Boiler cycling on and off - leak?

muftobration

I have had this problem since last winter, but it's now at the top of my maintenance list for my heating system. I fired up my boiler for the first time this heating season today to try out my new main vent (B&J Big Mouth). I decided to time how long it took for all the radiators to get hot, as well as any cycling on and off. Based on the timings, I think I have a leak. My thermostat was at 67 and I called for 70. Here's what I recorded:

1. Boiler shuts off and all radiators hot after 25m:22s
   
2. Boiler off for 1m:30s
   
3. Boiler on for 2m:16s
   
4. Boiler off for 1m:40s
   
5. Boiler on for 2m:09s
   
6. Boiler off for 1m:37s
   
7. Boiler on for 2m:04s
   
8. Boiler off for 1m:41s
   
9. Boiler on for 2m:04s
   
10. Boiler off for 1m:39s
    
11. Boiler on for 2m:05s
    
12. Boiler off for 1m:37s
    
13. Boiler on for 2m:05s
    
14. Boiler off for 1m:40s
    
15. Boiler on for 2m:04s
    
16. Boiler off. Thermostat satisfied after 53m:54s.
    

You can see it's like clockwork. From what I've read, the cycling on and off shouldn't be happening. My best guess is that I have a leak somewhere at a constant rate (hence the very regular timing), so the pressuretrol tells the boiler to make more steam. I can hear one of my radiator vents hissing very slightly, so I'm guessing that's the issue. Could something else be wrong?

System details
MegaSteam installed January 2018
B&J Big Mouth main vent (new as of today)
Basement pipes not insulated
5 radiators downstairs, 2 upstairs
Ventrite #1 vents set to very low settings (1 or 2) on all radiators

mattmia2

What does the pressure look like as it cycles? What is the differential on the pressuretrol? Sounds like the boiler is either over sized by a lot or one or more radiators has air trapped in part of it. Do the radiators all heat all the way across and top to bottom?

Jamie Hall

mattmia2 said:

What does the pressure look like as it cycles? What is the differential on the pressuretrol? Sounds like the boiler is either over sized by a lot or one or more radiators has air trapped in part of it. Do the radiators all heat all the way across and top to bottom?

Totally agree. That's not a leak. That's a classic symptom of an oversize boiler. By the timings, I would say oversize by about 20% to 40%.

ethicalpaul

Classic oversized cycle pattern, I agree. I agree so completely I wouldn't even post my agreement, but I wanted to ask a question that might inform your future actions.

So this was the timing pattern for a 3 degree setback.

What is it like during a normal call for heat? (Like, run the test again, but do it for the call for heat that immediately follows a test like the above.

Or even better, a normal call for heat after like a day of normal running (which will ensure all the insides of the house are up to temperature--70 in your case above)

PS: you're right, it shouldn't be happening, but that ship has sailed and the fault lies with the 2018 installers, who didn't properly size the boiler to your installed radiation

JUGHNE

What was the outside temp for this test?
IMO.....with little heat loss for the house there would be little condensation of steam in the rads, thus little pressure drop and the build up to cycle the pressure switch.

The rads were sized for the cold design day temp.
(and probably even oversized at that time)

Now would you get this short cycling pattern when it is perhaps 10 degrees outside?

True, boiler is probably oversized but then the radiation is also perhaps more than you need.

Maybe I am thinking upside down on this......

dopey27177

Try this, boiler at a cold start, set T stat at 75 degrees, fire up boiler, it should take about 25 minutes for all the radiators to get hot. Not to worry about reaching set temperature.

If you have a leak in the system it should show as loss of water or visibly see steam some where in the system.

As is stated the boiler is most probably over sized.

If you have an oil fired boiler you down size the the firing rate by installing a smaller nozzle in the oil burner. Looking at the time delays I would try a nozzle about 20% smaller.

If you have a gas fired boiler have the gas company down size the firing rate 20% and that should correct the problem.

Remember a combustion analysis should be done and he proper adjustments must be made.

