Heating Help

issues with on demand

@ September 7, 2010 10:50 AM in Solar and Rinnai

The on demand heaters can only turn down so far, and if the incoming water temp is 90'F or higher the things shut off the gas as output temps get too high. This is a glitch in these designs that render them unpredictable. We now prefer to install electric back-up tanks to complement solar because of this issue. And with on demand efficiency around 80% for the affordable units, and electric tanks at 92% eff., energy savings result. Make sure to add some insulation when using tanks to reduce standby losses.

beware

@ September 7, 2010 10:41 AM in solar with radiant-control strategy

Many solar/radiant jobs I've seen rely on a boiler for back-up, of course. But when there's no solar heat in the storage tank, the returning water from the radiant system heats the storage tank as well. Make sure to install controls to bypass the tank when the return water is hotter than the tank, or your gas boiler will heat the storage tank, wasting energy. Use a snap switch or differential controller operating a NO and a NC automatic valve at the same time on a tank by-pass.

too much heat

@ September 7, 2010 10:31 AM in thermal shock in a slab

I have a system like this. It is very efficient, and with so much mass doesn't run too hot. BUT, of the collectors ever stagnate, the fluid will melt pex in a heart beat. How do I know? Yup, it happened to me when snow covered my PV panel for the pump, and the snow on the collectors slid off. I highly recommend a dump load that opens if the fluid reaches 130'F. You could shunt it to a water heater or other load, mine is a hot tub!

How to size pumps in pressurized systems?

@ September 7, 2010 9:57 AM in How to size pumps in pressurized systems?

When sizing a circulator for a pressurized system, how do you figure the total head ? I have a flat plate heat exchanger with 14 ft of head @90 GPM, and a short run of 3" pipe. The system pressure is 15 PSI. Just pulling numbers off the pump curves doesn't seem to allow for the system being pressurized. Is there a correction factor, or a rule of thumb to go by? Or is the pump curve the best way to do it? Thanks for any light you're able to shed on this subject.