Rinnai M160CN CH vs. DHW operation..

uhmgawa

I've come across an M160CN boiler (CH + DHW) which appears to disable the CH external loop pump while the DHW is active. Rinnai's install manual would lead one to believe that would only be the case should DHW BTU consumption be that of the full boiler output. But with even a slight DHW draw the CH function appears to be overridden. Anyone know if this is the expected behavior for these boilers?

The M160CN is the maximum CH output of three (AFAICT otherwise physically/component identical) boilers M090CN, M120CN, M160CN) all of which claim a DHW output of 160K BTUH but CH outputs of (90K, 120K, 160K) BTUH respectively. Still I'd expect the M160CN to eek out some CH capacity when the DWH wasn't consuming full BTUH of the boiler.

GGross

Seems like it might be. Combi boilers won't provide heating to the system while they are making DHW, no matter the amount, they use a diverting valve built in so when there is DHW demand the boiler fluid only circulates to heat the DHW. I couldn't see any adjustable parameters for turning the pump on/off during a DHW call. Possibly the most effective solution if you want that pump to run while there is a DHW demand would be an external pump relay unless you are able to find that parameter in the rinnai install manual

With an external pump on a combi boiler you must have the system piped in a primary/secondary configuration or else you can possibly cause problems with the internal diverting valve

uhmgawa

This system has a secondary pump and primary / secondary hydraulic isolation via closely spaced TEEs. However even if the external pump remained powered during DHW mode the internal diverter operation is likely binary rather than modulating/proportional where the boiler output could be proportioned between CH and DHW circuits. Easier to design and control and possibly more reliable and lower cost. I found a delay parameter exists between stop of DHW draw and CH function resuming. Setting that to "0" seems to have mitigated the problem. I speculate this is likely typical for boilers in this class and why separate external DHW heat loops (heat exchanger & storage tanks) appear popular for these low mass modern boilers, when concurrent operation is required.

GGross

You usually set up DHW for priority (disable heating circulators) with tanks as well, not always, but in most residential applications that is how it is set up. But yes you would be able to set up a tank system, or a field fabricated on demand system to run with your space heating at the same time, assuming that the boiler is sized for both loads at the same time.

uhmgawa

Ironically the old "dumb" boilers with large thermal masses managed to approximate proportioning without any comparable intelligence or control. The predecessor for this install was a 80's vintage 4 section oil fired boiler with about 16Gal in the main exchanger and a coiled DHW tankless heater set into the top of the main exchanger. The DHW draw was unrestricted, conventionally tempered down to a semi-stable usage temp. The only control would be the aquastat hitting its low set point of 160*F which would halt CH circulation until the main exchanger came back up to 160*F, but otherwise permitted concurrent DWH and CH draw. In practice it works better than it seems it should. Then again those cast iron anchors were about a quarter ton bone dry.