Avoid combi-boiler short cycling with piping configuration?

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zapps

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I'm looking to replace my Rinnai tankless water heater that is used for hydronic heating (w/ wall-mount convectors) and domestic hot water. The system is open-loop so I want to use this opportunity to make it close-loop as well.

Most of the contractors that came to estimate propose Navien combi boiler. Reading through this forums and research on the internet however suggests that this would cause short cycling on the space heating side due to the heat load of individual heating zone is a lot lower than the combi boiler minimum btu.

One contractor argues that short cycling can be avoided by using primary/secondary piping (or pumping) configuration. He tries to explain it to me but I still don't get it. So I'm here looking for help. Is it true that there is some piping magic that can remedy short cycling issue with using combi boiler?

Some addition background info: It's a 1,800 sq.ft. townhome in Seattle. Heating is devided into 5 zones, each has one wall-mount convector (rated at around 5000 BTU based on its spec sheet).
 
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Dana

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I hope that's a combi boiler, and not a broiler.

Plumbing it primary/secondary has NO effect on short cycling, unless the hydraulic separator is high volume unit (such as a Boiler Buddy) with some thermal mass to work with. A small manifold or closely spaced tees does nothing for short cycling. For low mass boilers on low mass radiation it's all about BTUs-in from the boiler vs. BTUs-out at the radiation.

Without knowing the total amount of convector on each zone it's impossible to estimate the short cycling risk, but if it's one convector is rated 5000 BTU/hr at an entering water temp of 180F per zone you're basically screwed. The minimum modulated output of even the smallest combi-boilers are twice that, and at condensing temperatures (assuming it's a condensing combi) it'll only be delivering 1700 BTU/hr or so.

The basic napkin math on that lives here.

Even if the zones can all be combined into one zone you're still stuck with only about 8-9000 BTU/hr of radiation output at condensing temperatures, with even the smallest of the line NCB 120 delivering something like 11,000 BTU/hr at min-fire, and have barely enough burner at high fire to support a full-flow shower with any margin.

Bottom line, combi boilers are only for houses with low to moderate hot water needs, and HIGH heat loads. An 1800' townhouse in Seattle could easily have a design heat load comparable to the minimum-fire output of any combi-boiler big enough to support 2 bathrooms. Even at 180F water temps the five 5000 BTU convectors only adds up to 25000 BTU/hr, and that's likely to be twice your actual design load.

So just forget about low mass wall hung combi-boilers- it's guaranteed to be a lousy fit. If you want to figure out something that actually makes sense, run a room by room IBR or Manual-J heat load calculation, or at the very least a whole-house load calculation based on last winter's gas bills.
 

zapps

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Well, that's quite embarrassing. It's actually a combi boiler. Can't spell anymore *sign*.

Thank you, Dana, for your input. I learned about short cycling from your posts in this forum and on GBA in the first place. And it has been my understanding that I got a short cycling situation with combi boiler setup. However as the contractor seems knowledgeable and confidence that there would be no issue I would want to double check on that.

The short cycling will only be related to the primary loop satisfying. There needs to be a minimum of a 20 degree change in temperature to fire the unit so the heaters will need to soak all of the heat from the lines and the heat exchanger prior to the unit firing up. Again, the secondary pump will be pulling from that loop and this will reduce the short cycling potential of the system.
Above is what he mentioned regarding primary-secondary configuration. I still don't quite understand but maybe you do.

I'm not quite sure how zoning is designed in my place. But remember one contractor mentioned that when there is a call for heat from any of the zone, hot water gets recirculate through all zone regardless (but the convector fans in other zones would not turn on). If that is the case it would work like one zone is that correct? Are you aware of such design?

I also corrected the link to spec page of the fan convector unit I have. The convector is rated at 3900 BTU/hr @ 120F, 5000 BTU/hr @ 140F, and 7800 BTU/hr @ 180F entering water temperature. How do I determine the return water temp and how to determine the BTU at condensing temperature? I read your article but still don't quite figure that out. Nevertheless, I imagine it would not change the fact that it's still way below the minimum firing rate of this combi boiler.

 
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