Is the boiler oversized?

Molodious106 · January 17

Hi,

A family member recently had an 80 year old oil burning steam boiler replaced in her 1000sf home and the new boiler is running 15 minutes when cold then less and less with each subsequent cycle. It ends up running for only 2-3 minutes then turning back off for one minute. It comes on and off this way until the set point is eventually reached. It’s going off on high pressure. The pressure cutout is set for 2psi.

My research suggests the boiler is oversized. We had the installer measure the 6 radiators and they came up with 180EDR but the boiler is 396. They should have measured before selecting but here we are. I was not involved in the selection, only after there were issues. I understand smaller steam boilers only come in gas now, which is not an issue if a conversion is the best route.

We challenged their selection and they tried to defend it blaming the vents. They replaced all 6 vents at the radiators and the 2 main vents in the basement and the performance was the same.

Then they changed the burner to a 288 and it got a little better but not much change. Same boiler block.

I’m pretty sure we need something in the 200-210 range and that we are still significantly oversized. By I’m not fully confident in what the pickup factor should be for a small house. Is 10-15% ok?

They still want to test more things (they have been vague about what else there is to test) but I’m at a loss for what other possible explanations there could be for this. I want to make a hard push to swap in a smaller replacement boiler, but looking for some reassurance before doing so.

The distribution piping and radiators are all existing so the only part that’s new is the boiler itself and the near boiler piping.

Greatly appreciate any feedback from the helpful and knowledgeable folks here!

109A_5 · January 17

In a perfect world the EDR (Equivalent Direct Radiation, the radiators) should match the boiler's Square Foot rating.

What was the old boiler's ratings ? Was there a pressure issue then ? I understand that information may be long gone.

Going to a smaller gas boiler would probably help but may not totally solve the pressure issue unless the EDR matches the boiler size. Example Peerless only goes down to 233 Sq. Ft. You could add radiators (or increase their size), but that may not be very practical either.

If the contractor bails out and puts the whole expense for a Gas down sized conversion on the customer, I'd use a timer. To me the boiler size and type should have been worked out before the boiler change.

The timer is inexpensive and disables the excessive cycling that is really not needed to heat the home. The timer method is controversial and obviously not optimum or best practice, but it will eliminate the excessive cycling. And even with a timer there are different methods to implement it. And it needs someone capable of wiring it into the boiler control system correctly.

mattmia2 · January 17

a 30% pickup factor is subtracted from the number on the boiler so if the boiler is rated 180 it is actually outputting 240 ft^2 of steam. in reality you probably need more like 10% pickup factor so you probably need a boiler rated for more like 150 ft^2 of steam. You might not be able to find a boiler that small, but the closest you can get the better the system will run.

are the radiators all heating fully? if the whole radiator is heating the system is consuming as much steam as it can. the boiler is going to cycle off on pressure if it is bigger than the connected load.

you can downfire oil some in most cases but not by over 100%.

mattmia2 · January 17

~~looks like there is a weil mcclain and burnham steam max that is close, probably is a peerless too.~~

oops, i was looking at oil boilers but there were gas boilers mixed in too. there are small enough gas boilers, the smallest oil boilers look to be about 50% oversized

EdTheHeaterMan · January 17

When it comes to oil heat, the smallest boiler is sometimes too big for a small home. There is nothing you can do about that in most cases. Some oil fired boilers have a minimum and maximum firing rate. If it is possible to reduce the nozzle input to the combustion chamber without compromising operating functionality and efficiency you can get a little better performance. If, however, they put in the smallest heater and it is too big, there is nothing you can do about it. You can't install anything smaller than the smallest heater.

Your old coal converted oil heater may have had longer operating cycles because it was grossly inefficient. Now with a more efficient heating plan, you can't count on that inefficiency to make your system operate better. You can however place some electronic time delays on the thermostat/limit circuit to make the on cycles longer. Can we know the make and model number of the boiler you have now?

ethicalpaul · 12:07AM

Yes the boiler is oversized significantly. It seems like you know this, but that's the question in your post's subject so I'm answering you.

What is the boiler that is installed now? It sounds like it is oil but you didn't say directly I think. Can they downsize the burner even further?

EBEBRATT-Ed · 12:14AM

@Molodious106

If the 180 EDR is correct the boiler is double the size it needs to be. But as others have mentioned you probably can't buy an oil boiler that small.

The smallest oil boiler is a Peerless which is about 288 square feet of steam.

When comparing boiler sizes don't look at BTUs it just confuses things. If you have 180 EDR you pick a boiler with the square ft of steam rating #. The mfg already include the PU factor in the boiler sq ft of steam rating

Weil McLain has the gas fired EG-30 which is rated at 196 sq feet which would be the right size.

If you stuck with what you have you could install a timer as others have mentioned. It doesn't cost much. just need someone who can wire it. The way it works it the boiler runs until it goes off on pressure. Then the timer starts to time out and you delay the boiler from restarting for say 20 min. The rads continue to give off heat. 20 min later the boiler will refire unless the stat is satisfied.

Lets see a few pics of the install so we can see if the installer knows what they are doing

Jamie Hall · 12:15AM

The smallest Weil-McClain oil boiler is still oversized — but not anywhere near as wildly. From the way your contractor is behaving they either are completely clueless or trying to cover that they made a serious mistake in sizing the replacement.

Molodious106 · 12:51AM

Wow, thanks everyone for the fast and helpful responses, I am so grateful to have found this community today!

