Question re how much fuel is burned

NY2024

Does the “Input MBH” of a boiler refer to how much natural gas is burned per hour?

Using my own Burnham IN6 boiler as an example, is the 175,000 Input MBH mean than that the boiler is burning 1.75 therms of natural gas (i.e., 175,000 BTUs) per hour that it runs? The DOE Heating Capacity is 144,000 and the IBR Rating is 450 sq. ft. (i.e., 108,000 BTUs).

The EDR of my system is 302 sq. ft. (i.e., 72,500 BTUs). As the near boiler piping and mains are fully insulated with 1 inch fiberglass, would an appropriate pickup factor be 1.2x (instead of the 1.33x multiplier that Burnham uses in their “gross” to “net” calculations)? If 1.2x is appropriate, this would yield a “gross” EDR of 362 sq. ft. (i.e., 87,000 BTUs).

If I reduced the boiler to a Burnham IN4 with an Input MBH of 105,000 and a DOE Heating Capacity of 87,000 BTUs, would I theoretically see a reduction of 40% in the number of therms of natural gas burned? With the math being 175,000 – 105,000 = 70,000. Then 70,000 divided by 175,000 = 40%

Would I likely notice a change in comfort by reducing the boiler to a boiler sized to the EDR of the house?

bburd

"Input" MBH does indeed refer to the firing rate, which is the amount of fuel burned.

A 20% pickup factor is probably reasonable for your system.

A correctly sized boiler will be more efficient than an oversized boiler. It should need fewer repairs and have a longer life due to reduced short cycling.

However, with a lower firing rate it will have to run longer to heat the house. You will likely see a modest savings, perhaps 10%, and a bit more if the current boiler is very inefficient; but it will be nowhere near 40%.

From a comfort standpoint I doubt you would notice much difference, except that the system may be quieter and the temperature swings more gradual.

The IN series boilers are known for short lifespans, especially in areas with high chlorides in the water. If you go with Burnham again, consider the SteamMax which is a more robust design.

NY2024

Thank you very much! The IN6 I have dates back to 2004 (we just moved into the house this past July). It seems to be functioning 20+ years later, but I've read from numerous posts here that they often have much shorter life spans.

Do you have any recommendations for strips to measure the chlorides or any other relevant minerals/chemicals in the tap water in our area?

mattmia2

make sure it isn't leaking, that the vents are all closing properly, that valve packings and such aren't leaking so it gets little makeup water. There are filter cartridges designed for filling boilers that you can fill it through but making sure it isn't losing any more water than necessary is the most important step.

with a small system that is insulated 10% is probably a more realistic amount of loss to the piping.

EBEBRATT-Ed

Agree with @bburb

ethicalpaul

0% loss to piping with 1" insulation (you only fill all the radiators on long calls for heat so you don't need extra even though you will have it, plus radiators are usually oversized too.)

You won't have a change in comfort I think regardless.

You won't have a change in fuel savings that you will ever notice—regardless of the size of the boiler, 83% or so of the BTUs are going into your living space.

Cycling doesn't matter very much either in an atmospheric boiler—what are we talking about, some additional cycles of the gas valve? It's rated for a couple hundred thousand cycles. Some cycles of the flue damper? It doesn't hurt the boiler to "short cycle" it. Cycling is greatly preferable to building pressure.

mattmia2

there is more standby loss during the times when it isn't firing but still has a heat call. it isn't a huge amount, but it uses some more fuel than if it just fired continuously at a rate that matched what the system can consume. It isn't remotely close to an amount that would pay for replacing the boiler within the lifetime of the new boiler.

ethicalpaul

There's less standby loss if anything (immeasurably less but still) due to the very slightly smaller temperature differential. It's too small to even consider, but my point is whatever standby loss there is doesn't matter.

It doubly doesn't matter because the system never goes negative pressure.

Surely the fact that the burners aren't burning for a couple minutes saves many, many times more fuel than whatever it costs to get the boiler boiling again (which occurs within a few seconds of the burners reigniting).

