Oil boiler steam system short cycling

bkselly

My boiler runs for about 30 minutes initially before cutting out, then short cycling every 2 minutes

I initially thought I had pressuretrol issues, but have established that it does in fact cut-in at 0.5 PSI and cut-out at 1.5 PSI

I know typically one of the big concerns is poor venting, but I have established that all my radiators get fully hot before the boiler cuts out (so I assume it's not air pressure from poor venting but I might be wrong about that). It took about 15-20 minutes to get all the radiators from cold to fully hot (but the boiler did run earlier in the day so I guess the water in it was already warm)

What else should I be looking at? Would replacing my non-functioning main vent help, or do the radiators getting fully hot before cut out indicate air clearance isn't a problem?

Thank you

EBEBRATT-Ed

Does the short cycling ever stop? Is the water level stable? does it just short cycle once or twice and then stay off a while?

If you have a bad main vent it should be replaced

bburd

That sounds like normal operation for an oversized boiler, which is quite common.

If you want to check, you could add up the EDR of your radiators, and compare it to the boiler’s net rating in square feet of steam. We can help you if you need it. 

CaptPaul

@bkselly I have the exact same situation. I have measured 167.5 sq ft of EDR while the boiler is rated for 313. It’s an old, oil-fired, 1-pipe counterflow system with no venting other than what’s on the radiators. I may be replacing the whole system but if I don’t I will need to address the situation. I have been thinking about downfiring the boiler, adding main venting and possibly adding radiators (at least one has been removed).
Maybe there are other possibilities?

bkselly

bburd said:

That sounds like normal operation for an oversized boiler, which is quite common.

If you want to check, you could add up the EDR of your radiators, and compare it to the boiler’s net rating in square feet of steam. We can help you if you need it. 

I think this might be the issue. I have 6 radiators total, but there is a potential for 11 radiators total (5 pipes without radiators because certain walls were broken down so I guess the previous owners didn't want two radiators per one room).

For the six active radiators, I get 97 sq ft of EDR (or ~24,000 BTU/hr), but the boiler says it is rated for 421 sq ft (or 101M BTU/hr which I assume is 101,000 BTU/hr).

By my calculation - it is 4 times overpowered? Perhaps 3 times if accounting for the pipes/risers?

If that is the case, is there anything I can do to resolve the issue without replacing the boiler? It says it consumes 1.1 gallons oil/hr if that is relevant

I used this guide: https://www.weil-mclain.com/sites/default/files/field-file/Weil-McLain_BoilerReplacementGuide_WM2012-web_0.pdf

Big Ed_4

Twenty minutes and all the radiators are hot , perfect

Big Ed_4

Are your mains insulated ?

bburd

With that many radiators missing, does the system heat adequately on the coldest days? It might if insulation and windows have been upgraded. If not, you could calculate the current heat loss and add some radiators to meet it.

Oil boilers can usually be downfired to some degree, the minimum firing rate should be in the manual. This is a job for a trained technician with combustion test equipment. But it will not get you nearly to matching that small load.

bkselly

All the rooms with radiators are typically warm on normal days. On the coldest days -- we had a few days below 10F and the radiators couldn't get the rooms above 70F but that's comfortable enough

Any sense in downfiring per manufacturer minimum firing rate (with professional combustion testing) and maybe adding an additional 23 sq ft (5600 BTU/hr) in radiators (two missing radiators)?

Just trying to figure out if the short cycling due to oversized boiler is worth fixing (potentially by above method) or if the only proper answer is to replace for a properly sized boiler

bburd

If it were my house, I would do exactly as you just described. There is no reason to replace the boiler just to make it smaller; I don’t think oil boilers are made for loads as small as yours. It’s just going to short cycle. Regulars here tend to get upset about that, but we don’t live in an ideal world and probably 80% of steam boilers out there are too big. If your steam pipes are insulated, your venting is good, your pressure is limited to 1.5 psi and you keep the burner tuned, it should heat the house relatively economically.

Jamie Hall

The cycling won't hurt anything -- except maybe your nerves. With a boiler that oversize, though, there is another approach: use two timers on the system: one which will limit the run length to half an hour or whatever it takes to get to cycling, and the other to keep the boiler from restarting for 10 or 15 minutes or so after it has turned off.

bkselly

I have heard cycling increases wear and tear on boiler and affects longevity - is that not true?

@bburd - would doing these two steps also decrease amount of oil consumed to heat house, or is it more just about decreasing amount of short cycling

Jamie Hall

bkselly said:

I have heard cycling increases wear and tear on boiler and affects longevity - is that not true?

@bburd - would doing these two steps also decrease amount of oil consumed to heat house, or is it more just about decreasing amount of short cycling

Very little change in oil consumption -- a percent or two, perhaps. Just changes the cycle lengths.

bburd

Short cycling increases wear somewhat on things like the burner and controls, not the boiler itself or surrounding piping, which are most of the cost of a boiler installation. The net cost of slightly more frequent possible replacement of those components is so small compared to the cost of replacing a working boiler with one that is closer to the needed size that it’s simply isn’t worth doing the job until the boiler leaks and *must* be replaced.

Short cycling also may slightly increase oil consumption. If the rest of the system is working well… very slightly.

Downfiring should improve both short cycling and fuel economy a bit.

Adding radiation to meet the current heat loss of the house will further reduce short cycling, but will also keep the house warmer in very cold weather. The net effect on fuel consumption is hard to predict, but the house will be more comfortable and it’s easier to sell a house with an adequate heating system.

bkselly

Thank you all - very helpful!