Published: August 13, 2015 - by Ray Wohlfarth

Categories: Commercial Heating

Ray Wohlfarth Pic

For years, we have designed boiler rooms consisting of two oversized boilers with fully modulating burners.

The idea was that the boilers could "loaf" along at low fire because we were told that boilers were more efficient at low fire. I think it is time to rethink those strategies. According to the EPA, "Boilers are most efficient when operated between 50-80% and boiler efficiency drops significantly when operated below 50%". I know that this may require a paradigm shift but bear with me while I explain.

Fully modulating burners are designed to safely operate throughout its firing range from high fire to low fire. The most common turndown ratings in commercial boilers range from  3-1 up to 10-1. Turndown is how far the burner firing rate can be lowered and still effectively fire. For instance, a 3-1 turndown burner means that the  burner will be able to drop to 33% of its firing rate. A 10-1 turndown will be able to reduce its firing rate to 10%. High turndown is used to reduce the burner cycling and maintain a consistent temperature or pressure in the boiler. This is crucial if the boiler is used in an industrial process that requires a consistent temperature or pressure. It is not as crucial in a commercial space heating environment except to the control tech that can and will track the temperatures to within four decimal points of the setpoint.

When a power burner starts, it has a pre-purge period which ranges from 30 seconds to several minutes. During this pre-purge, the burner fan pushes air throughout the combustion chamber for a period of time required to provide four air changes inside the combustion chamber. This is to "purge" the combustion chamber and flue of any leftover fuel. Some of the older boilers required seven air changes which prolonged the process. Many of the older, larger boilers also had a post purge which pushed air through the combustion chamber after the call for heat ended. While this air movement through the boiler does take heat from the boiler and send it outside, it is less wasteful than oversized boilers idling at low fire.

Jacket Loss

Traditional estimates suggest that a boiler will lose between 2-5% of its rating through the boiler wall into the boiler room at any time that the boiler is warm.  Let us assume that we have a boiler with a rated input of 1,000,000 Btuh and it has a jacket loss of 3% from the boiler into the boiler room. That equals 30,000 Btuh for every hour that the boiler is warm. Now, let us consider what happens when the burner drops to low fire. The boiler will still lose 30,000 Btuh because the boiler jacket does not know that the boiler is at low fire. It just knows that the boiler is warm. If our burner is a 3 to 1 turndown and drops to a low fire setting of 330,000 Btuh, that 30,000 Btuh loss just Tripled our jacket loss percentage to 9%, dropping the efficiency of our boiler from 80% down to 71%. If we have a 10-1 turndown burner, our jacket loss percentage just jumped to 30% of our firing rate. Our boiler efficiency is now at 56%.

If you have another boiler in the boiler room that is not isolated from the system, you will have jacket loss from that boiler as well because the boiler is warm as well. This could drop our efficiency even lower. For example, if our system had two boilers at 1,000,000 Btuh and only one was firing at low fire or 33%, our jacket loss would be 18% or 60,000 Btuh and our system efficiency would be 62%.

The following will allow me to show you the heating plant efficiency with both standard and condensing boilers at low fire. We will compare two boilers each sized at 1,000,000 Btuh input at 80% and two at 90% efficient. Each boiler will have a 3% jacket loss into the boiler room.

Two 80% efficient boilers

Burner Turndown

3-1

10-1

Input @ low fire

333,000 Btuh 33%

100,000 Btuh 10%

Flue Loss @ 80% efficiency

66,600 Btuh

20,000 Btuh

3% Jacket Loss from 2 boilers

60,000 Btuh

60,000 Btuh

Net to Building

206,400 Btuh

20,000 Btuh

System Efficiency

61.98%

20%

 

Two 90% efficient boilers

Burner Turndown

3-1

10-1

Input @ low fire

333,000 Btuh 33%

100,000 Btuh 10%

Flue Loss @ 90% efficiency

33,300 Btuh

10,000 Btuh

3% Jacket Loss from 2 boilers

60,000 Btuh

60,000 Btuh

Net to Building

239,700 Btuh

30,000 Btuh

System Efficiency

71.98%

30%

 

According to Honeywell, a Low High Low burner is 15% more efficient than a modulating burner. A Low High Low burner uses a firing rate control like a modulating burner but only has two settings, Low or High fire. The burner will go between the two settings to meet the needs of the facility.

Combustion Analysis

A modulating burner will take much longer to check and adjust than an on-off or low high low burner which increases the maintenance costs. In addition, it is almost impossible to get the same efficiency throughout the entire range. The new linkage-less controls make the setup somewhat easier.

When adjusting the fuel to air ratio at low fire, a subtle change has taken place in the industry. Many of the burner manufacturers have found that when the burners stay on low fire for extended times, damage to the burner heads may occur. In addition, the fuel and air do not mix as well, leading to elevated carbon monoxide readings. To get better mixing and better burner protection, you will now see that many burners require more air at low fire. In some instances, it may be 20-40% more at low fire. This could lower the efficiency by 1-2% more. In addition, extended operation at low fire in standard boilers could allow the flue gases to condense, destroying the boiler.

I have found that several smaller boilers, piped primary secondary, with low high low burners can offer better seasonal efficiency than two large boilers with modulating burners in many instances. This also concurs with the findings of the International Energy Conservation Code which recommend isolation of the idle boiler(s). Each boiler room will be different so keep an open mind on your next replacement.

Want to learn more? Check out my books, Lessons Learned in a Boiler RoomLessons Learned: Connecting New Boilers to Old PipesLessons Learned: Servicing Boilers, and Lessons Learned: Brewing with Steam.