sizing indirect for solar post-heat

CC.Rob

Have a solar DHW system that needs boiler supplement in winter. Typical worst case lowest tank temps around 75F. Average temp rise required around 40-50F.



System is piped with a 3-way valve that allows seasonal SDHW only, and winter SDHW output to boiler-fired indirect (a tired old poor performer).



Existing boiler is 84k output with DHW priority control.



Question is whether to go with smallish 20 gal indirect (e.g., SSU-20) and shoot for basically continuous hot flow capability assuming fairly low temp rise, or go with larger indirect that does more 'standard' dump-and-recover service.



Thoughts appreciated.



TIA

Brad White

Personally

I prefer storage, a dump zone, so would opt for an SSU-30 or more likely a 45. Even larger and with a shuttle circulator to charge the final tank. That way, even with the larger volume vs. your relatively small boiler, the trim time will not be as long as if heating from a cold start.



You mentioned your worst case, but is your best case (high production) much above charging the 45 gallon tank? You do imply that summer solar is all that you need. I may not be picturing your arrangement correctly, but so long as the water moves in series, I like volume.

CC.Rob

setup

Thanks Brad.



The setup is:



cold domestic supply goes into solar tank (120 gal). Hot solar out goes to manual 3-way valve where it either goes direct  to taps (April-October), or to indirect 'cold' supply, where it is trimmed up and then out to taps.



That help?



I have played a little with running the summer production through the indirect with boiler shut down. Sort of raises effective capacity over the 120 gal solar itself, and keeps the bottom of the solar tank cooler and better stratified. But the 40 gal additional 'storage' can cool off depending on solar feed temp and time between major usages.

ABSolar

So what's the problem?

What makes you say your 40 gal indirect is a tired old performer?  Do you think the coil is blocked w/ sediment/ lime build-up?  Are your winter shower/ faucet temps insufficient?  It sounds like the solar system is correctly piped, and though we don't know the collector size and orientation, if the solar installer chose a 120 gallon as the pre-feed, then everything should be hunky-dory.

If the winter hot water temps are insufficient, and the indirect boiler system is functioning correctly and you have already tried increasing the boiler domestic priority temp, THEN you MAY have a problem in the system.  Please describe in more detail your problem/ symptoms....