New Weil Boiler

corgi11

Thanks for your answers but I cant comprehend a 150k input boiler and a 100k input boiler buring the same amount of gas at identical weather conditions in the same structure for 1 day.

ethicalpaul

Jamie answered this for you. The larger one should have a shorter duty cycle.

Your house loses a number of BTUs in an hour. The larger boiler can restore those lost BTUs quicker than the smaller boiler so the the larger one has to fire less.

The reason you get the correctly sized boiler is because a smaller boiler costs less and it may have a duty cycle that results in better performance and comfort.

WMno57

Correct sized boiler vs oversized boiler.
Correct sized boiler loses less on start up. Correct sized boiler loses less when off through chimney and basement walls. Correct sized boiler might even have a smaller flue.

Jamie Hall

corgi11 said:

I'm sorry but if there is no difference in gas used why would anybody bother to do a heat calc on a 1950s house and get a correctly sized boiler. So if my neighbor across the street who started out with gas in his house identical to mine ( half of this development was oil half was gas when they were built) and they just replaced their old 150k boiler with a new weil 140k the difference of our gas usage with me burning 100k on identical weather days makes no difference why bother getting a heat loss and just replace same size with same size? Only gas hot water heating additional nothing else.

The reason for doing a heat loss should be obvious: to get a boiler installed which is big enough to cover the heat demand of the house, but not unnecessarily bigger. Bigger isn't better -- it's just more expensive to buy, and may, because of shorter run times, actually be more expensive to maintain.

Heat demand may change with time. Just because a certain size boiler was appropriate when a house was built (and it may not have been) doesn't mean it still is. Additions? Storm Windows? Insulation?

corgi11

No changes to house. All brick, no insulation in walls, 3 inch insulation in attic.

Jamie Hall

You don't have to do a heat demand if you don't want to. It's good practice to, and good contractors will do it almost automatically. If you don't care to do it right, just put in the biggest boiler you can fit in and live with it short-cycling. But don't complain if you don't like it

corgi11

So basically you guys are saying in my particular instance it really didn't matter if I bought the Weil CGA 4 or CGA 5. The cost of each on the same weather day would be the same even though my heat load is about 62k. The cga 5 is 140k in 113k gross and 98k IBR net. Mine is 100k input 88k  gross and 73k IBR net. The wholesale cost of the larger one is only marginally more. So are you guys saying I made a mistake putting in the cga 4. The company that replaced it wanted to put in a cga 5 but the specs of the cga 4 were closer to what I needed so I had them put in a cga 4. Did I make a mistake? Or does it really matter in this instance.
Thank You Guys Very Much

Jamie Hall

You did NOT make a mistake buying the cga4. There are very few occupations where bigger is always better, and heating isn't one of them.

corgi11

Thank You Very Much Jamie

corgi11

Finally results after a full heating season. New Weil vs 70 year old American Standard Oil converted to gas.

Old 140k in converted to gas. New 100k in. Was able to go down to 5 in flue from 6 in flue. Because new boiler has no mass and I have copper fin and tube I had to raise cph from 4 to 5 to remain comfortable, used to have constant circ. We had a full week of 10 below temps here in Chicago and I was able to maintain 70 degrees at -10. That was design spec for my house. Basement is colder, not as much loss from jacket. Gas usage on similar degree days from last year to this year was approximately 35 to 40 percent less usage. Old boiler on plate before conversion 97k gross new boiler 85k gross. New boiler rated 85% efficiency. Both boilers were rated 73 k net hot water. Very very happy with new boiler. Thanks to you all for all your help and comments.