Approaches to reducing boiler size

2 pipe steam you can orifice the radiator inlets. One pipe steam you can't do too much.

If it were me, I would do a heat loss calculation room x room. This will tell you how much you are over radiated.

As you mentioned above normally, we are taught to add up the edr of all radiators, then the boiler MFGs add to that EDR a factor of 1.33 for a steam boiler for piping and pickup.

Many including me think this factor of 33% is too high. I would make sure any boiler installed covers the actual radiator EDR and then lower the PU factor depending on how over radiated you are .

I never said — and would never say — that the boiler should be oversized for the radiators. for a steam system In fact, I would nope that no responsible person would say that.

That it happens more or less routinely doesn't excuse it.

Let's try again. The boiler not ourput — most easily expressed in EDR — should never be significantly more than the installed radiation, also expressed in EDR. Because there is a built in safety factor in the EDR rating of the boiler, it can be less; with well insulated mains, as much as 20% less is usually satisfactory.

This is not rocket science. The job can be done in an afternoon with a table of radiator sizes and an adding machine.

Done correctly, you have a system which works together well. The boiler can reliably and evenly power the radiators. You can then control that with a thermostat so that the power delivered to the space by the system matches the power required to keep the house at the desired temperature — also evenly and reliably.

Provided the temperature swing from the thermostat — not the boiler or radiators — is kept reasonable, the efficiency impact from turning the system on and off, say once an hour, is minimal.

The efficiency impact of an oversize boiler in comparison to the radiation can, however, be significant, depending on just how bad the oversize is and how the boiler is controlled.

We're in a similar situation with two WGO-5 hot water boilers in a 100-year-old 4-unit condo building near Boston, and all original cast iron radiators. The boilers run about 25% of the time on a design day, similar to yours. Your boiler is making steam; ours is making hot water. Your boiler concentrates its heat into a few gallons of water to produce steam; ours distribute their heat into 100+ gallons of water each to produce lower-temp hot water. But both our systems run about 1/4 duty cycles on cold days.

Since these are non-condensing boilers running at 80%+ "dry gas" efficiency, the loss of almost 20% of the heat in the "dry gas" exhaust, plus another 7% or so latent heat of vapor loss (for oil) and 15% or so latent heat of vapor (for gas) is already "baked in" to the boiler, and those losses can't be recovered without changing horses to a condensing boiler with lower exhaust gas temps.

So those of us with non-condensing boilers are left with trying to make marginal improvements in overall system efficiency. For example, recovering some of the heat stored in the boiler's cast iron heat exchanger after shutdown. In my case, someof that heat gets recovered during a typical 2+ hour idle time, during which I have our circulators set up for a thermal post-purge until the supply temp drops to 90 F. But even without the post-purge, gravity circulation would continue to suck some of that residual heat out of the boilers. I've done some calculations, and I figure that there's at most maybe 10% of a typical cycle's (45 minute boiler run) heat left in the boiler water and heat exchanger. We recover some of that , but some of it goes up the chimney as air continues to draft through the boiler after shutdown. So maybe we lose 5% up the chimney and recover 5% in post-purge circulation.

Some people try to retain that last 5%-10% of residual heat by installing automatic vent dampers. The downside of those is that when the damper motor fails and the damper won't open, the failsafe switch prevents the burner from firing, and you have no heat. I've decided the marginal possible gain isn't worth the hassle.

But back to your original question, the reason boiler oversizing matters is that a larger boiler has more thermal mass in the heat exchanger and in the higher volume of water in the boiler, both of which increase the amount of stored heat that can be lost up the flue after boiler shutdown. If we had boilers that were, say, half the mass of our WGO-5, we'd have roughly half the number of BTU's stored in it after a cycle, and thus roughly half as much heat to lose up the flue. So maybe we'd go from losing, say, 10,000 BTU of a cycle's heat production up the flue post-shutdown to maybe 5,000 BTU, or a 5% overall efficiency improvement.

Our boilers run about 500 cycles per season, so 5,000 BTU saved per cycle would be 2,500,000 BTU, or about 18 gallons of oil (out of 600 gallons burned per boiler). That turns out to be about a 3% savings. It's something, but not much.

The numbers above are ballpark-ish, as there's no easy way to measure how much heat we lose up the flue after boiler shutdown. But based on the above, I'd be surprised if we saved more than 10% fuel by going to smaller boilers (even though I plan to do so when these fail).

