solar w/ combi boiler

kcopp

Anyone ever used or know of piping diagrams for a solar thermal DHW set up and using a Combi boiler to suppliment the hot water needs... Do you use a bronze circulator to move the water or do you have  the water tank feed the combi coil? I was thinking of using either a Embassy Onex or a TT excellence.

ty,kpc

hot_rod

a few options

The solar tank feeds directly to the coil or tank in thge combi boiler. You don't want to circulate this loop as you want the solar tank cool for best solar harvest when the sun comes up.



This is another option that allows all HW to come directly from the solar tank until it drops to a temperature too cold to use. Then a 3 way ZV allows the flow into the combi for temperature boost.



It does take more components and a control with logic to make the switch.



hr

kcopp

so the question....

becomes... will the boiler fire up even when there is 140F/ hot  water flowing through the coil/ tank. That would appear to be wasteful. I see the 3 way ZV would solve that issue but then also brings a more complex and costly set up to the customer. Personally I try to design systems as simple as possible... maybe I am trying to over simplify in this case. Thanks

hot_rod

depends on the logic

in the boiler control, I suppose. If the setpoint for DHW is 140F then I wouldn't think the boiler would fire? Also some only fire when a flow switch detects DHW flow, then look at the desired DHW setpoint temperature. Good question for the manufacturer of the equipment you are considering.

Some of the early combi boilers maintained the boiler loop at 140F.



Yes more control involves more complexity and more things to breakdown. It's always a fine line how much controls and technology is desired to achieve such a simple task as heating DHW.



hr

Fortunat

my solar preheats for my excellence

Hey Kcopp,



I'd skip the three way valve complication on the DHW side if I were you. It is more trouble than it is worth. I'm with you...simple = good.



I simply feed my preheated solar domestic hot water to my TT Excellence domestic cold line. To minimize the gas I use, I wound the Excellence DHW temperature setpoint down to 120. It comes set at 140, presumably to give the boiler a bit better DHW recovery, but since even in the worst case, I'm feeding the boiler 100 degree water from my solar tank, it never has trouble keeping up with our 2 bath house.



I ran my system this way with the boiler turned off from June til early October (here in Maine). At that point, I flipped the boiler on, but it's really still loafing cause the solar does most of the work.



Because I'm not worried about DHW recovery, I've also wound down the boiler target temperature on a DHW call to about 160 to try to squeeze a little more efficiency out of it. I'm going to keep cheating it down until I find I run out of hot water sometime this winter and then I'll bump it back up 5 degrees and leave it there.



Good luck,



~Fortunat

kcopp

that is an idea....

I could do that. There will be  2 people there most of the time w/ guests occasionally not a huge demand. I speced out 2 flat panels and a 80 gallon tank...Is it possible to oversize a storage tank?. Job is in NH..right on the state line w/ Maine... where in Maine are you located? 

Fortunat

Maine and New Hampshire

kcopp,



I wouldn't oversize the solar tank if I were you. Going to a larger tank without adding collector will tend to give you lots of luke warm water instead of a little less hot water. If your load is modest (like just 2 people) 80 G of hot water is preferable to 200 G of lukewarm water.

That is, unless you have the ability to make the tank a strongly stratified one, but that gets a bit more challenging.



I'm in the Portland area, but our company (www.revisionenergy.com) is throughout Maine and NH. Feel free to give us a call if we can help.



Best of luck,



~Fortunat