The average, 1,200- to 1,500-square-foot house is not what seems to be popping up out of the ground in most developments these days.

The majority of new developments are larger, three-or-more-bedroom homes with multitudes of baths, whirlpool tubs, hot tubs, baths with multiple showerheads, and body sprays (“human car washes,” as I call them), and the necessity of providing a source of heat to all of these things.

Many of these homes have multiple zones of radiant floor heat, air conditioning and heating coils, along with everything else. For these reasons, more and more of these homes are being furnished with hydronic heating.

The question for the HVAC contractor often is, whether to size the system with one heating boiler or to use two or more.


Surprisingly, many builders don’t have a particularly strong opinion on this matter and leave it up to the heating contractor to decide what is best. And although the prospective homeowner and the builder do in fact have a budget to try and maintain, the strong likelihood of being able to upscale the heating system budget does exist.

Any home with a heating requirement of 175,000 Btu or greater would therefore be a good candidate for two or more boilers in lieu of one. All that the heating contractor has to understand are the features and benefits of when it is appropriate to do this.

The greatest benefit of installing and operating a multiple boiler system is, of course, fuel savings. Typically, an average home will only have a requirement for the maximum heating capacity or design temperature load for approximately eight to 10 days of the heating season. Realistically, the demand will average 40% to 60% of the design load for better than 200 days of the heating season.

We cannot short change the system, however, by undersizing the boiler; the heat has to be available for those few design temperature days or for the large-demand indirect water heater, or the installer may get some nasty phone calls.

With the advent of dependable and accurate solid-state boiler reset and staging controls, these energy savings are easily accomplished.


Reset and staging controls are able to monitor outdoor temperature, indoor temperature, system supply water temperature, and indirect water heater temperature with simple and accurate thermistors.

They will only provide the temperature of water needed to maintain steady supply water temperature based on outdoor conditions or indirect heater demand.

These controls will always try and meet the requirement with one boiler first and then will stage fire the other boiler(s) as needed.

This means that the Btu’s used to accomplish this are right in line with the seasonal demands. This results, of course, in only using what is needed.

Using controlled demand on two boilers means that you will be inputting 50% or less of the system capacity most of the time, instead of always firing at 100% capacity on one boiler.


Other benefits may not be so obvious. The first benefit of a multiple-boiler system is redundancy.

With one boiler, if something doesn’t work, the heating system is off and life becomes chaotic for both the homeowner and installer. With multiple boilers, you are still operating at least at 50% capacity, and, in many cases, the reality of a boiler being down will only show when a design temperature day or two comes around, or when the 100-gallon whirlpool bath takes longer to get hot.

This simply becomes an inconvenience, not chaos.

Another benefit would be that if setback thermostats are used, they could also be staged to come back on at different times, resulting in increased fuel savings. Even with all of the setback thermostats coming back on at one time, the system will try and satisfy the demand with one boiler first. The others will stage on to keep up with the demand and then shut back down.

The third benefit would be the ability to satisfy the domestic hot water demands of the home without having to add additional Btu capacity to the system to accomplish this. Most of the staging and reset controls have a domestic hot water demand priority option to dedicate all of the available system capacity to domestic hot water recovery.

A 20- to 30-minute “off” period for the heating zones is generally not an issue, while the looming possibility of a cold shower certainly is. A good majority of reset and staging controls have a built-in “safe” mode to turn the heating zones back on in the event of an indirect water heater component failure.

Taking into account all of the possible benefits that exist with multiple boiler applications and the fact that generally two smaller boilers do not cost considerably more than one larger one, why would anyone install only one?


The biggest obstacle for the heating contractor to overcome is the fear of being the higher bidder.

I would always suggest giving two quotes to the builder or homeowner: one quote for a single boiler application and the other for a multiple system.

People are more aware of what is available and what is best these days, due in part to the Internet, magazine articles, and especially word of mouth. Heck, they just installed a “human car wash” in their bathroom, didn’t they?

They more than likely will not flinch at spending a few more dollars on a heating system that can put money back in their pockets for years to come and offer peace of mind that is hard to put a price on.

Remember, this is their “dream house”; it deserves a dream heating system.

Stanton is manager of training for Burnham Corp., P.O. Box 3079, Lancaster, PA 17604; 717-397-4701; 717-293-5827 (fax); (website).

Publication date: 10/14/2002