Boiler Short Cycling on Pressure

Diar

Cool. I'll certainly have it downfired. Would you start with 10% or would you downfire a little more, given that its an 80 series?

Thanks,
Diar

Jim_64

Hi,

My new one-pipe steam system is short cycling many times on pressure after filling all of the radiators with steam. Typically, the problem is worse when the t-stat changes from 65 to 70 in the mornings / evenings.

FYI - After installing a 16oz max vaporstat, I had to completely open up all of the Vent-Rite #1's on my radiators , in order to fill the radiators before the vaporstat cuts out at 16 oz and cuts in at 4oz. If I set the Vent-Rite #1s on some radiators to less than the max vent rate, then the boiler will cut out on pressure before the radiators gets filled completely.

The boiler is slightly overfired ( 1250 sq ft instead of 1100 sq ft ), but I have large and lengthy piping. In addition, I replaced the main vents with multiple Gorton #2s to match the sizes of my mains.

The current operation is about 20-30 minutes to fill the radiators and then 10-15 cycles of 1 min off / 2 min on.

With respect to the short cycling, I have the following questions:
1. Is the short cycling the boiler so many times bad? Will it effect the lifetime of my boiler setup / efficiency of the system?
2. Should I try to raise the pressure to prevent the short cycling? This would involve replacing the 16oz vaporstat with one with a larger range ( 0-4 psi ).

FYI - My previous boiler had a much higher pressure setting and didn't short cycle on pressure at all, so the house heated in a single run. However, the previous system was pretty inefficient / a very large boiler (2500 sq ft) that was significantly underfired at 1050 sq ft. And lastly, I have a fairly large uninsulated house in the Boston area.

Any help would be greatly appreciated.

Thanks,
Diar

jim_110

cycles

Diar Questions -Are your mains insulated -how large are they and how many #2 vents per main. Are your EDR and boiler numbers correct.Do you have the old pressuretrol.Do not raise the pressure yet, in some cases the small cycles are acceptable, but in a new system I would say no. Either the your boiler is too large or your system needs balancing.

Jim_64

>> Are your mains insulated?

Mostly, except the fittings.

>> how large are they and how many #2 vents per main.

100' 3" main with 2 Gorton #2s
75' 3" main with 2 Gorton #2s
45' 2" main with 1 Gorton #2s

>> Are your EDR and boiler numbers correct.

Yes.

>> Do you have the old pressuretrol.

Nope, it got removed when I added the vaporstat.

>>Do not raise the pressure yet, in some cases the small
>>cycles are acceptable, but in a new system I would say no. >>Either the your boiler is too large

Its a little overfired at 1250 sq ft instead of 1100 sq ft. Do you think it would make that much of a difference if I downfired it to 1100 sq ft?

>> or your system needs balancing.

How would I go about balancing the system? Do you think that I'm losing pressure too fast? Or just not heating the right rooms fast enough? Or are you talking about something else like steam velocity, etc?

Thanks for your help,
Diar

jim_119

Diar I hope the boiler was installed per specs eg. near boiler piping/ header and flushing. If so having a "Pro" adjust the fire down 10% to match your EDR is a start.If cycles smooth out but not optimum, I would use a pressuretrol ( less$$$) to raise the cutout to 1 1/2 lbs. Your system may respond and enjoy the higher range. If your system is full of steam after 20 mins ,given it's size,it appears you have enough main vents.

mel rowe

Diar, sounds like your new boiler was being throttled down by the radiator vents, before you opened them up. Now, from what you said, it sounds like all rads are heating up within the 20-30 min. time, but then the boiler continues to cycle on the vaporstat. I think that most of the vents have shut off and the stat is still calling for more heat. The key question here would be, has the set temp. on the stat been achieved? If it has, then why does the boiler continue to cycle? This could depend on the type of thermostat and how it is set up. If the desired temp. has not been achieved, what room is the stat in and what rads affect it the most? In this scenario, you might need to do some balancing of rad. vents, so that they are shutting off at about the same time and then you wouldn't be getting so many cycles at the end. I've been through some of this myself and my cycling problem was due to the heat anticipator setting on my old Honeywell stat was set too high and keeping the boiler on too long after the set temp. was achieved. I also had to do some balancing of vents. Now I get about one cycle at the end and then it shuts down. Just some thoughts, FWIW.

