I'm looking to replace my existing copper heat exchanger hot water boiler. The boiler is 135000 btu @ 80 % {out put around 100000 btus} efficient. I'm looking to install a modulating wall hung boiler. My question is . If I know the total heat output of all my heat emitters @ 180 degree water can I use this to size the replacement boiler. My home is well insulated with new windows. I live on long Island NY. Current heating is very comfortable. Also boiler recommendation would help.
Thanks
Sizing the boiler to the maximum output of the radiation is a TERRIBLE way to size a modulating condensing boiler! With modulating condensing equipment the minimum firing rate is more important than the maximum rate, and maxing out the heat emitters will almost always result in woefully sub-optimal oversizing of the boiler.
Most homes have on the order of 2x or greater heat emitter than the house needs at the 99% outside design condition. Even with non-modulating boilers ASHRAE recommends holding the line at 1.4x oversizing, which is enough boiler to operate in overnight setback mode even during a Polar Vortex disturbance cold snap.
The first order of business is to get a handle on the magnitude your actual 99% heat load (which can be reasonably well estimated based on fuel use & weather data from wintertime gas bills,
as outlined here.)
Then, measure up the radiation/heat emitter zone by zone to both assess the water temp requirement at design condition, and the required
minimum firing rate to avoid short-cycling on zone calls at efficient & comfortable condensing water temperatures,
as outlined here.
To make the heat-emitter analysis simple enough for napkin-math, Most heat emitter types will deliver only about 1/3 the amount of heat at an average water temp of 120F (the temp needed to hit mid-90s efficiency) that it delivers at an AWT of 180F (where the best any gas-burner can do is mid-80s efficiency, even if it's a condensing boiler). eg: Typical fin tube baseboard delivers about 600 BTU/hr per running foot at an AWT of 180F, but only about 200 BTU/ft at an AWT of 120F. Similarly, cast iron rads put out about 160-170 BTU/her per square foot of "equivalent direct radiation" at an AWT of 180F, but about 50-55 BTU per EDR-foot at an AWT of 120F.
To get condensing efficiency out of a mod-con it has to run closer to 120F most of the time, and 180F only during the coldest few hours of the season (if ever), but if it's short-cycling at 120F it's going to ruin the average efficiency and shorten the longevity of the boiler.
A hydro-air coil running 120F water would have somewhat tepid air output, cool enough that it could be objectionable if blowing sub-100F air directly on people at high cfm with a wind chill. If one of the zones is a large hydro-air handler it may even require up-sizing the boiler to deliver comfortable temp air, depending on the particulars.
Only when you have the heat load & heat emitter size numbers (broken down by zone if multizoned) it will be possible to make reasonable boiler recommendations.
