Short Cycle Problem

TomH42

<span style="font-size:12pt">Since becoming aware of this site and buying Dan’s book We Got Steam Heat, I have learned to clean out a pressuretrol and pig tail and install a main air vent that had been eliminated.  Now we have a new problem……</span>

<span style="font-size:12pt">The boiler is short cycling like crazy.  It will fire up for five minutes and then shut down for one minute and then fire back up for four or five more minutes.  It is not the thermometer as I have  turned it up to get a continuous burn only to hear the system cycle on and off.  I guess the good news is the pressuretrol is now working properly!!</span>

<span style="font-size:12pt">About the system, it is 18 years old and feeds 6 radiators (5 on first floor and one on second).  Twelve years ago we did a considerable renovation and eliminated two radiators (one on first floor, one a second.  This boiler is now supplying 25% less radiators than it was designed for.   I think this is the problem.  Dan’s book mentioned that an oversized boiler will short cycle.  I assume this is because steam builds up in the system and pressure rises too quickly bringing the pressuretrol into play. </span>

<span style="font-size:12pt">Because of the short cycling, the second floor radiator is not getting enough steam to heat the room.  This is where my wife and I sleep and it has been quite cold in St. Louis recently.  In fact the whole house is a bit chilly.</span>

<span style="font-size:12pt">Anyway the question now is what to do about it?  I have read that short cycling can wear out parts and waste fuel.  I would seriously consider adding a radiator back into the house if I thought it would reduce short cycling (and provide additional heat upstairs).  I guess we could also get a boiler that is sized for the current system but that seems drastic.  I could also adjust the settings on the pressuretrol but the idea is to have the system operate with low pressure right?</span>

<span style="font-size:12pt">Are there any other ways to address short cycling because of an oversized boiler?</span>

Jamie Hall

Things to look at...

First off, is this one or two pipe/vapour?  Although many of the principles are the same, not all are.  You mention that you installed a main air vent.  My first question would be -- is it really big enough?  That could be part of it.  Second, you mention that the second floor bedroom radiator isn't getting enough steam to heat the room.  Do you mean that the radiator isn't getting hot all the way across?  If this is one pipe, how are the radiator vents set up?  That is the principle control in one pipe for getting more (or less!) steam to a radiator.  If this is two pipe, however, make sure that the valve is all the way open.  If, in two pipe, the radiator is heating at all, the trap is not stuck shut (if it isn't heating at all, that's a possibility).  On the other hand, if the radiator is hot across, that's all the radiator capacity there is... blankets? (grin!).  Another possibility in two pipe, if the radiator isn't heating all the way across, is that some trap somewhere else on the same return is leaking or stuck open.  If that is the case, the radiator with the bad trap will be nice and hot (perverse of it) but the other radiators on that return will heat poorly.



The short cycling is probably not why that bedroom radiator isn't getting hot -- it's almost certainly a venting or valve problem (the short cycling, as you note, is because the boiler is producing more steam than can be condensed -- if the steam could get to that radiator, it would!)



If you have checked out all the venting and trap issues, and are happy, the next thing to do is to insulate all the steam lines in the basement -- often overlooked.



Then, if the boiler is still running as you say, you can usually have your serviceman put in a smaller flow rated nozzle on the burner, if it's oil, or downfire the burner, if it's gas.  There is a practical limit to this.  I don't know of any conclusive studies on how far you can go with residential boilers on either oil or gas, but anecdotal evidence suggests that it may be possible to downfire either one by as much as 25% to 30% below the lower end of the stated firing range of the boiler without starting to run into other, rather bizarre and counterintuitive, problems.  It might be that that would help a lot -- and it's a good deal cheaper and easier than a new boiler, if not as efficient.

TomH42

Thanks

We have a one pipe system.  The main line is insulated and the main air valve is a Gorton #2 which vents 2/3 of the units.  A Hoffman 75 vents the other third.  The radiator on the second floor warms the room quite well when the boiler runs continuously.



I will check airvents on the radiators themselves.  The boiler is fueled by natural gas.  Perhaps downfiring is an approach to take.

nicholas bonham-carter

1-pipe problems

if you have a 15 ounce gauge, you can see from the back-pressure during main-venting whether the total main-venting is adequate or not. aim for 2 ounces at a minimum. the low back-pressure enables the mains to fill first, and then the risers will all fill evenly, although with perhaps with a higher back-pressure because of their smaller vents.--nbc