Combi Unit vs Separate Boiler and DWH

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Reicherb

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I've had a pole barn built with radiant in slab piping. I'm working to spec the boiler and water heater. The heat load comes out about 44,000BTU. I'm looking for DWH for a half bath and utility sink. How much efficiency do I lose by going with a Combi unit that is much larger than the heat load calls for?

This is the unit I'm looking at:
https://www.supplyhouse.com/Weil-Mc...ombination-Wall-Mount-Gas-Boiler-65000-BTU-NG

It's cheaper than a smaller boiler and DWH.

Please advise. Am I on the right track with that unit?

Thanks.
 

Jadnashua

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With a combi unit, it needs to be running 24/7/365 to provide hot water, even during the summer.

With a modern condensing boiler (one that can cold start), depending on how much hot water you use, it might not run more than once a day during the summer.

Generally, you do not want to upsize your boiler to provide hot water. Set it up as a priority zone. As a side benefit, a boiler tends to be larger than the burner of most water heaters, so it recovers quicker, and, as long as you size the tank for your needs, you should not run out. A combi may have issues in the winter at some delivery rates if your incoming water is super cold (mine has been measured at 33-degrees after a cold spell). Having a tank means high volume, should you need it.
 

Dana

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With a combi unit, it needs to be running 24/7/365 to provide hot water, even during the summer.

With a modern condensing boiler (one that can cold start), depending on how much hot water you use, it might not run more than once a day during the summer.

Huh?

The AB-80C (referenced in the original post) is a condensing 80KBTU/hr combi with a 10:1 turn down. It doesn't have to "be running" any more than a mod-con with an indirect water heater. As long as the fairly modest max gpm of the potable hot water is OK (probably is, if it's a shop, probably not if it has a full bath with a tub), it will be fine.
 

Jadnashua

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Okay...I made an assumption based on a couple of things...at the input BTU, the thing won't have very much volume of hot water for the domestic potable side. But, it's closer to a tankless water heater with space heating capability engineered for higher duty cycles than a typical tankless system.

For light duty hot water needs, it should be fine. If you need more flow volume, forget the combi and install an indirect to a standard boiler.
 

Dana

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Okay...I made an assumption based on a couple of things...at the input BTU, the thing won't have very much volume of hot water for the domestic potable side. But, it's closer to a tankless water heater with space heating capability engineered for higher duty cycles than a typical tankless system.

For light duty hot water needs, it should be fine. If you need more flow volume, forget the combi and install an indirect to a standard boiler.


That's right. Combi boilers are best matched to situations with modest hot water needs and large heat loads, which is almost never a residence, but is often the case for commercial buildings or farm shop outbuildings.

Assuming the 44K heat load calculation accurately reflects reality (often not the case- most newbies overestimate by 50% or more, and even in the hands of pros the tools can sometimes overshoot by 25%), a boiler than can modulate down to 8000 BTU/hr=input will be just fine. On the coldest day of the decade it might have to crank up to 55-60K to cover the heat load, but most years it won't break 50K for more than a few dozen hours total.

ASHRAE recommends a 1.4x oversize factor, which is really quite conservative (rarely if ever needed). So 1.4 x 44K = 61,600 BTU/hr. At 90% efficiency that would be a boiler with a max input of 61,600/0.9 = 68,444 BTU/hr. Can't do much better than an 80K-in mod con (or combi) here.

In hot water mode the AB-80C won't support much more than a single low flow shower and would take seemingly forever to fill a standard sized tub in the dead of winter, which makes it pretty crummy for a residential application, but fine for a heated shop or barn building where you might need/want to wash your hands or clean up a muddy truck/dog with something other than freezing cold water.
 
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