Heat recovery unit suitability for modern AC

[wulianlian]

I am a college student doing a report on AC hot water heat recovery systems. Several AC professionals and alternative energy specialist tell me that in the 80's they recommended and installed many of these heat recovery units, but they no longer recommend them or install very many. They said that modern AC units make heat recovery less beneficial and can, in some cases, create problems with the AC systems. After looking into the question, I am having difficulty understanding what are the problems with the heat recovery systems as an add-on to modern AC units. Can some of you AC professionals out there please point out some of the specific problems as you understand them?

[Enthalpy]

For us with a different background, you might perhaps tell what you mean by AC?

[InigoMontoya]

Even better: Got a link to one of these heat recovery units?

[solar33480]

 

@wulianlian

 

I see you have posted this same message on a few forums. I hope you find what you are looking for.

 

@Enthalpy & @InigoMontoya

 

Not sure if it is legit to post a link here but I will try, see companies likehotspot energy.

 

Hope this is helpful.

[InigoMontoya]

OK, having glanced at the link (thanks solar)...

 

The theory is obviously sound, but I do believe the AC guys who say that it could give modern AC units problems. Modern AC units tend to assume a thermal mass at a pretty low temperature - I'll say 105 F - to cool the coils. If the ambient temperature isn't cool enough, well... Your coils don't get cooled and your refrigeration cycle takes a dump.

 

But that assumes using the heat in a normal hot water environment. By that I mean, using your water heater as the thermal soak.

 

I noticed on that link, however, that there was a model available for use as a swimming pool heater. Now something like that could be EXCELLENT. You're not likely going to heat up your pool to 105 F. It has much more thermal mass and it has a lot more area to bleed off excess heat (it's not small and wrapped in an insulator). Further, since your pool is most likely much cooler than the air temperature during the hottest part of the day, the refrigerant cooling cycle should be more efficient / more effective thus resulting in an AC that works better and/or more efficiently.

 

In summary:

 

For use on a traditional home water heating system: I think it's a bad idea.

 

For use on a swimming pool water heating system: Brilliant.

 

 

 

edit: By the way, my primary reason for liking it on the pool is that it could help the AC run better. For actually heating the pool, if you're interested in low cost heating the best I ever saw was a guy who simply bought about 100' of black rubber hose and flaked it out on his roof. His pool water was then run through the hose with a 12 V RV water pump (which was in turn run by a solar panel). Cost him almost nothing to put together other than the solar cell for the pump and if anything his pool was TOO hot.

Edited August 4, 2011 by InigoMontoya

[CaptainPanic]

It's a heat pump, and heat pumps work with something that is called the Coefficient of Performance (COP). It means that for every Joule they heat or cool (it's the same), they must put in X Joule of energy, usually in the form of electricity.

 

A good air conditioning (AC), or a good refrigerator have a COP of more than 4.

Heat pumps either heat up something, and must 'dump their cool' somewhere (more correctly: they must find a source of heat somewhere). ACs or refrigerators remove heat from something (they cool it), and must dump that heat.

 

And in all cases, you have 2 temperatures: the target (which either needs to be heated or cooled), and the 'outside', where you have heat available, or where you can dump heat. What really matters for the efficiency of the system is the temperature difference between the two.

 

If you use an AC to make hot water, then obviously, one temperature goes up (the place where you can dump the heat is now hot water). That will reduce the efficiency of the AC. However, it does give you hot water for free.

 

It might be argued that if you live in a hot climate where ACs are used all the time, that you might as well build a solar boiler to allow the sun to heat your water. Those have no limitations to how hot the water can get (they can reach almost the boiling point).

 

InigoMontoya correctly says that heating a swimming pool means the hot side remains relatively cool (unless you're making a hot spring)