Issue: 2/02

What's the first component you envision when someone mentions hydronic heating? Chances are it's a boiler, the benchmark heat source for the vast majority of hydronic space-heating applications.

PM Engineer conducted a survey of 1,000 randomly selected readers to gauge their preferences on boilers. The survey was conducted during the fourth quarter of 2001 and received a response rate of 15%.

Ninety-nine percent of the responses where from readers directly involved in specifying, recommending or purchasing boilers for hydronic space heating applications: 84% indicted they specify boilers for light commercial systems, 74% for larger commercial and industrial systems, and 49% for residential applications. This article summarizes the responses from that survey.

Which Fuel?

When it comes to fuel types, natural gas was the overwhelming winner, with 74% of the residential boilers specified using it versus the 17% that use fuel oil. Propane came in third with 8%, and electricity followed with 1%. Solid fuel (wood/coal) boilers where used in less than 1% of residential systems.

The lead enjoyed by natural gas extended even farther when it came to commercial systems: 82% of light commercial systems used natural gas versus 13% fuel oil. Propane picked up 5%, with electricity again at 1%. In larger commercial systems, natural gas powered 82% of the specified boilers, versus 14% for fuel oil. Propane dropped to 3% in this category.

Air In/Exhaust Out

Atmospheric combustion combined with chimney venting was specified by 83% of the respondents. Atmospheric combustion paired with power venting was specified by 82%. Interestingly, 64% of respondents indicate they had specified sealed combustion systems. More boilers in the latter category have become available over the last few years. Concerns over indoor air quality and potential backdrafting of toxic gases, especially in tightly sealed buildings, has certainly helped build the market for sealed combustion systems.

Keep It Dry, or Run It Wet

How about condensing versus conventional (non-condensing) boilers? In the residential market, the survey indicated a near dead heat. Conventional (non-condensing) boilers were specified in 51% of the systems versus 49% using condensing boilers. The strong market growth in low temperature radiant heating has likely helped condensing boilers gain market share over the last decade.

Outside the residential sector, 70% of light commercial systems and 84% of larger commercial systems were specified with conventional boilers.

Is One Boiler Enough?

Multiple boiler systems (MBS) are another "hot" topic in hydronic heating these days. PME asked its readers at what space heating load would they consider using an MBS instead of a single high capacity boiler.

The first threshold number seemed to be 150,000 Btu/hr. Below this, only 6% of respondents would consider a multiple boiler system. However, with a space heating load between 150,000 and 200,000 Btu/hr, 16% indicated they would consider using an MBS. Twenty-one percent more jumped in when the capacity requirement fell between 200,000 and 250,000 Btu/hr, and 43% of respondents indicated the load would have to be in excess of 300,000 Btu/hr before they would consider using multiple boilers.

The reasons cited for specification of multiple boiler systems were no surprise. Fifty-six percent of respondents said the ability to provide partial heat output in the event one boiler was inoperable justified a multiple boiler system. Forty percent cited the higher seasonal efficiency of an MBS versus a single larger boiler. Only 4% cited the size and reduced weight of smaller boilers as a reason to use an MBS.

When it came to piping multiple boiler systems, 60% of respondents indicated they prefer parallel primary/secondary piping. This arrangement allows each boiler to be isolated from system flow when it is not firing. It also allows the same (system return) water temperature to enter each operating boiler. Another 26% indicated they piped multiple boiler systems in a "2-pipe" configuration with valves. Series primary/secondary piping was used by 7%, and straight series piping by 5% of the respondents.

It's worth noting that terms such as "series primary/secondary" and "2-pipe with valves" may conjure up different schematic images for different people. Fortunately, the concept of piping several boilers together in one series circuit is not widely accepted.

Incidentally, the piping and performance issues associated with multiple boiler systems will be discussed in much more detail in the September 2002 issue of PME.

Boiler Output Ratings

I once heard a marketing person quip, "Why is it that whenever you ask an engineer a question, the answer always begins with, 'It depends...'?" Surely some engineers, mixed in with Washington bureaucrats and traditionalists in the hydronic industry, helped develop the current potpourri of boiler output ratings.

With boilers under 300,000 Btu/hr, you can currently choose from the IBR Net rating, DOE Heating Capacity, or a couple of different fuel input ratings (one for gas, the other for fuel oil). Each rating gives the same boiler a different heat output value depending on the assumptions involved.

