Heat Pump on Generator

[89MustangGX]

Currently have a propane furnace and there have been occasions where we have lost electricity and it easily powers through my 30a generator interlock setup, along with the rest of the house.

Have been considering adding a heat pump. My understanding is that there will be some temperature where the heat pump would transition back to the propane furnace. I also understand the heat pump will use much more electricity.

My question is, will I need to upgrade to a 50a generator/interlock in order to heat the house if I need to run on generator power at heat pump temperatures? Or is there some way that the heat pump could manually or automatically switch off to propane?

 

[kbeefy]

There are dual fuel units, but since you already have a propane furnace I wouldn't bother with them.
How big? a 12-18k would probably be fine on the 30A, a 24k+ would probably be better on a 50A to give you some overhead for other loads.

 

[89MustangGX]

Honestly don't know how big yet. Just trying to get started wrapping my head around everything. Never even considered it might be ok on 30A. I'll have to do some more homework. The home is 2700sqft with strong afternoon sun and lots of windows if that helps ballpark anything.

Does this mean there likely isn't an easy way to force the system onto propane so I will be looking at a generator upgrade as well?

 

[u3b3rg33k]

assuming you have a non-inverter heat pump, a bigger generator is definitely better. how big is the actual generator? nevermind the transfer stuff, if the generator can't put out enough surge to start the compressor, it doesn't matter. also what fuel is the generator? 30% efficient generator * COP of 2.5 = 75% efficient, if the propane furnace is condensing (and the generator burns propane), it'll cost less to just burn propane while the power is out.

 

[89MustangGX]

Generator is 8k surge I believe, and it runs on gas. Was also considering getting a dual fuel generator if I was going to upgrade, but that's a whole other question.

I'm not so much concerned with the running cost, more concerned with ease of use. Not unusual to have a couple small power outages a year, but last year we had one outage that was 4 days I believe. Sure was nice to have the interlock and be able to run anything as long as I kept feeding it gas.

 

[nadogail]

With out the name plate data for the equipment we have been asked to comment on, an intelligent answer will be difficult to come up with.

Generator will need to be sized to the load. If you want lighting as well as HVAC, you will need to include your anticipated lighting load. Will you be cooking and how will you be doing it?

There are too many missing pieces in this puzzle.

 

[89MustangGX]

Thanks for the replies. I think GJ might be overthinking this or I didn't ask my question right. My question is, is there an easy or automatic way to force the furnace to run on propane if I don't have enough generator power? Or wanted to conserve gas in a shortage situation for example?

 

[The Cobbler]

you would be better off switching off the heat pump & running on propane. you need to ask that question of it can be manually switched to propane . I see no reason why there couldn't be an override on the system to do that

 

[larry4406]

Thanks for the replies. I think GJ might be overthinking this or I didn't ask my question right. My question is, is there an easy or automatic way to force the furnace to run on propane if I don't have enough generator power? Or wanted to conserve gas in a shortage situation for example?

Yes, you simply walk to the stat, and flip it to emergency heat, and it will force the propane furnace to run and not the heat pump assuming you have a properly set up dual-fuel furnace heat pump.

Not sure how you would manually intervene and prevent the gas furnace running as backup heat when the heat pump can't keep up. If you shut off the gas valve, I think the furnace would try 3 times to ignite, then go into lockout likely shutting off the system (I have no facts on this conjecture).

I have two rural houses under construction now both with dual-fuel furnace heat pumps and generators. These are conventional propane furnaces yet are integrated with a heat pump for AC and heat. These are Lennox systems and Generac generators.

A typical heat pump uses resistive heat elements for back up (emergency) heat. These kick in typically when there is more than 2F differential between demand on the stat and actual.

On a dual-fuel furnace with heat pump, there is not resistive heat elements for backup (emergency) heat; the propane furnace is the backup heat. Same deal, when there is more than 2F differential between demand on the stat and actual, the propane furnace kicks on.

We have run low voltage wires to the furnace and have intercepted the red wire to the heat pump and run this thru the low voltage contacts of the transfer switch to load shed control the outdoor units.

 

[yeldogt]

With an automatic system you have to have a unit big enough to run the HP .... typically people do a bit of load shedding on the house.
Obviously -- there is no point of having a propane generator killing itself outback to provide electric to a HP when the propane can provide direct to the furnace. This is where the internet is great --- once you get the notification you can adjust the thermostat to remove the HP from operating. There are also relays that can be added ... but -- that's another level.

In your case it's all manual. Just flip the HP's power off and when the manual generator is running the thermostat will be functional and you can hold off the HP

Typically there is no electric back with the HP/ propane setup. The modern duel fuel thermostats have way to input the fuel costs (propane and electric for the HP) and they determine the best way to operate. Remember modern upper end HP's today can work down to very low temps and often it's just a question of cost.

At my place in NJ we had a problem a few years ago while doing some remodeling and the hot water heat had to be turned off and the HP had no problem keeping the place warm in 10 degree weather .... house has ducts for AC. It was much more money to operate as we have natural gas for the boiler .... but it worked. Propane is a different story ... if the place in NJ had only propane we would use the HP a lot.

Even with hot water heat we turn the HP on a lot in the shoulder season -- times when you just need a boost in the Am or evening

 

[u3b3rg33k]

I'd think most thermostats would have the "emergency heat" setting on a dual fuel do the backup (propane).

 

[yeldogt]

I'd think most thermostats would have the "emergency heat" setting on a dual fuel do the backup (propane).

They do .... but, the thermostat is not going to know the power is coming from the generator.

 

[u3b3rg33k]

They do .... but, the thermostat is not going to know the power is coming from the generator.

if you've got to go outside, plug in the generator, start it, and flip the interlock, manually setting the thermostat to backup/EM heat is NBD. if it was a big generator with an ATS, a box of relays could be warranted.

 

[justinjoyal]

I don't know if yours is, but I would not run a modern heatpump on a non-inverter Generator. The electronics don't like dirty power all that much...

 

[89MustangGX]

I don't know if yours is, but I would not run a modern heatpump on a non-inverter Generator. The electronics don't like dirty power all that much...

Are there inverter generators that will deliver 50A? I know there are a couple that will get upwards of 10k watts but still have 30A plugs on them.

 

[captaindiode]

Don't over complicate it. I have the same setup, heat pump with gas furnace backup. My furnace runs when the outdoor temp drops to 37 degrees or less. If I am running on generator, I manually flip the t-stat to emergency heat, which locks out the heat pump and runs the gas furnace as long as there is a call for heat.

If I loose power in the summer, I can't use the heat pump for cooling, but that is a trade off I accept.

 

[89MustangGX]

It seems pretty straightforward now. I just didn't realize there was such a thing as an emergency heat setting before. Much more clear now.