[Alden_Sloe]

Looking at the numbers for a heat pump with gas furnace I'm wondering if there are any furnace designs that allow the airflow to be split to either the coil or the heat exchanger. The constriction of the heat exchanger seems to be a significant burden on the system and in mild climates it's almost never used. I guess there might be a bit of an issue with some defrost modes but that doesn't seem like a show stopper. Again, defrost is (or should be) a very small percentage of run time.

Alternately, has anyone every developed a system where the gas is used to bring the line temp of the refrigerant back up to where it would be at say 50 degrees outside temp before it circulates through the coil? Maybe some sort of boiler type arrangement since heating the lines directly with a gas flame would probably be a bad idea.

 

[beenthere]

Sanyo(I think it was them) had a mini split that used a gas burner to provide the heat for the heat pump.

Didn't go over real big.

 

[Cold Feet]

I suppose you could use a separate furnace and air handler ducted in parallel with diversion dampers and controlled by relay logic. I don't see what you'd gain by going Rube Goldberg, though.

 

[Alden_Sloe]

No, that would be too expensive. I was thinking that if the heat exchanger was separate from the air mover you wouldn't be duplicating the fan and control logic. All you'd be adding is a large damper that directs the supply air to either a heat exchanger or the AC coil depending on demand for heating or cooling. You'd get better performance on both because the AC wouldn't be constricted by the heat exchanger and the furnace when operating on gas wouldn't be pushing through the coil.

Simpler and perhaps even more efficient would be using a gas boiler arrangement on the refrigerant linesinstead of electric heating strips at the coil. Then you still benefit from the long cycle time and even heat of the heat pump and you effectively take care of the defrost issue. A really efficient system might incorporate the boiler with domestic hot water and/or radiant heat in areas like the bathroom. Under mild conditions and with the right "brain" it might even work as or in conjuction with a hot water heat pump which seems to be the latest trick pony.

 

[amd]

The constriction of the heat exchanger seems to be a significant burden on the system and in mild climates it's almost never used

If the correct blower speed tap is selected based on the manufacturer's chart and static pressure, you won't have a problem. (or airflow selected on a variable speed blower)

I suppose you could use a separate furnace and air handler ducted in parallel with diversion dampers and controlled by relay logic. I don't see what you'd gain by going Rube Goldberg, though.

That would be more trouble than it's worth.

 

[Alden_Sloe]

If the correct blower speed tap is selected based on the manufacturer's chart and static pressure, you won't have a problem. (or airflow selected on a variable speed blower)

It's not really a problem I guess. That's what we have and it works great. I guess it's sort of an artificial problem created by the government specs for the tax credit. The difference in efficiency between a dedicated air handler and a furnace are enough in most cases to just barely miss the spec required on 4 ton units.

However, it seems like a "boiler" arrangement could be designed with a significant increase in efficiency, comfort and possibly for a very competitive price. Heat exchangers, especially on the high efficiency furnaces aren't cheap and they tend to be a failure point. It would also allow more flexibility in the install. The furnace + coil tend to be pretty long and often have to be squeezed into a space designed for just a furnace. Add on air filtration and it can be a real problem in retro fits. I would think the boiler could be located anywhere along the line set and that might make venting and gas supply a lot easier. Especially in installs where gas wasn't the prior heat source. In fact the boiler could be electric, oil, propane, etc.

 

[beenthere]

Water source heat pumps have been around longer then I can remember.
Commercial, and apartment building commonly use them, with a boiler providing the heat.

 

[Alden_Sloe]

So I guess the answer is if you're going to the expense of putting in a water based heat exchanger then why bother with the air source heat exchanger?

If I'm thinking this through correctly, in the summer the heat pump is cooling the house and preheating the domestic hot water. In the winter the heat pump is using the incoming water at ground temperature as a heat source which gives you nice cold tap water or is modulated by some other energy source reheating the water. Is the reason they're not used much in residential the cost or because of limited water usage making them impractical? Eliminating the giant fan, the maintenance on the outdoor coil and all the defrost related expense would seem to offset most of the expense of the water source heat exchanger. Not so? I would think they would be super efficient approaching geothermal without the tens of thousands required to bore or trench a geothermal system.

 

[beenthere]

Thats what the water tower is for.

Thd being able to use a boiler for heat, and air to cool it in the summer. That you describe. Would add a fair amount of cost to the unit. Making it a long long time to recover the install cost.
If at all.

 

[amd]

It's not really a problem I guess. That's what we have and it works great. I guess it's sort of an artificial problem created by the government specs for the tax credit. The difference in efficiency between a dedicated air handler and a furnace are enough in most cases to just barely miss the spec required on 4 ton units.

However, it seems like a "boiler" arrangement could be designed with a significant increase in efficiency, comfort and possibly for a very competitive price. Heat exchangers, especially on the high efficiency furnaces aren't cheap and they tend to be a failure point. It would also allow more flexibility in the install. The furnace + coil tend to be pretty long and often have to be squeezed into a space designed for just a furnace. Add on air filtration and it can be a real problem in retro fits. I would think the boiler could be located anywhere along the line set and that might make venting and gas supply a lot easier. Especially in installs where gas wasn't the prior heat source. In fact the boiler could be electric, oil, propane, etc.

Okay, now I get it.

Most high efficiency furnace second heat exchangers are covered by a 20 year if not lifetime warranty; I don't think there's any evidence which demonstrates that they're a common failure point. (Ignitors, inducers, and circuit boards, present in all furnaces are probably much more prone to failure)

With respect to government rebates/credits, if the additional cost of meeting the minimum specs required exceeds the tax credit, you might as well not play the efficiency ratings game. You'll be better off with a basic system which meets your needs.

 

[Alden_Sloe]

Yes, the government specs do create an artificial incentive which probably doesn't make sense otherwise. Sometimes that's what's needed though to simulate a breakthrough. Just from a sales standpoint I'm guessing 4 ton makes up a large portion of the market and the manufacturer that can deliver that $1500 credit is going to have a significant advantage. Realistically it's probably going to be through better coils than any break through innovation. Still, the retrofit market is likely to be a huge sector over the next several years as we work through the new housing glut and a more flexible coil/heat exchanger/air mover offers flexibility important when trying to squeeze all that into a space that was previously just a furnace. From a home owner perspective having them separate is nice if one fails or needs to be changed. That 20 year warranty on the heat exchanger means nothing when the labor cost ends up being almost as much as a new furnace. Had a friend in that situation just a few years back. Possibly a victim of a bad install but he lives at a ski area and the plasticized steel heat exchangers failed on a number of units in his development within five years of being installed.

I have no understanding of how the commercial water source units work. Cooling tower? Sounds like a nuclear power station :eek2:

 

[beenthere]

Cools the water for the heat pump in the summer.

You have probably seen some. But don't know it.