Aux Furnace Heat vs Aux Electric Heat

by **bob350** » Tue Oct 13, 2009 10:31 am

Hi All. Newbie here. I'm sure as with most newbies, I am trying to figure out the best way to get a geo system for my home.

I've read many posts, and am actively getting quotes and trying to figure out who I trust, what system I think is the best, etc.

One installer just threw me off by suggesting a backup aux NG furnace instead of electric grid as everyone else offered. His reasons made fairly good sense, but wanted a "second" unbiased opinion.

The main benefit to an aux furnace seem to be that if you have a power outage, you can run the furnace much easier than the geo. I've been told that for a geo heat pump, I'd need a 15-17kW generator. I have a 5kW generator now that keeps me up and running with the basics now (heat, well pump, hot water, lights, fridge, microwave, etc). At first I thought I could disable the aux electric and be good, but I am gathering that the main current draw isn't the elec grid, but the compressor. So that thinking is bust. Correct?

So by going with an aux furnace, I can maintain some level of heat in the house without buying a $2500+ generator and upgrading my transfer switch, etc. Does this make sense?

I live in MI, so winters can, and do, bring very cold weather and power outages due to ice, etc. I am very very keen on keeping my family out of a hotel during an outage!

Oh yeah, is it impractible to consider keeping my 17 year old (but still working) furnace and installing the geo next to it?? Haven't asked the contractor this yet, but I can imagine he'd rather sell me more equipment...

Comments, suggestions, information always appreciated!

by **bob350** » Wed Oct 14, 2009 4:59 pm

So either this logic is so far off base that no one bothers to reply...

Or maybe no one knows???

Seems like if a contractor is recommending this, someone has done it or thought about it.

Anyone out there....

by **urthbuoy** » Wed Oct 14, 2009 5:09 pm

I think, by definition, you are talking emergency heat. And, really, it can be anything you want it to be - including your old furnace. This just comes down to some control and ducting issues.

Back to auxiliary - typically your auxiliary electric is on a different breaker than the heat pump, so you could disable it in that manner.

by **bob350** » Wed Oct 14, 2009 5:40 pm

urthbuoy wrote:typically your auxiliary electric is on a different breaker than the heat pump, so you could disable it in that manner.

Thanks for the reply. Basic question here... does the high demand for power come from the electric grid heater, or from the compressor?

If from the aux electric heater, then I can disable like you say and use a small generator to get the Geo up and running during a power outage. I'd then only run into issues if it's really cold out...

by **geome** » Wed Oct 14, 2009 6:22 pm

I'm not an expert, but be careful turning off the aux breaker(s). One of them might power the blower and shouldn't be turned off if this is the case.

For the experts - could the blower power come from a totally independent breaker (so the aux breakers could be turned off without a problem)?

If on generator power, you could disable aux heat at the thermostat, but this may disable your emergency heat too (so would turning off the aux breakers). Wouldn't want pipes to freeze if the geothermal system stopped working, but that being said, I wouldn't size the generator to include aux heat (but we have gas logs as a backup heat source).

From what I have read here, LRA (locked rotor amps) is the reason a good sized generator may be needed (even without aux heat running). There was another thread here that may give you some more info on this - "Where is my domestic hot water?" Also, a link was just posted in another forum for a product that may help (but I am not certain): http://www.waterfurnace.ca/Engineer/Int ... 0Sheet.pdf

by **bob350** » Wed Oct 14, 2009 6:31 pm

geome wrote:From what I have read here, LRA (locked rotor amps) is the reason a good sized generator may be needed (even without aux heat running). There was another thread here that may give you some more info on this - "Where is my domestic hot water?" Also, a link was just posted in another forum for a product that may help (but I am not certain): http://www.waterfurnace.ca/Engineer/Int ... 0Sheet.pdf

Yes, I saw that link also. It seems that if the LRA is reduced by that much, the required power to start the compressor would be reduced significantly. Seems to me that a much smaller generator would then work. Can anyone shoot any holes in this theory?

by **mark1745** » Wed Oct 14, 2009 7:56 pm

Bob:

I would be surprised if a 5kW generator would be able to start the compressor. What size geo are you going with (in tons)?

If the idea is back up emergency heat I think your contractor is right, put in a split system with the gas fired furnace. You can run the furnace on back up power, done & done.

I don’t think the compressor in the geo will start. The engine will bog down & the top of the sine wave will fall apart. Very hard on the compressor, generator… expensive to find out.

Another thing is you might get a off peak electric rate because you have dual fuel. The Utility can turn off the geo, you go on gas = low electric rate. You would have to ask your Utility if they offer off peak .

Also what type of fuel is the gen-set? If its natural gas no too expensive to run, but if its gas, wow, that will be a fuel bill.

Straight electric back up heat is expensive to run. You can size your gen-set to the kW needed. The heat is 3.414BTU’s / watt. Example 10kW strip heater is 34140 BTU’s/ hr. That’s about 13.5hp, so you can see the contractor is right you need a big gen-set to run it. The geo might be 2-6kW but the start up current needed will be much more A electric motor can use up to 300-600% starting current for a split second.

A 100,000BTU furnace is about 40hp, but the power to run it is about 1-2kW or 10-15 amps 115volt circuit, very easy for your gen-set to run.

Sorry for the general numbers.

Mark

by **AMI Contracting** » Wed Oct 14, 2009 8:21 pm

Bob,
I install geos in SE MI and am not a fan of split systems though against nat gas (depending on electric provider) prioritized fossil heat might be cheaper than electric supplemental.
Do you often have lengthy electricity interuptions or are you simply suspicious of geo and want a security blanket?
Fact is with nat gas and lengthy power outages you may not be a good canidate for geo.
15K would be minimum generator for 3 ton system, what size is proposed for your home?
good luck,
Joe
ps if we are slow to respond around here, it's cold out and we work long hours at first cold snap.

by **bob350** » Thu Oct 15, 2009 5:00 am

I've had quotes for 3 ton to 5 ton, but it seems 4 ton is the right number.

Thanks for the info regarding starting the compressor. That seems to be the limiting factor rather than the aux electric heat in the heat pump. I'm not considering the furnace because of a lack of confidence in the geo, but rather my lack of confidence in the power grid. We seem to get power outages once or twice a year (just had one for 5 hours two days ago), and they can last for several days at a time. Although they are sometimes in warm weather they are often in the winter. So, I want to be able to heat my house during one with a generator.

The generator I have now is a gasoline portable 5kw. I'm fine paying for the gas during the outages since my main goal is to keep my well, heat, fridge, and lights going.

My impression of the split system is that it provides aux heat on cold days when the geo can't keep up and to be able to heat the house during power outages with my small generator. If I need a $3k-$5k generator to run my heat, I'm going to have to add that price to the system that I choose. Adding a furnace seems to be much cheaper!

Don't know about the dual fuel savings. Interesting point. I'll see if I can figure that out.

Thanks for all the replies. I'm trying to make a decision and get going hopefully within the week.

by **geome** » Thu Oct 15, 2009 5:36 am

Not positive about a gas furnace, but I believe there will be installer settings on the thermostat that will work with an outdoor temperature sensor. If the outdoor temperature is above the outdoor temperature setting, the geo will function. Below that outdoor temperature setting, the furnace will run. Make sure the thermostat that you get has the temperature lockout setting (temperature) that you need (all thermostats don't have this feature and they all don't allow for the same range of temperature settings). Your installer should know what is the right unit for your area.

Experts: can both systems run at the same time if desired and installed accordingly? Is there a benefit to doing this that may exceed the cost? Would doing this interfere with bob350's plans to use the furnace with a backup generator?