sizing efficiency of modulating boiler

Kevin Callahan

Hi everybody,

I'm about to replace a very old oil converted to gas boiler in a 2400 sq. ft., baseboard heated house in Anchorage Alaska. I've decided to go with a modulating condensing boiler even though the high temperature return of baseboard will not permit the exotic maximum efficiency of, say, radiant heat. The increased efficiency is still worth it to me, and I do not want to be locked into a cast iron, less efficient system for the next 30 years. I will also be putting in an indirect hot water heater.

The best available local choices seem to be either a Weil-Mclain Ultra or a Lochinvar Knight. (I was quoted an additional $3600 for a Vitodens 200, more than I can justify.) I favor the Knight for the stainless steel heat exchanger and ease of use.

My question is sizing and efficiency. My house probably needs something over 100k btu to cover very cold days. A smaller Knight may not quite cover this, but possibly it would. I know correct sizing is important, but is it really critical with a modulating boiler? What are the factors involved?

I understand that a larger boiler has a higher low btu level (for example, a Knight 105 goes down to 21k, while a 150 goes only to 30k) and this makes some difference. On the other hand, manufacturer efficiency graphs (for example, with the Knight and the Viessmann) seem to indicate no great loss of efficiency, or even marginal gains, running such a boiler at less than full load. However, I am not sure that my newly acquired, seat of the pants understanding here is correct.

As a practical matter, my choice seems to be between
choosing a smaller boiler such as a Knight 105 or Ultra 105 or not taking any chance of running out of heating headroom on a -25 day by going to, say, a Knight 150 (net output of 134k btu). If the efficiency penalty is simply that the system can only cycle down to 30k btu instead of 21, I would go with the larger boiler. But because I only know enough to be dangerous, I thought I would ask you guys.

Thanks!

hot_rod

If you

have done a heat load calculation to come up with that number, also measure how many feet of baseboard you have in the home. No sense buying more boiler power then you have baseboard to get the heat to the space.

Consider adding more baseboard footage, if possible, to use the lowest supply temperatures.

All the brands you mentioned are fine choices. Lochinvar will have some wall mounted versions out this fall. All the smaller sizes of wall hung Knights vent with 2" which may be an additional plus for you.

hot rod