Short cycling after cutout on fast pressure drop

dgn

I couldn’t quite find a similar post on The Wall so here goes...
On a recent Burnham IN7 boiler replacement I installed a vaporstat and 3 psi gauge replacing the pressuretrol. Cutout is 12 oz. with a subtractive differential of 8 oz. I replaced all 18 vents on radiators with Gorton’s from sizes #4 to #C depending on radiator size.

What happens on 2-deg. overnight setback (or if I change temp by 1-deg during the day) is that most rads get fully hot, pressure builds on gauge to about 12 oz and vaporstat shuts the boiler. Then the pressure on gauge drops to almost 0 in about 45 seconds and the boiler starts up again. It takes about 5-7 minutes for the pressure to build again, vaporstat cuts out at 12 oz again, 45 sec. later the pressure is about 0 and boiler starts again. Repeats multiple times.

This doesn’t seem right to me seeing the pressure drop so quickly after cutout. Isn’t this short cycling bad for the boiler?

However, during the day when the temp setting is constant and there is a call for heat, there is no pressure buildup, which I know is good.

Some of the new vents close on steam once they get hot, however others do not -- they hiss. I thought all vents should close on heat. At least that’s what I read and what the Gorton literature states. Could SupplyHouse have sold me some bad ones?

One radiator, the largest, is not quite hot all the way across when the cutout happens.

Some system stats:

• 2-pipe, air vent system with a few 1-pipe rads just as described in “The Lost Art of Steam Heating Revisited” as well as here. All rads go to wet returns.
  
• The IN7 is rated at 542 EDR. All radiators in house have approx. 418 EDR which puts it about 30% over the boiler rating.
  
• Boiler input is rated at 210 MBH. At the suggestion of a steam plumber here on HH, I measured the gas meter over one minute while the boiler was on, did the math, and came up with 216 MBH input -- just slightly over-firing by ~3%.
  
• All basement pipes are well-insulated with fiberglass.
  

Fred

The connected EDR is about 30% less than the Boiler rating, making the boiler well over-sized. It is going to short cycle as the boiler will produce more steam than the radiators can condense. Look at the possibility of down-firing the boiler or adding a two stage gas valve.

KC_Jones

You basically have a 72% pick up factor so you are going to short cycle like that, no way around it. Should have installed the next size down and even that is slightly oversized.

Vents could be bad out of the box it can happen. Contact the manufacturer about that and see what they will do.

What is your main venting like? A "C" vent on a rad is fairly aggressive and sometimes an aggressive vent can cause issues with closing properly. If you get water in the vent from the steam moving too quickly and carrying water the vent isn't going to close. If there is water in the vent it's going to have difficulty getting to steam temps to close.

dgn

Yup, unfortunately all my self-educating and reading came after the new installation. So the pickup of all the piping is taken into account in the numbers?

Also, I have no main vents. In describing the basement setup to a steam guy, he said I had an "octopus" of piping down there so suggested slightly larger rad vents. There are 2 mains of about 20 ft. each plus 3 other horizontal pipes go to risers headed to other radiators. Funny, but the longest of the mains w/o a vent starts getting heat to the 3rd floor radiator in a just a few minutes.

Mostly #4,5, & 6 vents. Only one "C" which goes to a 24 sq. ft. rad on the third floor which is the slowest to heat.

@Fred Willing to do some of my own work but most definitely will get a pro if I go the down-firing route.

Fred

You need main vents on those two mains. Definitely have a Pro do any down-firing or two stage gas valve. He needs to do a combustion analysis as part of that process.

dgn

What I forgot to add was that it takes close to an hour of the boiler being on to get the pressure to raise which seems to indicate the system (pipes & rads) are condensing steam at a decent clip until some vents start to close and the pressure builds. Also, that there is no pressure when the boiler cycles on & off to maintain a constant temp.

What concerns me is the fast pressure drop on cutout when the pressure does rise after the setback recovery.

Fred

The fast pressure drop, on short cycling is normal. I might add that if the boiler runs for an hour before it starts to short cycle, if you had main vents, you'd spend a better portion of that time supplying steam to the radiators rather than pushing air out of the mains, into the radiator run-outs and out the smaller radiator vents.

dgn

So here's a hypothetical situation:
If I had a near-perfectly sized boiler, then wouldn't the pressure never rise? Wouldn't the cutout on the vaporstat (or pressuretrol) never be reached?

So, the system in my house is oversized so the cutout is triggered -- ONLY on setback recovery -- not on steady T-stat setting. Doesn't that mean it's not too oversized?

Finally, I'm really curious when the cutout pressure is reached (12 oz. in my situation), about how long is the expectation that the cut-in pressure (4 oz.) be reached? In about how many minutes? In Dan's book I never found this.

I appreciate all responses since I'm trying to further my education here.

Jamie Hall

What you are seeing is reasonably normal behavior. Yes the boiler is perhaps a little oversize -- but not by enough to make me, at least worry about it.

On the rapid pressure drop -- that's perfectly normal. The steam is condensing in your radiators, and it does it quite rapidly. The principal system I care for, for instance, is set up with a cutout of 7 ounces and a cutin of 4 ounces. It also has a post purge and a prepurge with a short time delay in between. The vapourstat cuts back in before the post purge is finished -- perhaps 30 seconds -- never mind the time delay and the pre purge.

I would be rather surprised to see a reasonably sized system take more and 45 seconds to a minute from cutout to cutin.

