Very Early Performance for PEX Water Heater


This is the very first temperature log for the $1000 PEX solar collector solar water heater  -- it covers the first few days of October 2008.


For the longer term performance plots...





Storage Tank Temperature -- October 2 to October 9:



The red trace is for a thermal sensor located near the top of the tank, and near the heat exchanger coil.


The blue trace is for a thermal sensor located about 1/4 up from the bottom of the tank.


The storage tank holds about 160 gallons of water.


The differential controller shuts the pump off when the tank temp reaches about 139F -- this is to protect the pump and the heat exchanger coil from being exposed to water that is too hot.


The tankless water heater that the solar tank feeds is setup to warm water up to 108F if its not already this warm.  Since the solar storage tank did not drop below about 112F, the solar has been providing 100% of the hot water needs. 


Our incoming water is at 50F, so even If the tank temperature were to (say) drop down to 80F, the solar would still be providing half of the hot water heating -- that is, it does not have to be above 108F to provide useful water heating energy.


I should mention that the way the plumbing in our house works now, the dish washer is on a separate hot water heater and does not (yet) get solar heated water -- so there will eventually be a bit more demand than the plots show.  I have been trying to take extra long showers to make up for this (the sacrifices I make for science :).  We are also not heavy duty hot water users -- we don't use hot water for laundry at all.


You can see that some days had very little sun, but the large tank easily carries heat over to the next sunny day (so far).  The outside temps have been fairly warm (70's) until today, when the high was in the 40's.  Snow predicted for tomorrow.


The tank heat loss rate at night is about 0.3 F per hour, which agrees pretty well with the calculated heat loss rate of about 0.25F per hour.  This seems too high to me, and I will add some more insulation on the outside of the tank that should reduce this by about half.


The tank never seems to develop much temperature stratification.  I'm not surprised that it stays well mixed during collection periods, since the pump pulls water directly from the tank (no heat exchanger).  I'm a bit surprised that water draws don't result in more stratification.


I added sensors today for outside air temperature and for temperature inside the collector.