Dual Fuel Heat Pump

Rich W

Condensation happens when warm/moist meets cool. The outlet of the evap. is very high RH. Plus, look at the ductwork just after the A-coil with non-insulated ductwork. Quite often you'll find plenty of condensation dropping out of the basement air onto that cold metal...Is this air or water source?

John H.

Duel Fuel Heat Pump

Is There any reason the evaporator can't be installed in the return air of a dual fuel system to allow the heat pump to run down to lower temps and use a 2 stage furnace to stage in additional heat as needed similer to elect furnaces. this would allow for additional energy efficency.
The Heat exchanger shouldent swet durring the cooling mode since there are no pilots now.

Brad White_81

Makes sense in principle

in that the "evaporator" (heating season condenser that is) will impart it's lower temperature heat first. Maximize the less expensive energy first of course.

The pitfalls I see are:

1) The secondary heating source had better modulate very well or have as small a staging as you can find. You may take in say 95 degree air off the refrigerant coil and boost it to who knows where- Remember all you need is a topping-off, maybe 10-15 degrees on average up to the full load. If you cannot "trim" to the load you run the risk of short-cycling or over-shooting your space temperature.

2) You would have to know your balance point, at what OD temperature your heat loss and refrigerant-only output aling and maintain indoor temperature. The better to size the modulation range -if you can even get it. And the better to control bringing on the secondary heat.

One good thing about electric secondary heat is, you can with some application use an SCR infinitely variable controller for dead-nuts trimming to space temperature.

My $0.02

Brad

John H.

My Home 1st

I will try this in my home 1st. The secondary gas heat will start coming in after the space temp balance point and the supply air temp from the heat pump will be lower at that point and dropping. I plan on running down to the economic balance point & plan on using a plenum temp controll of some type to cycle the gas so it wont hit the furnace high limit. I still have a few details to work out but I wanted to see if there was something Major I was missing.

Jeff Lawrence_25

Major Pitfall?

Most furnace manufacturers used to deny warranty claims if the evaporator coil for the AC system was before (in an airflow situation) the gas furnace heat exchanger.

S Ebels

I see what.....

I see what you are trying to do but I think the issue that Brad brought up will be a problem. That being, the temp rise across the HX of the gas furnace when you already have 80-90 dgree return air. Jeff also has a good point. Restricting the negative pressure side of the duct system by installing the coil in it, will be the equivalent of running with a dirty filter. Not good either.

Carrier and Trane both have "dual fuel" systems out that use an air to air HP as the first stage of your heating system. They place the evap in the normal spot though, downstream of the blower. In Carrier's system you can program the control to cut out the HP at whatever outdoor temp you desire and then the gas furnace will stage on from there. I haven't done one yet but it looks interesting provided your electric rates make it feasible.

Christian Egli_2

The power of vapor cycles

Feeding a hot air furnace hot air to begin with spoils heat exchange efficiency in a big way. Modulating the fire like Brad suggests is the way to improve heat exchange efficiency and solve the pesky comfort problems all at the same time.

Or, rather than touching the burner, find yourself a thermostat that sends extra power to the heat anticipator when both the pump and the fire are on. But this won't improve exchange efficiency numbers, though.

I operate a setup that does about the same thing. It is an office, with air ducts. The air handler in the ceiling consists of the return duct and make up air, the filter, the refrigerant coil, the fan, the steam coil, and the supply pipes, in that order, and life is easy.

The original set up was to have AC and steam heated air. When we needed to change the AC machine, we went for a heat pump combo (that on its own is a sick joke here in the middle of the winter) and now there is more than one way to get heat in the office. The AC/heat pump has its own thermostat with no electric reheat. When outdoor conditions no longer permit the heat pump to keep up, the old steam coils come on and save the day. The steam is still controlled by its own old thermostat and its own slow acting zone valve. The two thermostats coexist happily together, I don't know that people fight over the settings. I suppose switching the steam call to the white line could do just as well, depending on thermostat model. Switchovers are manual.

This setup causes no change in performance in the steam boiler.

What I was thinking is that you could perhaps install a boiler to do its own thing and then, (either a steam coil... or a hydro air coil to fiddle with the air duct temperature. There might be a control benefit to this setup. And if you need a new hot water heater anyway, a slight bump up to an indirect tank and small boiler could do just the trick. Easy.

Or, just buy the ready made combo units S is talking about. Easier.

Brad White_82

Yes but

only in cooling mode not in heating mode when the RH will be much lower. Even in cooling mode (just thinking out loud) would one expect condensation on the downstream heat exchanger? I would think it would be above dewpoint. And if if a condensing type does it matter? Curious what you think.

Christian Egli_2

Breaking a sweat

Supply air ducts should be insulated; we do that in order to prevent condensation on them from the outside when cold air is flowing inside.

A frozen cold metal heat exchanger because it is sitting in a stream of cold air (regardless of this air's dew point) will sweat on the outside, the fire side... and rust. This should be particularly true in a damp basement.

I also think a cold mass at the bottom of an old type smoke stack could cause draught problems and stinky summer reversals.

And a non rusting heat exchanger with a condensate drain and on a power vent should do just fine.

