amps would be just fine, even if we were making regular use of electric heat and had every light and appliance in the house running full tilt. The black and white proof in the detailed load calcs and the good reputation of this electrician convinced me that this seemed OK. We had this electrician get the permits and do the wiring work, keeping the existing 200 amp service. We've never experienced lights dimming on compressor startup or any other signs that we've got load issues (of course the breakers on the 80 amps of aux heat have normally been off). But I can see how the electrical service for the house can easily become the limiting issue on aux heat sizing.

Where do you dump 20 GPM at those temperatures? Just curious.

Someplace where no one minds an iceberg forming, I'd guess.

pulled up duluth......22 days under 0*F. There are smurfs that aren't that blue! LOL
I thought about open loop when I inquired but didn't expect it with those lows......
Watch out for polar bears and pet your penguin for me ;)

All that said your situ is still unique and not applicable to the OP for many reasons. It is however a great illustration of design variability.
j

Two wells, about 150 feet apart. Injection well handles it without trouble.

Best investment on a system that far north might be a bunch of luggage and a U-Haul truck...go South!