Which Ultra?

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We just calculated total heat loss of client's home at 149,719btuh. Which WM Ultra should we use?

Alan(CaliforniaRadiant)Forbes

Looks like

they have two sizes close to your heat load:

The 155,000 BTU at 92% AFUE will put out 142,600 BTU's.

The 230,000 BTU at 92% AFUE will put out 211,600 BTU's.

I'd go with the 155 because if your heatloss calc's. are correct, 1) the boiler will seldom see a design day and because of this, most boilers are oversized and 2) boilers are more efficient with long burn times.

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Jim Davis_3

Ultra Specs

According to Weil-McLain spec sheets the Net IBR output rating on the 155mbh is 123mbh. The 230mbh is 183mbh. Just for curiosity, how you can be 92% AFUE when your outputs are less than 80% of the input??

Alan(CaliforniaRadiant)Forbes

IBR

"The IBR rating of a boiler derates the tested thermal output delivered to a water stream by 15%. It implicitly assumes 15% of the boiler's heat output is lost from piping between the boiler and the load, or is required to raise the system's thermal mass to its normal design load temperature."

(John Siegenthaler, "Boiler Biases", PM Engineer, 2/2002)

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hr

If you are sure

all the heat load inputs are correct? Older homes are harder to calculate without seeing in the walls and pining down an infiltration number. But if you feel comfortable with the load number size to the smaller one. There is usually a bit of fudge in most heat load calc programs. A RPA comparison, years ago, showed something like a 30% over kill in some programs.

On the other hand, all Ultras modulate and would adjust to the varing load quite well.

A heat load calc is just an estimate for a point in time.

hot rod

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