p/s, vs injecion, and buffer strategy (diagram)

1bourbon

Thinking about a primary/secondary system with a buffer tank and variable speed injection for an oversized fixed-fire CI boiler that also does DHW duties.

Attached diagram is borrowed from tekmar's application guide for the 361 control, which would be used in this case. It shows a 30-40 gallon buffer tank (four-port style like a boilerbuddy, Heat-Flo, or HTP SSU-xxB series) as a blue rectangle. My drawing skills aren't so good....

Would probably delete valve V1 as unnecessary. P2 would go on the return, and the boiler supply could be piped into the bottom of the primary side of the buffer to avoid short-circuiting the flow path through the buffer.

Any thoughts on this setup would be appreciated. Thanks.

Jamie_5

are the heating zones operating on ODR?

Maybe I'm wrong, but I assume the variable speed injection means you're supplying lower temperature water to the heating zones. Is the VS running on reset? If not, why not supply lower temperature water to the buffer tank and protect the boiler with a simple thermostatic valve? That would allow you to get rid of the VS altogether and lower the temp of the buffer tank water, thereby simplifying and reducing standby losses from the buffer.

If the zones are operating on reset, never mind. I think V1 is there to control the injection loop and necessary for that.

1bourbon

ODR, yes

Yes, the space heating will be doing ODR. Good point about V1.

Thanks.

Unknown

If your mix temperatures are consistently under DHW temperature, you might be able to use the DHW tank as a your buffer and inject out of that (best with a shell in tank design tank).

1bourbon

cool

Yes, the mix temps will be ~80-150 (probably 80-130).

I assume this would require pulling the injection loop off the DHW supply/return.

Which tank-in-tanks have high volume? Looking for 25-35 gal as buffer size based on Siggy buffer sizing calc.

Thanks.

Uni R_2

The DHW side stores BTUs as well

Given that it is a water to water heat exchanger with storage potential on both sides of the stainless steel liner, any size tank would provide a lot of buffer capacity.

I would upsize the overall tank size somewhat from the DHW requirements and add a mixing valve to keep the output steady, while using a wide differential... at least in the summer time when the boiler gets stranded with how many ever gallons of really hot water every heat call. I'd try a reduce the cycling there.