My steam coil is fully equipped to deal with inside condensate... not that I have ever seen any. Does this -thinking aloud- seem reasonable?

Brad White_82

I was thinking along those lines, Christian

yes of course we insulate ducts for the reasons you mentioned. But even if 100% RH, there will not be any condensation on any object warmer than the dewpoint of the air. This is the condition I would expect in that environment.

But the psychrometrics say to me that because the drybulb temperature is at or above the wetbulb temperature by definition, the mass of metal downstream would be hard pressed to assume the full drybulb temperature let alone the wetbulb temperature -dewpoint if at saturation.

Of course unless one is controlling that (the finite temperature of the downstream mass of metal), there are no guarantees!


Not advocating the practice just seeking the realities of applying a lower temperature heat source upstream of a (hopefully fine-tuneable) secondary heat source).

Always a pleasure, Christian-

Brad

Rich W

Brad

I don't know what type of furnace we are talking about. An 80% will condense on the burner side of the HX when the burner is off (haven't seen any insulated heat exchangers I think the motor will too. If this is a water source HP, what about some kinda reverse de-superheater to add heat to the supply water. Maybe a storage tank with a built-in HX and PS pumping to inject a few extra btu's when needed. Thinking of an earth coupled loop here. Seems much easier to modulate this than a bang-bang gas valve. Why not let the heat pump do what it's good at? Need more info. I like the fact that you're trying John- now I have to get off my butt and try this.

Good luck

John H.

great ideas

> Feeding a hot air furnace hot air to begin with

> spoils heat exchange efficiency in a big way.

> Modulating the fire like Brad suggests is the way

> to improve heat exchange efficiency and solve the

> pesky comfort problems all at the same

> time.

>

> Or, rather than touching the burner,

> find yourself a thermostat that sends extra power

> to the heat anticipator when both the pump and

> the fire are on. But this won't improve exchange

> efficiency numbers, though.

>

> I operate a setup

> that does about the same thing. It is an office,

> with air ducts. The air handler in the ceiling

> consists of the return duct and make up air, the

> filter, the refrigerant coil, the fan, the steam

> coil, and the supply pipes, in that order, and

> life is easy.

>

> The original set up was to have

> AC and steam heated air. When we needed to change

> the AC machine, we went for a heat pump combo

> (that on its own is a sick joke here in the

> middle of the winter) and now there is more than

> one way to get heat in the office. The AC/heat

> pump has its own thermostat with no electric

> reheat. When outdoor conditions no longer permit

> the heat pump to keep up, the old steam coils

> come on and save the day. The steam is still

> controlled by its own old thermostat and its own

> slow acting zone valve. The two thermostats

> coexist happily together, I don't know that

> people fight over the settings. I suppose

> switching the steam call to the white line could

> do just as well, depending on thermostat model.

> Switchovers are manual.

>

> This setup causes no

> change in performance in the steam

> boiler.

>

> What I was thinking is that you could

> perhaps install a boiler to do its own thing and

> then, (either a steam coil... or a hydro air

> coil to fiddle with the air duct temperature.

> There might be a control benefit to this setup.

> And if you need a new hot water heater anyway, a

> slight bump up to an indirect tank and small

> boiler could do just the trick. Easy.

>

> Or, just

> buy the ready made combo units S is talking

> about. Easier.

John H.

Great Ideas

I did oridginaly plan on using a variable speed 90% 2 Pipe 2 Stage FAF and a 2 stage heat pump this way.
I was concerned about the coil before the blower but the total static will be the same just moved to the return instead of the supply. Most air handlers have the evap in the inlet of the blower so I dont think it should make a difference on a FAF. Trane will have a variable speed 3 stage unit out later this summer maby I will wait for that for better staging with the heat pump.
I am not to concerned about voiding the warranty since it is in my home. I am mostly interested in the efficency and safety although if it dident void the warranty and worked effecently this would be a great selling feature to our customers.

Christian Egli_2

Mammy-yyyyy, he made me step in the puddle!

Yup, your thinking out loud is sounding clear. I agree fully, you'd be really hard pressed to find cooled air ducts sweating inside a home(and even outdoors), so, indeed why worry about the heat exchanger. But now, if your furnace is in the bathroom and you're taking a steamy hot shower... you will have the suitable conditions for pipe dew. Ha! , A warm and humid basement would do that too, sometimes.

Would this problem more realistically apply to a high velocity system? I've stuck my foot in the mouth before.

And, oh by the way, this idea that the AC coil must be placed after the fire... means that in an upflow system, the coil sits on top of the furnace. Right. Everything stays nice and dry until the drip tray rusts out or clogs up and, in a playful mood, spills all the condensate down the cleavage area of the precious furnace; now it's all wet. Ohh. So sorry.

It's always fun playing with you Brad. Thanks

John Mills_5

I've seen...

a couple of furnaces that had the evap in the return. One in a basement, one downflow in a closet. Neither were very old but the heat exchangers were so badly rusted they were pretty much shot. Look at gas packs. Heat exchangers are in the cold air supply on most and they tend not to last very long where the same or similar exchanger in a gas furnace will go 20-30 years.