Fred

Yes, if the boiler were perfectly sized, it would produce steam at the same rate that the piping and radiators could condense it and you'd never build enough pressure to trip the Pressuretrol or Vaporstat. The boiler would run until the thermostat was satisfied and the Pressuretrol/Vaporstat would only be a Safety device (as originally intended).
The boiler is over sized enough that when it runs for extended periods it starts to short cycle. In your case, that is currently on a modest set-back. That may change as the outside temps get colder and depending on how tight/insulated your house is, how well your steam pipes are insulated, how good the main venting is and what the heat loss actually is. The longer the boiler has to run, the more likely an over sized boiler will start to short cycle, be it from set backs or temp changes/colder weather.
I have my over-sized boiler with a Vaporstat set just like yours. I don't use setbacks so as to avoid short cycles. I typically run at about 2 ounces of pressure, on a normal heat cycle. If I decide to change the tstat up a couple degrees, I will get two or three short cycles before the tstat is satisfied, and the fact it is 0 outside.
When I hit a short cycle, (and I can run about 45 minutes before that might happen) it will shut down and drop to Cut-in in a minute or less. Run for maybe 4 minutes and then shut down on pressure again, for about a minute. How long it takes for the drop just depends on how fast the steam in the system is condensing. That is a variable with each system.

dgn

@Jamie Hall Am I correct in understanding that you're saying 45-60 seconds from cutout to cutin is fine? My concern was the fast off & on of the boiler. Once back on it could be 10 min. to next cutout. And, again, that's only on setback recovery, not temp maintenance.

Jamie Hall

dgn said:

@Jamie Hall Am I correct in understanding that you're saying 45-60 seconds from cutout to cutin is fine? My concern was the fast off & on of the boiler. Once back on it could be 10 min. to next cutout. And, again, that's only on setback recovery, not temp maintenance.

So long as everything in the burner has stopped spinning before it tries to start again, the burner doesn't "know" how long it's been. It's actually in some ways better -- as the boiler has never had a chance to cool down at all (the post/pre purges don't do that -- all they do is get rid of leftover combustion gas or odd vapours).

dgn

The down side I see to this is that the boiler is off for a minute, not burning fuel for a minute, so not much of a savings, even though the rads are hot and so don't need steam. I'd like to see the boiler off for a spell while the rads give up some of their heat.

@Fred - Yes, looks like we have similar situations.

Fred

@dgn , I use to use a 4 degree setback at night. I learned that setting the tstat and leaving it alone eliminated 95+% of my short cycles and I noticed no difference in my fuel (natural gas) costs, considering the time it took to bring the house back up to temp in the morning.

Jamie Hall

dgn said:

The down side I see to this is that the boiler is off for a minute, not burning fuel for a minute, so not much of a savings, even though the rads are hot and so don't need steam. I'd like to see the boiler off for a spell while the rads give up some of their heat.

@Fred - Yes, looks like we have similar situations.

You have, quite innocently, wandered into a can of worms. As I said to someone else recently, you will find quite a variety of comments on the problem of cycling with slightly oversize boilers. All of them are well intentioned. Some of them are well informed.

I've given you my view, developed hands-on over many years; now sit back and enjoy...

dgn

@Jamie Hall Wow, you're leaving me hanging with that tease! With all the reading I'm doing I'd like to do more than just open the can of worms. Haven't yet found comments directly related to this type of short cycling. Will continue to dig but you're welcome to point me in a direction. I appreciate you sharing your experience.

Yes, bottom line is that I have decent heat.

Jamie Hall

The fundamental debate is between those who believe that it is better to turn the boiler off after it shuts off on pressure and hold it off for a period of time -- such as you suggested -- to allow the residual heat in the radiators to do some useful work, and those who believe that it is better to simply let the thing cycle on pressure alone, as yours is doing (and, not wholly incidentally, the system in the main property I supervise).

Neither side in the debate has any hard data to support its position; getting such hard data would be exceedingly difficult, as there are entirely too many other variables, most of which can't be controlled.

Both sides tend to get a bit passionate about their positions, which tends to detract from the clarity of the debate.

My own feeling is that it does no harm to the equipment and, in fact, may be better for the boiler since it doesn't undergo temperature swings as often, to let the burner do the necessary modulation to match burner input to radiation output simply using the pressure control. This is partly based on the recognition that a BTU is a BTU, whether it is developed by modulating the burner on pressure or by a timer, and the same number of BTUs have to be dumped into the space in any event.

I also have a known bias (review some of my other posts!) for keeping things as simple as possible, and it is really hard to beat a single vapourstat as the modulating control for simplicity.

Ideally, of course, the burner input and the boiler efficiency would be such that the net output of the boiler to the radiation would exactly match the output of the radiation, and the output of the radiation would exactly match the heat loss of the space. Even more ideally, this would be done by modulating the burner input firing rate; some gas fired hot water boilers can, in fact, do this over a fairly wide range. One can get off on some pretty wild tangents regarding the potential loss of efficiency from continuous modulation, as those do, vs. pulse modulation, which is what we do with the vapourstat. As I said above, I know of no hard data on the pulse modulation side -- and precious little on the continuous modulation side.

It's all rather interesting and sometimes thought provoking.

dgn

@Jamie Hall Thanks so much for laying out the issues in this debate. Excellent explanation. Easy to see how things could degenerate into a flame war in this HH or other HVAC forums. Now I can understand how it's better, at times, not to open the can of worms.

EBEBRATT-Ed

I won't get into this one other than to add some thoughts on night setback.

A lot of people look a a boiler or furnace as an "air heater" but it's a lot more than that.

When you set the temperature back yes the air in the house is cooler but so are the walls, the rugs, the chairs, the floors, ceilings etc etc. All those items loose heat when the temperature is set back and.........all those items must be heated back up when the system comes out of night setback. So depending on how well the home is insulated and sealed against drafts and the mass of it's structure and contents determines if there is any energy savings