@winnie said "Is this boiler oversized enough that it will make a significant difference to fuel consumption? The system seems to run just fine."

You may have missed my previous post before asking this question. My short answer is that, after a lot of data collection and calculation, I don't expect more that 10% fuel savings from reducing our boiler size/capacity by half. I'd be pleasantly surprised by better savings, but I don't think it's going to happen.

I must have some sort of weird setup. I have a mercury T87 thermostat. Vapourstat. 2 LWCOs. More radiation than needed for the house (although at -12 this morning… and the boiler sized to fit the radiation..

No other controls.

No insulation to speak of (house built from 1780 to 1893)

And the building, at the thermostat, stays within half a degree either way of the setpoint, in design cold or fall and spring.

How odd…

If it's still working 100 years from now, I'll take it. Well, 96. Cedric's system's birthday.

This year my system piping turns 100 and my boiler is at 69 years.

@Jamie Hall

I was complaining bitterly when I left the house this am as well. I don't think I can remember a colder winter. I know when I was young if I remember 1960-61 was bad.

My old man had a 41 Plymouth that would never start when cold. He used to have this great big heat lamp he would plug in and hang by the distributor when it got cold out. He always said the "distributor is too low to the ground" LOL. Then he got a 48' Nash. That one liked the cold even less. In fact it wouldn't start and the old Plymouth which was sitting in the garage unregistered actually started. So the Plymouth pushed the Nash around the block until it started.

Your values of 245 connected EDR and 450 sq ft I=B=R NET steam indicate that the boiler will routinely reach its pressure limit during normal operation, assuming all other components are functioning properly. The oil burner producing this 450 sq ft steam output is currently firing at a rate of 1.20 GPH.

Early in my career working with oil burners, I learned that down-firing a boiler can improve overall operating efficiency in several ways. A reduced firing rate lowers fuel consumption per hour of burner operation. While the flame temperature is slightly reduced, resulting in marginally less instantaneous heat transfer to the boiler water, the reduced flue-gas volume slows the passage of combustion gases through the cast-iron heat exchanger. This increased dwell time allows more heat to be transferred into the boiler water, which in turn lowers the stack temperature and reduces stack losses. Provided the stack temperature is not allowed to fall excessively, flue-gas condensation will be minimal or nonexistent. This operating principle applies equally to hot-water boilers, steam boilers, and warm-air furnaces.

By reducing the burner firing rate from 1.20 GPH to approximately 1.00 GPH, the I=B=R NET steam rating is reduced to roughly 375 sq ft. If the burner can operate for approximately three minutes before steam production begins, the stack temperature can then be evaluated. For example, if the stack temperature measures 550°F at a 1.20 GPH firing rate and decreases to 400°F at 1.00 GPH, a further reduction to 0.90 GPH may be considered. At that firing rate, it is important to confirm that steam is still produced within four minutes and that the stack temperature remains at 350°F or higher.

Once the firing rate is optimized to produce steam within four minutes or less while maintaining a minimum stack temperature of 350°F, the boiler will be operating at a higher level of thermal efficiency.

Have your oil burner service provider check the firing rate of your burner as it stands now. Someone may have already reduced the firing rate. That would be a good thing. If not, then try the 1.00 Firing rate and again the 0.90 Firing rate to see if you get good combustion test numbers. Once you get good results then make a note of those settings with the actual nozzle rating and pump pressure and combustion test numbers with Smoke Spot reading Draft over the Fire and at the exhaust of the boiler (before the barometric draft control) and of course the CO PPM and Oxygen and Carbon Dioxide percentages with the stack temperature associated with those readings.

By lowering the firing rate you will in fact lower the Input, and the NET output accordingly, resulting if getting closer to the actual EDR of the building.

EDIT:

You can go too low with the firing rate. Just because 1.20 reduced to 0.90 may end up with a lower fuel bill, does not mean that you can keep going lower. There is a point where you get too small of a flame and end up burning lots of fuel for hours and hours and not make enough steam to get the heat to the farthest radiators, or to satisfy the thermostat.

