Condensing boilers

Uni R_2

All houses fit into one of two categories. Some houses will never have return temps over 130° (lucky buggers that own them) and the rest of the houses will exceed those return temps for a fraction of the heating season. The big question is how cold does it need to be for that to happen. Then try and assess what portion of the heating season that represents (wundergound.com will help for seasonal analysis).

You could probably do a reasonable estimate of that number by doing a heatloss calculation on SlantFin's software. I'd first use the real design day and then once you have the heat loss, I'd raise that design temperature until it is about 70% of what it was. Heatloss calculations are very conservative. Then you could see what water temperature you would need. Take away 10 for a rough ΔT adjustment and you have an idea of the maximum temp that you should expect near minimum conditions. You could then play around with the outdoor temperature until you get to a 140 supply. The difference in temp between that outdoor temp and the 70% outdoor temp is what you need to add to design day. That will be a rough guess as to how low the ODTs can go and still allow the boiler to condense.

The beauty of that cutoff number is that every time you do anything to improve the insulation or infiltration of the home it will continue to drop that outdoor temperature where the boiler stops condensing and add more efficiency to the boiler.

Just remember that the temperature profile in winters are kind of like a snow capped mountain. Only when the load gets high enough at the peaks will it experience the highest return temps. You'll have wonderful efficiencies in the foothills of fall and spring.

Paul_84

Condensing boilers

I recently purchased a house in New England with a very old oil based boiler. The house is 3700 square feet with 6 zones of forced hot water. I am planning to convert to natural gas and am considering installing a condensing boiler (Triangle, Burderus, etc).

My questions are whether it makes sense to spend the extra money for the more efficient system because I have read that the efficiencies are not captured when water return temperatures are above 120 degrees. Also, should I spend the money to install pumps for each of the 6 zones or one pump for all of them?

The incremental cost of the high efficiency boiler is about $4000?

Any advise you could provide would be greatly appreciated.

Thanks