Fifty-one percent of those specifying boilers for space heating systems under 300,000 Btu/hr indicated they used the IBR Net rating. This rating derates the tested thermal output delivered to a water stream by 15%. It implicitly assumes 15% of the boiler's heat output is lost from piping between the boiler and the load, or is required to raise the system's thermal mass to its normal design load temperature. This may be high or it may be low; in fact, "it depends" (sorry) on the system as well as the boiler.

Another 28% of respondents indicated they use the fuel input rating to select the boiler's capacity. They presumably also assume some nominal combustion efficiency to deduce the thermal output of the boiler. If not, they're likely to undersize the boiler by 15 to 20% (i.e. a 100,000 Btu/hr gas input to a boiler operating at 85% combustion efficiency delivers 85,000 Btu/hr to the water stream).

The DOE Heating Capacity rating was used by 17% of respondents. This rating assumes all heat released from the boiler, including jacket heat loss, becomes useful heat input to the load. This is a reasonable assumption for systems where the boiler is located in conditioned space, especially if it uses sealed combustion.

Is It Big Enough?

Call it what you will--safety factor, fudge factor or CYA multiplier--the simple fact is that no specifier wants to find out the boiler they selected can't keep the building warm on a cold day. To guard against this, most specifiers feel some additional heat output above and beyond the calculated load is justified. A wide range of opinion exists on how much extra is merited.

Eighteen percent of the respondents indicated that no oversizing factor was used when they specified the boiler, while 40% indicated they used a 10% oversizing factor, 31% indicated that a 20% oversizing factor was justified, and only 2% were willing to take it to a 50% oversizing factor.

Is One Temperature Enough?

Outdoor reset control (ORC) allows the water temperature supplied by a boiler to increase or decrease based on outdoor temperature. The colder it is outside, the higher the water temperature supplied by the boiler. The goal is to make the rate of heat delivery equal to the rate of building heat loss while maintaining more or less constant circulation.

Controls that reset the boiler's temperature are very common in Europe. In fact, it's hard to find a modern European boiler that doesn't have ORC built in as a standard feature. The popularity of ORC is increasing in the U.S. market, but its overall implementation lags significantly behind that of the European market.

Thirty-six percent of the survey respondents indicated they specified ORC for residential boilers, while 79% specified ORC for light commercial jobs, and 70% for larger commercial projects. The potential savings from reduced fuel usage are certainly greater in larger systems. This is undoubtedly one reason ORC has a higher acceptance in this market sector. Personally, I think 36% usage of ORC in residential systems is encouraging. Higher fuel prices, or price spikes such as those experienced last winter will surely help sell ORC in all market sectors.

How Long Should a Boiler Last?

Do you know someone who religiously changes the oil in their car every 3,000 miles, but wouldn't consider servicing their boiler until that fateful day when it's a bit chilly inside and turning the dial on the thermostat doesn't fix the problem? Some building owners think of a boiler as practically a permanent part of their building. Certainly all want the longest possible service life for their dollar. So what do we tell them when they ask about the life expectancy of a boiler?

We asked heating professionals what they feel is a reasonable service life for boilers of various constructions. The following numbers indicate the mean expected service life in each of three categories:

Residential Systems 1. Cast-iron boilers: 29 years 2. Copper tube boilers: 20 years 3. Stainless steel boilers: 29 years 4. Steel boilers: 21 years

Light Commercial Systems 1. Cast-iron boilers: 26 years 2. Copper tube boilers: 19 years 3. Stainless steel boilers: 27 years 4. Steel boilers: 22 years

Larger Commercial Systems 1. Cast-iron boilers: 26 years 2. Copper tube boilers: 19 years 3. Stainless steel boilers: 28 years 4. Steel boilers: 25 years

The range of expected service life varied for each type of boiler, and in each of the three categories. However, based on these responses, it's safe to conclude that boiler specifiers expect significantly more life from cast-iron and stainless steel boilers than from those constructed of copper and steel.

The Bottom Line

The final question in the survey asked which of several considerations were most important in deciding on the specified boiler.

A whopping 86% of respondants indicated the reputation of the manufacturer bears on their specifying decision. This was followed by ease of servicing (71%) and life expectancy (68%). Bringing up the rear was low purchase cost at 31%. Although some wholesalers may disagree, these responses indicate that quality and reliability hold major sway over up-front cost in the minds of many boiler specifiers.