I think the controls they installed back then needed to be able to detect and maintain single digit ounces of pressure in the system and be able to react to changes due ONLY to changes in demand. That would be impossible with the radiation ever filling because filling spikes the pressure unrelated to demand well out of range making the whole control scheme unworkable. So lots of extra was installed to be able to heat at ultra low header pressures without filling even on design day.

The open windows idea may be true, but quantifying exactly how much extra radiation open windows required would obviously be challenging. My point is that significant extra radiation would have been required even with the windows closed. I also don't think filling all that what we know to be extra radiation to pressure at any time makes any more sense today than it did back then.

Back then requires some qualification. There were controls on some coal fired boilers which were sensitive to pressure — not all coal boilers had them, and oil boilers which came in relatively early never did. In fact, the fundamental effort behind all the assorted widgets and patented whatnots for vapour pressure steam systems was not to control boiler pressure in itself, but to control the differential pressure between the steam mains and the dry returns. Some of these gadgets were disarmingly simple — the Hoffman Differential Loop itself has no moving parts, although it does require a vent at the Loop (one single vent, thank you — the only one in the system) but others look like nightmares from Rube Goldberg's shop.

It is quite true that many structures were over designed for the "design day" or whatever. This was not entirely sloppy practice, nor was it always motivated by leaving windows open. Rather, in any cases it was quite deliberate, so that there was enough power available to actually raise the space temperature if the occupants desired to do so on a cold day. Folks back then weren't as fanatical about saving energy as some are today — but they were pretty fanatical about being comfortable when and as they wanted to be!

Rather, in any cases it was quite deliberate, so that there was enough power available to actually raise the space temperature if the occupants desired to do so on a cold day. Folks back then weren't as fanatical about saving energy as some are today — but they were pretty fanatical about being comfortable when and as they wanted to be!

It was not uncommon at that time to continue the old custom of winterizing a house when travelling in the winter rather than leaving the house above 50f or whatever we do today. Quite a recovery when you return.

Yes I would tighten that deadband and adjust down the pressuretrol. If you are doing recoveries then I might suggest that you try breaking those up and if it is very cold outside even eliminating them.

For efficiency, control the boiler any way you want — but whatever yo do, don't let the off cycles be long enough to let the boiler metal cool. For cast iron, maybe two or three minutes. For steel keep the off cycles as short as the burner recycle will allow.

On steam, never let the off cycle pressure drop to zero.

P.S. That is, what if boiler was properly sized, but an addition added more EDR therefore undersizing boiler output needs after doing the math

Regards

RTW

I'm really not following you on this one. Running the way you suggest here will be pressurized by the time the tstat is satisfied and the shortest call for heat time. On average days this then guarantees long off times to the next call for heat and therefore much colder starts on every new burn cycle.

As an example, on an average day my boiler might need to burn 25% of the time tops. If I run the way you suggest (as it did when I moved in), it will run some amount of reheat time and then 15 minutes of steam filling radiator time, banging off the vaporstat a couple times and satisfy the tstat. Then it would sit off for 45 minutes to an hour and face another reheat. All we need is 15 minutes or so actual steam production an hour. If that is done in one straight run it guarantees even longer off periods which you say you don't want doesn't it?

My first move was to install a vent damper on my 12 inch flue. Game changer. My next move was then to install a simple delay timer which said the boiler can only run 10 minutes on and 10 minutes off no matter what during a call for heat. This was more than needed on an average day but would do the job also in the cold which I knew took a 50% burn time. Even just this was way better and I assure everyone no less efficient. Call times lengthened obviously, heat more even, no pressure ever and time between calls shortened so maximum cool down time actually shortened. The boiler was never off as long as without the timer and total preheat times with the damper actually less. That was 20 years ago or so. I've added control enhancements since.

The next game changer was vacuum. Vacuum extends steam production after the burn stops many minutes, and starts steaming seconds on every new fire even with the boiler having been off 30 minutes due to the fact that burns start always in the deepest system vacuum. Steam literally never stops flowing into the radiators. The net effect of this is also game changing, and gets more an more evident as demand increases.

It is a legitimate question how on earth we ever got to filling completely and controlling these systems with pressure when they were designed originally intending never to have any. 100 years ago pressure was rightly regarded as the enemy and system manufacturers bragged about how low they could get it. That and that somehow today some claim that there is a perfect fixed speed boiler size for these systems that magically running only on high is just right for all demands that will vary by many multiples.

1000.