Diar

The near boiler piping is to the boiler specs for a Weil-Mclain 480 ( 4" header, 28" above the water line ) and was skimmed once a few weeks after installation, but the water is still a little cruddy.

I'll see if I can get the burner downfired as you suggested.

The only thing I'm confused about is why many Pros on this web site contend that you should be able to heat in ounces ( 10oz or less ), when my vents have to be fully opened to fill before my Vaporstat cuts out at 16 oz. Is there something fundamentally wrong with my system OR is it typical to need higher pressures in a larger system like mine?

Thanks,
Diar

Steamhead (in transit)

Diar, if all the rads are full of steam

before it starts to short-cycle, the next step is to down-fire it. Have a pro do this. The experience we've had with the 80 series tell us they appear to be rated rather conservatively, especially on Vapor systems.

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Steamhead (in transit)

I'd start at 10%

but for proper combustion, 20% is the lowest I'd go. Any changes to the burner MUST be properly done using a digital combustion analyzer, making sure the positive draft over the fire is maintained. Also, avoid getting the stack temp so low that the flue gases condense in the chimney.

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Diar

Sounds good, but your comment about condensing flue gases reminds me that I just had my chimney lined this summer and they didn't put insulation between the new 10" liner and the clay liner of my old chimney. Would this effect the performance of the boiler significantly / my ability to downfire the boiler?

In addition, I have a forced draft flue, which is a closed system. Does a combustion analyzer require this to be openned up or can the analysis be done from someplace else on the boiler?

Thanks and sorry for all the questions. I just want to make sure that the burner is adjusted safely, since I had it adjusted last year and I guess luckily, the smell test was enough for me to know that it was adjusted incorrectly.

Diar

Diar

The cycling problem occurs mainly when I transition from 65 to 70 in mornings and evenings, since simply having the radiators full doesn't raise the temp in my house too quickly.

With that said, I am disappointed in my thermostat ( a Honeywell CT3500 without the ability to control or know the anticipator setting ), which as far as I'm concerned lies about the current temp. I'm going to replace it soon.

I'll try to balance out the radiators, but I really like having heat get to my bedrooms at the same speed as my living room downstairs where the stat is located.

Thanks,
Diar

mel rowe

Diar, I think the pros on this site would tell you that the cycling you are getting as the system comes off the nighttime setback is to be expected. The rads are filling and the vents shutting off long before the temp. setting on the stat is satisfied. At that point the boiler continues to steam, the rads are taking in very little steam, and the pressure builds up until the Vaporstat cutout setting is reached. Then the cycling starts. If you have noticed, there have been many discussions both for and against the use of setback thermostats with steam boilers.

Other than the times when you are coming off a setback, as long as the rads are getting hot and the vents shutting off at about the same time, a couple of cycles at the end of the normal heat cycle has been described as not a great concern. But,if you are still getting a lot of cycling in this condition, there are several alternatives that come to mind. One is to make sure that the room with the stat is not the coldest room, in order to slightly shorten the overall cycle time, although this might not give you the temperature you like in the upstairs area. Another possibility would be to have a thermostat that would give you the ability to better control the length of time the boiler is running. With a thermostat like this you could adjust the overall cycle time, until the boiler cuts off when you want it -- before the excessive cycling starts. And of course other posters have also mentioned downfiring the boiler as a possible help. Cheers.

Jim_64

Ok. I've got more accurate timings from this morning. It takes 20 minutes to fill the radiators in my bedroom, but it takes about an hour to fill the system with steam. I would assume that this means that I need to balance the system, so all the radiators get filled at the same time.

Since I believe I have the right main venting ( I got 5 gorton #2s on my system currently as specified on this site ), maybe if I turn down the venting rate on the radiators ( all are set to 8/full on a vent-rite #1 ), especially the rads close to the beginning of the mains, then the steam distribution might become more even.

Hopefully, this might also reduce the run time.

FYI - I'll be sure to read up on the other posts on setbacks .

Thanks,
Diar

Jim_64

Hi Steamhead,

If you read my latest post:

http://forums.invision.net/Thread.cfm?CFApp=2&Thread_ID=43996&mc=13#Message302232

I screwed up on the amount of time it takes to fill the system after the nighttime setback. It actually takes 60 minutes and then the cycling starts for another 30 minutes.

Do you think I should still downfire the system OR should I proceed with balancing / changing my setback / ... ?

