Upsizing a boiler for Indirect DHW

Cyclist77

So with assistance from a fellow on this site and a heat loss calculator I am closing in on the size boiler I need. But now I would like to know how much,if any, upsizing of a boiler is needed for indirect dhw.

We are 2 in the house with normal hot water needs. I will throw this into the mix. I have a lead on a brand new Weil-McClain Aquapro 30 gallon tank for $400.

Info that might be helpful

2000sqft house in NE Ohio, double 2x4 walls with R30 ,6000 HDD's,30,000btu/hrs heat loss@0f.

Leaning towards Weil-McClain CGa boiler.

Thanks!

Hot_water_fan

No oversizing needed.

hot_rod

Which boiler are you considering? The smallest may be a bit to small. The next size will be oversized, probably end up short cycling on low load conditions.

30,000 for 2000 sq ft is 15 BTU/ft. Sounds a bit low? But it is well insulated?

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EdTheHeaterMan

If your boiler was using a tankless coil (like the discontinued CGt boiler) I might upsize it to a 4 section boiler. But with your indirect tank, that is not necessary. Your DHW recovery will be a little longer with your CGa-3 boiler. You may be a little undersized with a CGa-2.5 boiler. Same 3 section boiler with a smaller burner. But the CGa-3 will be a good fit for your heat loss. You should be using the NET rating for selecting a boiler based on the heat loss. The Input rating is used to select the proper size gas pipe.

jesmed1

OP has radiant floors so will want relatively low water temps at low flow rates. Does a buffer tank make sense with the CGa-3, or is it not necessary?

HVACNUT

The CGa-25 will heat the space fine. But WM used 234K BTU's for testing on their water heaters, where the -25 is only putting out 32K. Maybe the design experts here can help you get the best recovery with the smallest boiler.

hot_rod

30K is a bit smaller than most tank type water heaters burner, so expect performance similar to a 30 gallon gas water heater.

With a low temperature radiant, a boiler protection valve may be required.

Or with a 4 way motorized mix valve use the return protection feature.

You will need to run 150 or so and mix down to the radiant temperature.

So similar to driving with one foot on the gas and one foot on the brake when using a conventional boiler on a low temperature distribution :)

Cyclist77

A Tekmar or Taco 4 way mixing valve is in the mix. Pun intended!

Cyclist77

Hot Rod,

Yes the house has 11" exterior walls and lots of insulation in the attic space. In the early 80's there were a few of us that got interested in super-insulated homes.