Your boiler is probably too big.
In fact, I will state categorically that a substantial majority of existing multifamily boilers in New York City are at least twice as big as they need to be. How can I make such a claim? Consider the evidence:
• Even though heat-timers are set at “A” (the lowest setting), many buildings are still overheated.
• Many burners are set to fire at 50 percent of capacity, yet still short-cycle.
• Radiator surveys often show that a much smaller boiler could meet the load.
• Absurd levels of overheating are found in almost every building.
Historically, many boilers have been sized unscientifically. Instead of taking the time to perform a radiator survey, installers have relied on such quaint approaches as the “cube method” or rules of thumb like “two boiler horsepower per apartment.” Frequently, about half the boiler’s capacity is for the steam load and half for the domestic hot water (DHW) load.
Even if a proper radiator and domestic hot water survey has been performed, the resulting boiler would be grossly oversized for all but a few hours of the year. This is because the only time the entire boiler output is needed would be on that rare moment when it is the coldest day of the year and it’s also the morning DHW peak.
Smaller boilers would fit in well with the new New York City No. 4 and No. 6 oil restrictions. Existing regulations do not allow a boiler firing less than 10 gallons per hour (gph) to burn No. 4 heating oil, or less than 20 gph to burn No. 6 oil. Ten gph of No. 4 is roughly 35 boiler horsepower (bhp), and 20 gph of No. 6 is about 72 bhp. So, if you were to install a considerably smaller new boiler, the fuel decision becomes much easier. For example, if your existing boiler is 120 bhp and you reduced it to 60 bhp, you could burn No. 4 but not No. 6. If your existing boiler is 50 bhp and you shrank it to 25, you could burn No. 2 but not No. 4.
How much smaller can my boiler be?
If you decide to install a new boiler, it can almost always be much smaller than the existing boiler, which is oversized with respect to both the steam/hydronic load and the DHW load; it only needs to be sized for the larger of the two loads.
This last point is the key. Over the course of much of the day, there is a limited need for DHW in most multifamily buildings. The extra boiler capacity is really needed only on cold days between about 6:30 and 8:30 A.M., and to a lesser extent, 8 to 10 P.M. Monday through Friday. That’s only about 600 hours a year, out of 8,760. That’s very inefficient. The boiler must be able to meet the steam load or the DHW load but rarely both at the same time. So it can probably be half or less its current size.
What about those 600 hours, you ask? Well, what about them? During those DHW peaks on cold days, the DHW load will suck all the heat out of the boiler, leaving little or none for steam. But so what – you won’t notice it.
Let me give you an example from a typical winter night. The boiler controls essentially shut down the equipment late in the evening and bring it back online early in the morning. But, over the course of those eight hours, the room temperature drops only about four degrees, and over two hours drops only about a degree.
On colder nights (say about five to 10 degrees outside), the boiler does not stay off all night. But even then, the temperature drops only about one degree per hour when the boiler is off.
What that means is that on a really cold day, during the DHW peak, you might lose a degree or two for a brief period. I think you can live with that.
You’ll need a radiator survey and either a measure of the DHW load or an estimate based on fuel bills. Once you know which load is the larger of the two, the boiler size can be selected.
Change it now?
So, should you change your boiler ASAP? Well, no. But if you are considering replacing it for other reasons, now is the time to size the new one correctly (by the way, don’t be in a big rush to replace your boiler just because it’s old. See “Ten Energy Myths Debunked,” Habitat, September 2010).
If your boiler is in decent shape, you can reap some of the benefits of a smaller boiler by reducing the maximum firing rate of the existing burner. City regulations allow reduction as low as 50 percent if the boiler manufacturer approves. Before you proceed, make sure the boiler inspector won’t give you a violation.
If this is such a great idea…
Now, you ask: “If this is such a great idea, why isn’t everyone doing it?” One word: fear. Co-op boards, which are typically unversed in the intricacies of heating system design, are afraid of being cold. In the heating industry, change comes slowly, old habits die hard, and if it means less money for boiler installations, why even bring the subject up?
And so, the waste continues, and will continue, until some co-op board somewhere has the courage to take a more scientific approach and see how it can help save money. Are you that savvy co-op board?