Also set your pressure controller at 1 PSI on and 2 PSI off.

If you have a vaporstat set at .5 psi on and 1 psi off.

Te lower the steam pressure the quieter the vent valves will be and the less strain on them.

Jake

gfrbrookline

What is your building made from, wood or brick/masonry. If it is brick like mine the building has so much mass to heat the boiler will cycle with any setback as the bricks take a lot longer to warm up than a wood framed building.

If it is wood frame I agree with @dopey27177 and @Jamie Hall .

PMJ

@muftobration,

You don't have a leak. Your cycle chart looks that way because at over 1000 to 1 steam volume to water volume as soon as the burner goes off the system pressure crashes downward initiating a new firing in a very short time just as your numbers show. Considering that there is never a reason to be at any pressure at all, I think it is long past time for some thinking outside this pressure box. No one should be thinking they are stuck dealing with pressure cycles just because a boiler is large as they are usually told.

Any interest in making your own control?

Nobody wants pressure, nobody needs pressure, but yet we still install controls that operate only on pressure and then tell people if that "control" operates too much, the boiler must be too big.

How about this for a concept:

1. Put a remote temp switch on feed pipe at the last rad to get steam in the system.
2. Run every burn until that switch sees steam plus adjustable timer amount. The fill level will then be to the same amount no matter how cold(or not) the start but you stop the boiler before the system is so full that pressure can develop. It never needs to be that full of steam to heat the place anyway.
3. Have the next burn start(if call still in progress) when that same sensor opens again plus a different adjustable timer(wait) amount. Each new burn will then run to the same stop(fill) point in #3.
4. Cycle this way until call is satisfied.

There will never be any measurable pressure, recovering from setback or not. Evenly spaced cycles will be the result, many fewer per hour than in your chart. Cycles then will actually vary themselves(both burn time and off time) with the weather without touching the timers. You start filling steam again each successive cycle when the heat coming from your radiators has slowed to a point of your choosing - not at some pressure point where all radiators are still already fuller than they ever needed to be in the first place. Your heat will be much more even.

Oh, and this control doesn't care at all how oversize your boiler is.

mattmia2

Even better would be to install one way vents that allow the system to go in to vacuum as the steam condenses and lower the boiling point of the water in the boiler on the off cycle. That allows for longer on/off cycles while keeping the boiler steaming. Operating the valve (and possibly other accessories) more means they wear out faster. Longer cycles means more reliable.

Just a simple timer would probably help too.

pecmsg

Are you using the boiler control or a vapor stat?

PMJ

mattmia2 said:

Even better would be to install one way vents that allow the system to go in to vacuum as the steam condenses and lower the boiling point of the water in the boiler on the off cycle. That allows for longer on/off cycles while keeping the boiler steaming. Operating the valve (and possibly other accessories) more means they wear out faster. Longer cycles means more reliable.

Just a simple timer would probably help too.

You are correct on both counts. One simple delay on timer would help a lot.

I do what I described on my 2 pipe system with vacuum which is another whole step up performance wise.

Jamie Hall

I was thinking you would chime in, @PMJ , and wondering where you'd gotten to.

I gave up on this debate a long time ago, so I'll not add to it -- except for one thing. You absolutely must have a pressure control on the boiler. Set it for what you like, but it has to be there. It is a required safety control, if only by code and your insurance company.

PMJ

Jamie Hall said:

I was thinking you would chime in, @PMJ , and wondering where you'd gotten to.

I gave up on this debate a long time ago, so I'll not add to it -- except for one thing. You absolutely must have a pressure control on the boiler. Set it for what you like, but it has to be there. It is a required safety control, if only by code and your insurance company.

Agreed, and I'm not advising anyone to remove it.

Every year I see the same situation - folks with systems running banging off the pressure limit whatever it is. All they are told here is to make their boiler smaller somehow.

So I just throw out there occasionally that one might think about a much less expensive and more effective control solution instead. I'm not expecting it will gain any more traction this year than in the past.