@109A_5 Yes, unfortunately the old boiler is out and I didn't a chance to look for a nameplate before it was removed, I was late to the party. It sounds like the system would run two cycles pretty regularly when trying to overcome a typical delta T of 6 degrees or so, and occasionally a brief 3rd cycle if it had gotten colder overnight and it was ~8 degrees below setpoint in the morning. So it was probably oversized but it may have been less apparent due to inefficiency as someone on this thread noted. The timer idea is clever, it just feels like a big concession when this was our opportunity to get it right. I understand the idea in principal though, and I won't rule it out, thanks for suggesting it.

@mattmia2 I did not know pickup factors were baked in, wow, thanks! Yes, all radiators heating fully.

@EdTheHeaterMan Understood about oil. The burner on there is the smallest they make and I'm told it already has the smallest nozzle on it so can't go lower than 288. It's a megasteam MST288. It sounds like my family member expressed a preference for oil-burning since that is what the old one was, and the installer was so eager to get the order that they quickly made the sale rather than taking care to match the capacity to the load. I wish I'd been involved sooner, I could have pushed for a right-sized gas fired selection instead.

@ethicalpaul Yes, oil MST288 and it's down-fired as much as possible now. Thanks for the validation!

@EBEBRATT-Ed Thank you I will look into the EG-30, sounds like it could be a suitable option. My preference would be a straight swap for something like this, hopefully we can make it happen. The timer is a solid backup plan. I'll see about getting some pics.

@Jamie Hall Yes, I think they are covering at this point and dragging it out. If they were dealing in good faith, they would state the remaining hypotheses they intend to check or test, but they are unwilling or unable to do so, so it's a dead end. And so far everyone hear has been in agreement, and with the collective experience of this group, I now have the conviction to push appropriately here.

Thanks Everyone!

mattmia2 · 1:12AM

you say that the new boiler is 396 ft^2 of steam but the mst 288 is 288 ft^2 of steam. It is still about 1.5 x the size it needs to be but it isn't as bad as if it were 396 ft^2.

We see a lot of boilers that are very oversized although if it is the mst288 there aren't really better options for oil.

Molodious106 · 1:31AM

I might have been unclear there. It was originally the MST396 and after we went back and forth about it for several weeks, we were able to get them to put the MST288 burner on it instead, which we understand is the lowest they can go for this boiler. It's more important to size it correctly than to commit to a particular fuel source.

109A_5 · 1:35AM

109A_5 · 1:41AM

EdTheHeaterMan · 1:52AM

When a replacement boiler costs a lot over $10,000 and you already paid a lot over $10,000 for a new boiler recently, then you may want to rethink your statement "It's more important to size it correctly than to commit to a particular fuel source." Remember not everyone has the option to change fuel at a reasonable price. To get a Gas boiler in some of the homes that I used to service you needed to get LP gas and that has about 30% less BTU per gallon and costs 50% more per gallon. That is not an option for folks on a fixed income.

You have the ability to get natural gas but is it worth purchasing a complete new boiler? This is not rocket surgery. You have a steam boiler that is properly heating the home. And the manufacturer says not to fire the boiler lower than 0.75 GPH. I have more than once put a smaller nozzle in a heating system that is rated for 0.75 GPH and had it operate properly. You need an oil heat mechanic with experience that can try to fire it at 0.65 GPH and see if the stack temperature is above 400° with good combustion readings. Get that and you will be even closer to the Sq Feet of steam that your radiators can handle. Add to that some electronic timers to extend the cycle time and you will be as good as any gas boiler will get for a lot less than the price of a new gas boiler.

Making this contractor replace the oil boiler with a gas boiler without getting paid for installing it is not a great idea in my opinion. If however you are willing to pay for a replacement gas boiler then more power to you.

ethicalpaul · 2:54AM

In addition to the hints above, if you keep the thermostat at the same setting you will have much less cycling than if you set it back at night.

Jamie Hall · 4:54PM

You have the boiler. Now the question is how to tame it. I and others have made some suggestions. One thing doesn't seem to have gotten said explicitly is that the fuel type makes no difference at all — only the heating capacity (BTUh rating).

hot_rod · 5:20PM

Even a correctly, or exactly sized boiler is going to short cycle as 80% of the heating season you are below design conditions. Not much you can do about that with a fixed output boiler.

The more a system is zoned, the ore potential for short cycling of the boiler.

Buffer tanks are one option, controls to hold the boiler from firing, as long as you are not trading comfort for that option. High mass distribution maybe handles that forced off time better.

Once it is up and running observe the cycling on a mild day to see the frequency.

A few posters here have built data loggers to track the cycling.

Excessive cycling drives down the boilers efficiency.

The math looks like this if you are interested. Run Time minutes the boiler is firing/ minutes it is off before next on cycle

8 minutes on, 15 minutes off = 23 minute time period 8÷23= .34 or 34% Run Fraction

Plug that into this graph to determine the boilers efficiency. 84.5% being about the best efficiency. So that puts you at or just under 80% efficiency

So boilers with 5 minute on, 5 minute off, 5%as an example, really drive efficiency down.

A modulating oil fired boiler is what the industry needs :)

Screenshot 2026-01-18 at 10.08.53 AM.png

Jamie Hall · 5:58PM

Uncommonly useful graph — hadn't seen that before. Thanks, @hot_rod !

It also helps illustrate my point that a steam boiler which is cycling on pressure, provided that duty cycle is at least 50% on, isn't losing much efficiency during a given thermostat cycle. and provided the off time is short enough that the boiler doesn't cool.

Now if you hold the boiler off long enough for it to cool down — even as little as 10 minutes or so for a smaller boiler — then you start over…

Is the boiler oversized?

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