10,000 BTUs

Joe B.

I have a customer who has a Weil GV-6 installed with a net of 133,000 BTUS. It's has two zones of cast iron convectors and a 40 gal indirect. The owner said that in the 20 yrs of the boiler, the house has always been comfortable. He even has some convectors turned down. We would like to install a Weil Ultra but the 155 series is 10,000 BTUS short and the 230 series is about 50,000 BTUs over. The owner is worried about the fuel efficiency I am worried of under sizing. I said the best approach is to do a heat loss calc to be sure. He just wants the 155 series. Is 10.000 BTUS really going to matter?

Eugene_Silberstein

Just on the information....

Based solely on the information you provided, you are probably fine with the smaller equipment. If the system is 20 years old, it is doubtful that there was a comprehensive heat loss calculation performed on the structure. Also, mentioning that some of the terminal units are "turned down", indicates that a reduction in system capacity of less than 8% will not have a negative effect on the overall system performance.



If this were my house, I would have absolutely no reservations about downsizing.



Good Luck!

Jean-David Beyer

As a homeowner who cares about these things, I would risk it.

"He just wants the 155 series. Is 10.000 BTUS really going to matter?"



I have an Ultra-3 rated at 80,000 BTU/hr INPUT. I am the only one who did a heat loss on my house, and it calculates out at about 30,000 BTU/hour when it is 14F below design temperature. I cannot go any lower because mine is the smallest Ultra 3 in the product line. I have it adjusted so that the largest (radiant slab) zone does not cycle too rapidly except when it is very warm out (but below the warm weather shutdown temperature). The other zone is too small,  really, so it cycles more rapidly than I would prefer, but I hope it is acceptable.



If W-M had made a 40,000 BTU/hour boiler, I would have had to do a more accurate heat loss calculation, but on balance I would have risked it. Remember the design temperature is the one where 97.5% of the days are warmer than that. So if the heating season is 100 days, it should be colder than that only 2.5 days if I understand the meaning of design temperature. 10,000 BTU is only 6.5% under if he needs 165,000 BTU/hour. I know my heat loss calculation is not accurate to 6.5% accuracy



So I think you are absolutely right that this person needs an accurate heat loss calculation. Even when last year it was much colder than this year, mine went below design temperature for only a few hours for each of 2 or 3 daya,  and the heat stored in the slab coasted over that and I never noticed it. But this house will always have enough heat. Even with my present outdoor reset settings, it will have no trouble whatever if it goes 8F below design temperature. 



You must know that if you go a little under the house will not drop to 0 inside; you may have a little trouble going all the way up to the desired temperature. So if the customer is trying for 70F and it gets too cold outside, and it goes up to only 65, he will be upset, but it is probably not life threatening. And I just made up those numbers. You could probably calculate the correct values if you had an accurate heat loss calculation. I do not know how accurately a good professional calculates a heat loss. I assume he can do the math right, but the imponderables of what is inside the walls seem to me to preclude extremely accurate heat loss calculations. I wonder how much margin of safety a competent professional uses these days when the cost of oversizing the boiler is probably almost as bad as undersizing one.