Will sizing be affected by adding insulation?

QuintonS · February 19

currently the house I’m in has a 150k btu steam boiler. EDR total for the 1 pipe radiators are 605. Average run time on it is about 7 hours a day during the winter according to my ecobee. This house has no insulation in the walls. I’m planning on adding R13 cellulose to at least get something in there. Will I have to get a smaller sized boiler if I do this? I’m trying save money but also maximize my return on investment and not spend money on stuff I don’t necessarily need.

bburd · February 19

Since this is an existing steam system, you would change the boiler size only if you plan to add or remove radiators. The boiler rating in net square feet of steam should match the EDR of the radiators. Changes in building heat loss are considered for existing hot water and hot air systems, but not for steam.

ethicalpaul · February 19

they should be considered for steam too but you won’t find an installer who will do so.

But definitely “round down” to the smaller boiler if your EDR lands between two models.

dabrakeman · February 19

If the existing boiler is still in good condition I would certainly try to manage it as is even if well oversized vs replacing it. You would have to look at your payback based on present value of money of the initial expenditure and figure out how long it would take to pay back if had X% lower bills. How oversized is your boiler now?

ethicalpaul · February 19

Yeah I agree with dabrakeman. Don't rip anything out, but when you have to replace it due to failure, is what I was referring to.

jesmed1 · February 19

Insulating the walls will be good. But your heat loss from the walls is only maybe 25% of the total, and by adding insulation maye you'll halve that heat loss, so now you've reduced your total heat loss by 12%. It isn't going to change enough to warrant a new boiler.

Highest payback will be tightening your envelope against air infiltration. Have a blower door test done if you haven't already, find those air leaks and fix those. That's the low hanging fruit.

Jamie Hall · February 19

You might be able to get away with a smaller boiler — so long as it is still big enough to fill the radiation when that is needed. Particularly with 1 pipe steam, though, getting adequate balance can become very difficult if the boiler power output is less than the power requirement (both measured in EDR — square feet of steem

What will change with the insulation is the run time of the boiler. That will be reduced, regardless of boiler size.

Will sizing be affected by adding insulation?

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