Monday, January 17, 2011

Tiny heat pump, with benefits

I've blathered on about our heat pump enough that by now all of you should know how they work:  They use electrical energy to move heat.  Of course, this means that there has to be a heat sourceEolian's heat pump extracts heat from sea water.  And household heat pumps typically extract heat from the outside air.

But here's a heat source you may not have thought of:  condensing water.  A dehumidifier removes moisture from the air by condensing it.  Annnnd... condensing water releases  970 BTU/lb, 1010 BTU/pint.  So is that a lot of heat?

Dehumidifier capacity is typically rated in pints/day.  Ours is a small one, rated at 25 pints/day, or a little better than 1 pint/hr.  Using the numbers above, our little heat pump will deliver 1052 BTU/hr.  It does that while drawing 1.6 amps - that energy (about 600 BTU/hr) also gets delivered into the boat.

So let me put all that into clear perspective for you:
Our little tiny dehumidifier, which draws only 1.6 amps, is the equivalent of an 484 watt electric space heater.
We keep the dehumidifier in the head; this explains why it gets so warm in there when we take a shower.

Of course, these numbers assume that the dehumidifier is working at capacity, and that only happens when the air is very humid, like it is in the head when someone is showering.  At other times, the heat output will be less.  But like all heat pumps, the efficiency will never fall below 100%.

So think of that dehumifier as a tiny heat pump with these side benefits:
  • It keeps the boat interior dry
  • It provides an apparently never-ending onboard source of distilled water
I consider ours to be one of the best $125 we've ever spent.


Crew of Tanker 05 said...

OK where did you find that one for 125.00. The cheapest one I have found was 146. Thanks for posting the brand. when I find a source I will purchase. Thanks great blog.


Christian Holm
SV Summer Solstice

bob said...

Actually, that one is the one we have. But we bought ours nearly a year ago, on eBay. Inflation continues to take its toll.


Crew of Tanker 05 said...

Thanks I am ordering mine today.


Anonymous said...

Interesting? What do you do with the water extracted?

bob said...


Mostly I dump it into the head. Distilled water is a pretty good solvent, and I am hoping it will do some good in keeping the lines clean of lime.

We also use a gallon or so per year for topping up the batteries.

The water may be free of dissolved solids (it really *is* distilled water), but it is not free of pathogens, so we don't drink it. I guess we could boil it tho...


bob said...

Remember the Mars lander? The one that crashed on the far side of Mars because some NASA engineer made a mistake in converting units?

Yeah. Well I did the same thing, the consequence being that I over-stated the heat production. The numbers above in the post should now be correct.

NASA and I apologize.


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