Dual Fuel System

[Ralphs1]

Is there any reason the evaporator cannot be put on the return side of a 90% gas furnace to allow a Heat Pump to operate down to economic balance point. Then staging 1st or 2nd stage of the gas furnace simmilar to a air handler with resistance. There are no more pilots so the heat exchanger should not sweat durring A/C operation.

 

[RossABQ]

Interesting idea; but expensive. The evaporator would probably have to be corrosion resistant, depending on gas quality. I would guess some sort of face-and-bypass damper arrangement would be needed on the evaporator to control head pressure on the heat pump, as a 90% furnace still has a fair amount of energy in the flue. All in all, a control guy's dream!

What do you do at outside temperatures where the heat pump doesn't need any furnace assistance? Dual evaporators?

At some "sweet spot" this could be very efficient, outside of that it might not be very good.

 

[Ralphs1]

Evap in the return air side of the furnace not combustion air unless I am mis-understanding you. I have a 2 stage Heat Pump that basicly is the primary heat for my home untill the thermal balance point that is when the gas furnace will start to stage in to boost the air temp with the heat pump at the same time.

John

 

[RossABQ]

You would be robbing Peter to pay Paul, if you take heat out of the return air.

 

[MintJulep]

I think when Ralph says "evaporator" he really means condenser when the system in operating in heat pump mode.

Is the question really: "Could you put a heat pump's 'hot' coil in series with a furnace?"

Is there a specific reason why you are considering only the return side?

I don't see why not. Plenty of heat pumps have supplemental electric resistance heat. There is no shortage of gas furnaces with "A" coil evaporators for cooling.

 

[RossABQ]

That makes more sense, but I don't believe the economics (life cycle costs) support such a system.

The condenser needs to be in the RA side so the heat from the furnace isn't actually absorbed by the condenser.

I used to own an all-electric house with heat pump/resistance assist, in a neighborhood with gas service. I cast about for a solution to astronomical heating bills and poor performance that retained the heat pump. There was nothing that was economical considering the first costs (even solar assist/storage systems at a time when there were huge tax credits). A high-efficiency gas furnace was the solution.

In Ralph's case, you can retain the HP in a boost configuration ahead of a furnace, if you think it is in good condition and electricity @ SEER is cheaper than gas heat (per BTU delivered), which is the case in some areas. Or just use it as a central A/C system. Either way I suspect you'll end up buying a gas furnace capable of handling the full heating load, which is what spoils the economics.

 

[Ralphs1]

The Heat Pump (16 SEER 2 stage) is more efficient to operate down to around 5 deg than the gas furnace. Although it will not produce enough heat for the home below the thermal balance point.
That is when is the furnace (93% 2 stage) starts to stage in to supplement the heat pump. Instead of cycling off the heat pump and then operating the furnace independitly. I do have the condenser ahead of the furnace for exactly the reason you described.

I guess I am really questioning is there any reason why the indoor coil cannot be upstream of todays modern furnaces as they are normaly downstream of the furnace weather in heat pump or a/c mode. I know the older furnaces had a pilot warming the heat exchanger and could cause to sweat and rust if the coil was upstream durring a/c. But modern furnaces do not have pilots any more.

Thanks
John

 

[RossABQ]

I just can't imagine there is any advantage to that kind of set up, unless you live in a very moderate climate and will use the HP for 90% of the time. But then that means the furnace is hardly used yet costs the same as if it were the primary source.

You might think about using the hot water heater and a supplemental heating coil, instead of a 2-stage furnace. It is a lot cheaper and may be all you need if it's a moderate climate and you just need the occassional boost.

Once you hit the balance point tho, you should shut off the HP and go to resistance or gas.

 

[AbbyNormal]

Indoor/outdoor coil easier on the air source

Be a problematic set up in climates with higher dewpoints and people installing the equipment in the attic spaces.

Unless it is sealed combustion, that HX will fill up with air with a dewpoint higher than the leaving dry bulb temperature of that coil.

Gas heat you should be blowing air through the coil in cooling, so by default the heat pump is not going to work that way.

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[Ralphs1]

But with a sealed combustion furnace in a normal basement instalation the indoor coil should operate ok in the return durring cooling.

I dont see the difference between pulling the air or pushing through the indoor coil same cfm.

A electric air handler as well as most rootop units the indoor coil is on the return side of the blower and heat exchanger or resistance heat.

 

[AbbyNormal]

Blowing it through means it is down stream of the HX and the blower

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[AbbyNormal]

the coil is downstream of the hx

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[Ralphs1]

This is what I have sorry for the confusion.

 

[RossABQ]

The typical squirrel cage fan in furnaces is better at pushing than pulling.

It appears you already have this system installed and running; how well does it work? Are you asking because there is a problem? I would be curious what your utility bills are with this setup (if you can compare to previous years), and where you are (what climate).

 

[Ralphs1]

The furnace has variable speed drive blower. It operates great and I am not having any problems with the system.

I just installed the system and have just recently moved into this home (northern Indiana Just a few miles from Chicago) a few months ago so no previous utilty bills to compair to.
I am also applying for a different rate plan with my electrical supplyer that offers lower rates durring the winter months for Heat Pumps.
I have run all different scenarios with my Wrightsoft heat load software and this is the most energy efficent setup.

I was asking thease questions just to try and head off any possable future problems I have not forseen. I figure if I am going to experiment I should do it in my own home.

So far this winter the system has heated the home on only 1st and 2nd stage of the Heat Pump and only 1 time that I know of the system 3rd stage(1st stage gas furnace) has operated with the exception of when the Heat Pump goes into defrost.

 

[AbbyNormal]

Furnace fan board in the air stream?

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[AbbyNormal]

heat pump heat probably won't cook your blower motor, not sure what codes are enforced there but look up "Refrigeration Coils" in NFPA 54.

I only have the 1996 version here but refer to section 6.3.7

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[Ralphs1]

I dont think the board or motor will be a problem due to the same motor and boards are used in airhandlers with Heat Pumps and the indoor coil is up stream of there motors.

I will check with the mfg for downstream listing. re: section 6.3.7 although (just my opinion) since the furnace is sealed combustion and designed to condensate I wouldent think there woud be a problem especially since the cold discharge air will be crossing the stainless secondary heat exchanger 1st.

Thanks for the info.

 

[AbbyNormal]

Some furnaces have the board mounted on the blower housing.

A draw through air handler does not have gas ignition controls and it's little control board will be in a small integral cabinet and not in the air stream, thinking about the board getting exposed to cool air, and then the blower either timing off after the cooling call ends or being run continuous to take advantage of what appears to be an electronic air cleaner.

One thing though, I doubt the furnace was designed to operate in that manner with high return static and pulling the cold air through, you have what appears to be a fairly deep trap.

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[AbbyNormal]

Same principal on the HX too, gets cooled down then constant fan. The secondary HX is stainless, how about the primary.

Take the "V" out of HVAC and you are left with a HAC(k) job.