Of course I agree that sizing the boiler correctly is preferred, but it just doesn't matter efficiency-wise in any way that I have seen.

mattmia2

this isn't a 1970's standing pilot boiler, it is a couple minutes for the burner to file again once it gets a call. there is opening the vent damper, then lighting and proving the pilot, then firing the main burner. some boilers have a pre purge in the cycle too. By the time all that happens it is steaming less but still losing heat up the vent and to the basement. some of the heat in the basement gets in the house but a lot of it leaks out around the top of the foundation and through bypasses to the roof.

ethicalpaul

I agree with everything you said. But note what I said: despite the delays you list, the boiler starts creating new steam before the steam pressure in the system goes down to atmospheric pressure.

Therefore the whole time there is steam available to the radiators.

Therefore the heat loss you describe does not matter—and don't forget, the heat loss is happening while the burners are firing, too. I tried to make this point before but it's admittedly subtle.

I admit at this point I'm not sure what your point is (nor mine!). Are you saying that an oversized boiler costs more to run than a correctly-sized one?

I would agree with this in terms of additional start up time from warm/cold due to the additional water and metal that must be heated. But during the heating cycle, I don't see it. In fact, since the larger boiler has a larger mass-to-surface-area ratio, the larger boiler is going to lose LESS heat to the boiler room on a percentage basis than a smaller boiler.

Long Beach Ed

Theoretically, the smaller boiler would be cheaper to operate assuming your pick-up factor was correct. That actual determination would be made by examining the mass of the system - radiator and piping mass and average temperatures. As is stated here, your factors seem typical.

If you underestimate the pick-up, you will have radiators that don't heat and a higher bill trying to heat them on the coldest days. I've found the Independence to be somewhat generously rated, so consider this when shaving the pick-up factor of your system.

I do agree with Paul here that the fuel savings with the smaller boiler would be insignificant, and certainly not justify the boiler's replacement.

Hot_water_fan

FYI your home’s HEAT LOSS determines how much fuel you’ll consume. Not appliance size.

A boiler 2x the size will run half as often, but consume the same fuel (more or less).

LRCCBJ

https://forum.heatinghelp.com/discussion/comment/1852770#Comment_1852770

Sadly, a boiler 2X the size will run 1/2 as often as calculated by numbers of hours run but it will run 5X as often based upon CYCLES RUN. This is where the efficiency disappears. The cycles are determined by by the Pressuretrol. If set correctly, I might be in error with the 5X………..it could be greater.

dabrakeman

Most pressuretrols (as opposed to vaporstats) let the pressure reach near 2psi on lowest setting before initiating cycling which is too high for efficiency, noise, comfort and livelihood of vents.

Jamie Hall

Cycling is annoying. I get that. On older motors it can reduce life span. I get that. On some very old burner ignitors it can redeuce life span. I get that.

What you can't say, as a blanket statement, is that it significantly reduces overall efficiency. Not to say that it can't; sometimes it can — but for steam it depends on the pressuretrol or vapourstat settings, and for hot water it depends on the aquastat settings. For hot air it depends on the furnace thermostat settings.

That is not to say that there isn't a small efficiency loss inherent: that is the stack loss — the hot air (or residual exhaust gas, for a post purge) which is lost out the stack. That is real, but just how much it is depends critically on how hot that gas is and how long the post purge is. It might be as much as a few thousand BTU for a big boiler and a long post purge/pre purge cycle.

Now where efficiency is lost is if the boiler or furnace is allowed to cool significantly. While the heat required to bring a boiler or furnace up to operating temperature is not strictly lost, it is effectively lost in most installations. Thus to optimize the efficiency of a boiler or furnace which must cycle to match its power output to the load demand one wants to minimize the "off" time of the cycle as much as possible. This can be done by setting the differential in the pressuretrol or vapourstat or aquastat or whatever to a small, but reasonable value (in situations involving a post purge/pre purge, it is very likely that that timing will be the control)

Obviously it is preferable to size the boiler or furnace so that its output matches the load as closely as possible This is the principle behind modulating burners, which control fuel input in relation to the load and rely on more or less sophisticated controls to do that. Modulating burners are simply not available for oil fired systems in smaller sizes, as control of spray pattern from the nozzle is difficult (at best!). Modulating systems for larger boilers are — simply vary the number of burners firing — and modulating for gas is common enough. Sp — one modulates the power output by turning it on or off. No harm to that…