Cohousing-L Mailing List: Re: Re: Heating systems

[Roger Diggle (diggle]

> we at the EcoVillage at ithaca are currently evaluating them (specifically
> "ground-source heat pumps") as an alternative to our "baseline" design of
> high-efficiency gas supplementing passive-solar and energy-tight homes.

>    (3) Environmental impact is interesting. Our rough analysis
> suggests that we may "generate" about 1/2 ton LESS CO2 per household
> per year if we go the GSHP instead of high-eff natural gas (HENG ?!).

When considering environmental impact, don't forget that, unless you have
selected unusual equipment, the heat pumps have HCFC22 in their bloodstreams,
along with a pint or two of HCFC22 contaminated refrigeration oil.  There
will be leaks of the HCFC22, itself a greenhouse gas and an ozone depleater. 
Personally, I think of the ozone depletion problem as a more proven one than
global warming.  In addition, when considering life-cycle costs, the heat
pumps will undoubtedly create higher repair costs.  Either system will work,
and neither is BAD, so to some extent, it's arbitrary.  Still, without
knowing the details of your situation, I'd lean toward natural gas and skip
the air conditioning.

However, if you will be using solar panels, and aren't afraid to get fancy
(and do some experimenting, probably), you could use heat pumps to assist
your solar panels and increase the amount of heat that they collect.   But
I'd be very surprised if you could get the bank to help pay for it.

> We would really like to find a heat-pump mfg that can include full DHW 
> (domestic hot water) in the package. Many can support DHW but I
> am told that only a newer technology called "de-super-heater" or something
> like that (!) can manage the higher temps and consistent supply that full
> DHW requires.

De-superheating sounds more mysterious than it is.  If you're a chemist or
physicist, your working definition of superheat is different than that used
in refrigeration mechanics.  In any standard refrigeration system, the heat
is moved (pumped) by boiling a liquid at low pressure in the presence of the
heat to be removed, then taking the resulting vapor someplace else and
ditching the heat.  Typically, the refrigerant to be boiled is squirted into
one end of a tube (in a heat exchanger) and flows, boiling madly, to the
other end of the tube, where a pump (compressor) is sucking madly away
getting rid of the resulting vapor.

Now, the pump is made in such a way that it can suck on vapor just fine, but
if it inhales any liquid, it gags and sputters just like you or I might.  It
might even have a serious coughing fit and die.  So the refrigeration system
is arranged such that ALL the liquid boils before it gets to the end of the
tube.  So the vapor goes down the last 10% of the tube, still picking up some
heat, warming above its boiling temperature, and expanding.  The temperature
increase between the place where the last of the liquid boiled away and the
place where the compressor inhales it is called the superheat.  (In an air
conditioner, the typical superheat is about 20f.)

In order to begin liquifying the refrigerant to use it again (so you can
squirt it back into the tube), you have to get rid of all that additional
heat (superheat).  When people talk about desuperheaters, they are talking
about fancier-than-normal gizmos for removing  it.  If there is anyone left
whose eyes haven't glazed, and who wants to know about the gizmos, I will
supply info.

Roger Diggle       diggle [at] macline.com


- sent via an evaluation copy of BulkRate (unregistered).


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