Thanks,
Diar

chuckNJ

setback and preassure cycling

another HO here. For any given set of all the other parameters, deeper setback is going to produce a longer period of preassure cycling during recovery. Shallower setback will have a shorter recovery time and thus less time to cycle after the system is full of steam. Setback is one of a couple variables in the equation of what constitutes "optimum" operation.

It seems to be a judgement trade off to determine where a system is "best" once you are between the point of clearly overfired (building preassure before the entire system is full of steam) and clearly underfired (not able to fill the system with steam). Maybe we will get some more Pro comments on this.

Within this range, more fire means faster pickup (more "responsive") and also means you will reach the point of preassure cycling faster and your preassure cycles will be shorter for any given preassure differential. Less fire, the opposite. I suspect that the combusion cycling issues that affect this are different for oil and atmosperic gas burners (pros ?), but in both cases more cycles is certainly more wear and tear on the burner control mechanisms. More preassure (necessary for a larger differential and thus longer cycles) costs incrementally more fuel to raise the boiling point....

I have a Honeywell Vision 8000 stat and as others have said here, it seems to work quite well with steam. I'm running at 2 cycles per hour. (note that changing the cph setting appears to reset the "adaptive recovery" learning process, which does take many days to optimize to a relatively stable behavior).

Hope this is useful, and we get some more Pro observations on these trade offs.

ChuckNJ

mel rowe

Diar, forty minutes between the bedroom rads and the rest of your rads filling with steam and shutting down is way too long. You are on the right track if you work on adjusting the vents and getting them all to fill and shut down at about the same time. This might take some effort, but will be well worth it. Afterward, let us know if you still have excessive cycling at the end of a normal boiler cycle. Chuck, I've been interested in how a stat such as the Vision 8000 works. I don't understand how you can set a temperature, yet also impose a number of cycles per hour. Does the system reach set temperature on the first cycle? It is just confusing to me. Would appreciate any clarity you can provide. Regards.

chuckNJ

Mel

Hi Mel, I know less than I would like to. The stat manual is not very helpful and I got no input from Honeywell when I posted similar questions here several months ago.(probably the lawyers..) So what I know is just what I have actually observed myself. The vision 8000 stat cph setting does not affect or modulate the run time, at least on the settings of 1 and 2 cph which are the only ones I have tried. The setting simply determines how many heat calls per hour the stat is allowed to make. I found the 1 cph setting caused me perceptable comfort variation as the space cooled before the hour was up. 2 cph seems to level my temp control out nicely so I have not gone higher. I suspect, but can not prove without much more contolled testing, that the temp differential of the stat is also reduced as the number of cph is increased. Raising the cph setting too high on a steam system will tend to create short cycles that may not affect the space evenly if your room to room balance is not perfect, may occur unnecessarily due to small temp changes around the stat and there is some also some small overhead on heating both the boiler and the load back up for each such cycle. Hope that is useful.

mel rowe

Thanks Chuck. That Honeywell stat is very interesting. I was just wondering during a given hour after the boiler comes on, how long does it stay on and then how long is it off before coming on again. Also, when it steams, does it have time for the rads all to get hot all the way across, or only partially. I am assuming that it is controlling the temp. between the set point and a small differential temperature that is affected by the cooling rate of your house, as opposed to the way my older Honeywell works, with a heat anticipator setting. My boiler comes on at about the set temp. and by the time it shuts off all the vents on all rads, the temp. probably increases by about 3/4 degree. This takes about 30 min.. Then the system is usually off for 60-75 min. depending on outside temp.

Diar

still short cycling after downfiring

Over the winter, I've tried to balance my rads, but when I turn down the venting rate, the boiler started to short cycle faster, before all of the rads are filled.

I eventually opened up my attic rads, which are not in a living space, and this cut the short cycles down to 1-2 per cycle. This also allowed me to balance the system better, but I was heating the attic of course.

I just had the boiler ( WM 480 ) downfired from the factory setting of 1250 sq ft steam to 1100 sq ft to match my connected load ( or the approx load given a 1.33 pickup factor ).

I was hoping that this would solve the short cycling, but nope. I shut off the attic rads and the system started short cycling like crazy again. In addition, when I tried to balance the rads, it started short cycling faster, just like before.

Should this be attributed to my vaporstat, which only gives me a max of 16 oz?

