Quote:
Originally Posted by Daox
The problem is that you can only use the water down to about 85-90F before you need to start heating it back up. DHW needs to be warmer than that even. If you have too large a tank, it might never get up to the temperatures that its really useful which is probably 110F+.
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This is where a small homemade heat pump can really help...
The heat pump can extract heat from your 85-90F storage tank and deliver heat to you in the range of 115-120F, as you probably need. My experiments have shown that the COP of such a heat pump working in the 80-90F range is quite high... on the order of 6 or 7.
As an added bonus, the chilled water left in the storage tank will present a higher delta T to the solar heat gathering part of your system, and increase the efficiency of your collectors when the sun starts shining.
Of course, when the solar collectors are supplying enough hot water to properly run your floor heating, you would not want to divert heated water through a heat pump.
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Most hydronic floors like yours and Gary's are designed and built for cheap fossil fuel and have wide tube spacing, on the order of 12", and also have insulation that seemed quite reasonable when fuel was cheap. If hydronic floors were built to take advantage of lower feed temps (tube spacing on the order of 6 inches), you could still use 80-90 degree water.
Quote:
Originally Posted by Daox
Also, with a larger tank, you have more heat loss due to more surface area.
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Actually, larger tanks have a smaller surface area, per unit volume than smaller tanks, and lose heat more slowly.
This is why a cluster of bees can survive a cool night outside, but a lone bee will die.
-AC_Hacker