Is it possible that either non-ideal placement ( its above the water line, but not by much ) of the vaporstat is the issue?

Would I just be better off going back to a pressuretol/vaporstat that gives me more headroom / up to 4+ lbs?

mel rowe

Diar, is the "short cycling" you are talking about occurring on a regular boiler cycle, or just on the first cycle after coming off the nightly setback?

Diar

Its short cycles every time, but the severity is less when not coming off nightly setback. I changed my setback from 5 to 3 degrees in the fall ( 67 at night instead of 65, with 70 for wakeup ), with an improvement. To give you an idea, the boiler short cycles for an hour ( after a 30-40 min warmup ) after setback.

The basic issue is that the radiators are not filling completely when the boiler cuts out on pressure. I've tried to balance the vents on the radiators and I had 5 gorton #2s added to the mains ( 2 on a 100ft 2.5" main, 2 on a 75ft 2.5" main and 1 on a 45ft 2" main. ), but the only way I could get all of the radiators to fill was by opening the adjustable radiator vents to max. The issue with opening all of the vents to max is that it leads to the upstairs getting real hot before the thermostat is satisfied, so I'm getting hot, but its still short cycling.

To be more specific about the way I am trying to balance the system, if set some of my adjustable vents to 4 which others are on 8, the boiler goes off before some of the ones on 4 as well as 8 as full.

In general, I guess I don't even know if the radiator vents ( choice between a vent-rite #1 and vent-rite #11 ) effect the ability for a boiler to operate on a vaporstat / at low pressure.

Any help on this issue would be greatly appreciated.

Diar

Here are more specifics from my last run that wasn't on setback.
1. Took 45 minutes to shut off on pressure. Most of the rads were hot, but not full / vents hadn't shut yet.
2. Shut off on pressure on each cycle, but most of the vents on the rads didn't shut off, but still good heat on most rads.
3. Cycled four times for 15 minutes until the thermostat shut off.

nj

Short cycling

> Here are more specifics from my last run that

> wasn't on setback. 1. Took 45 minutes to shut

> off on pressure. Most of the rads were hot, but

> not full / vents hadn't shut yet. 2. Shut off

> on pressure on each cycle, but most of the vents

> on the rads didn't shut off, but still good heat

> on most rads. 3. Cycled four times for 15

> minutes until the thermostat shut off.



I have had a similar but not identical experience with "short cycling". I was on this site several years ago. One gentlemen, a heating conractor from NJ, had me check the anticipator setting on the digital thermostat. It allows you basically to set the sensitivity of the temperature swing. After some careful trial and error and calculation, believe it or not, I determined that setting back the thermostat at all was not worth it. If you leave the thermostat at 70 all time, and set the anticipator cycle accordingly, you can cycle the unit to go on, fill the system with heat, turn off, and the radiator will remain warm enough with residual heat to keep the house very warm. When it is super hot outside, the unit will end up cycling on pressure but the heat loss in the mains seems to be greater in this scenario and if you downfire the unit so that it doesn't excessively cycle y ou will get a good result then too. Believe it or not, I calcualted that keeping the temperature the same only increased my heating bill about 7% and kept the house much much warmer and more comfortable. I don't think setting back the thermostat is worth it especially if your house is drafty, which is usulally the case with steam heated homes (due to their age). these systems were designed to run with the windows open!

nj

Make sure your thermostat is has a steam setting. Many honewywell thermostats do not but some do.
I used the White-Rodgers thermostat. It has a digitial anticipator setting between 5 and 40. Set at 5 causes riduclous short cycling. Set at 40 is a temperature swing of .8 degrees --a little too much. try setting 30. This thermostat costs about $100.

mel rowe

Diar, I'm just a HO, not a Pro, but have some limited experience that I'm very willing to try and relate to your problem. I am a bit comfused, so let's start from the beginning. What is the calculated EDR for your system? On a normal cycle (not after a setback), how long is the total cycle from burner on to burner on again? In other words, how long does it fire before the stat shuts it off, and then how long is it off before the stat turns it on again? And I realize this depends somewhat on how cold the weather is, but just give me an estimate. Next, what is the boiler's gas flow (in cubic feet per minute) when operating, and what is your average daily gas usage during cold months? This will tell us what percentage of the time the boiler is operating and give us a feel of its relative capacity versus requirements. I have more questsions, but let's just start with these.