Short Cycling - How to Diagnose and Fix?

davelasers

I'm new to home ownership and definitely steam, but learning quickly. Just finished "The Lost Art of Steam Heating Revisited" after "We Got Steam Heat!". I think I enjoyed them too much!!

This first winter my heat system started with water hammering and one baseboard convector not working. Heat is working great now and fairly quiet but my boiler is short-cycling after the system heats up and and I'm unsure how to proceed. With the current setup, once the convectors are warm, the boiler will turn-on for 2 minutes and then off for 1 minute, until the Thermostat is satisfied.

What I've done so far:

• I replaced all existing air vents (Vent-Rite #1 and main - Vent-Rite #75) with Hoffman #40 and main vent with Hoffman #45. They no longer leak and drip and all convectors are heating up fully.
  
• My plumber put a tee and nozzle on the safety release port so that I can skim the system. This made the water hammering go away. I have probably repeated this about ten times but not seeing any additional changes and the water is pretty clear, although I see rust particles in the glass when the boiler is running. Should I continue to skim?
  
• I have installed a Nest Thermostat, initially with 2-wires. I now have a common wire added as well and last night enabled the "True Radiant" setting as well. This replaced an old Honeywell mercury/bimetal Thermostat.
  
• I have replaced the 30 PSI gauge, but this one seems worse than before. I just added a brass pigtail to this section and put a tee for the 30 PSI gauge, but also added a 5 PSI gauge, which seems far more accurate.
  
• I have used the 5 PSI gauge to calibrate the Pressuretrol to cut-off ~2.0 PSI and cut-in ~0.5 PSI.
  

What I know:

• It's 1-pipe steam that was originally oil (built in 1950).
  
• There are 11 in-wall convectors of various sizes on the first (and only) floor.
  
• The boiler is a natural gas Burnham installed in 1991 (~28 years old). It is rated 108,000 BTU gross.
  
• The "A" dimension is approximately 28 inches (between the water line and bottom of return pipe).
  
• All the convectors are sloped correctly and the air vents are elevated above the convectors.
  

What should I do next? Advice and suggestions are greatly appreciated.

nicholas bonham-carter

Definitely get some larger main venting in place, like the big mouth. They have no floats, so if your supply piping is bad, and you are blowing water up into the mains, you may need to use Gorton 2’s instead.
You can determine how well they will react by running the boiler for a few minutes without the main vent in place. Be ready to hit the switch when steam first comes out the hole. If you see evidence of a lot of water spraying out, then you will need the Gortons for sure. Keep the pressure down below 1.5 psi, (for basic functionality), or 5 ounces , (for comfort, and economy).
A good low pressure 0-3 psi gauge, graduated in ounces.—NBC

BobC

Your main vent is probably too small, how long is the main and what size pipe?

The boiler might be able to be downfired to lessen the short cycling, what model is it?

Do you know what your total EDR is?

Bob

davelasers

BobC said:

Your main vent is probably too small, how long is the main and what size pipe?

The boiler might be able to be downfired to lessen the short cycling, what model is it?

Do you know what your total EDR is?

Bob

Hi Bob, Thank you for the quick reply.

The main pipe is about 80-90 feet in total. The pipe is 2-1/2 inches.

The boiler model is IN6.

I don't know what the total EDR is. The convectors are Tuttle and Bailey with 3 pipes and lots of fins. Attached is a picture of a 21"
one.

I have 11 Tuttle and Bailey convectors just like this with different lengths

• 17" - 1X
  
• 21" - 3X
  
• 28" - 1X
  
• 32" - 4X
  
• 48" - 2X
  

Is it possible to estimated the EDR from this??

Mark N

It seems like the short cycling starts after the convectors are hot. Not sure how to set cycle rate on a Nest. With a Honeywell thermostat the usual setting is 1 cph for steam. Convectors are lower mass than radiators and heat up quicker. With a thermostat that you can set the cph you could try 2cph or even 3cph. This would most likely end the short cycling when the convectors are completely hot. More main venting is always good to a point. Need to know the length of the main and what size pipe.

davelasers

nicholas bonham-carter said:

Definitely get some larger main venting in place, like the big mouth. They have no floats, so if your supply piping is bad, and you are blowing water up into the mains, you may need to use Gorton 2’s instead.
You can determine how well they will react by running the boiler for a few minutes without the main vent in place. Be ready to hit the switch when steam first comes out the hole. If you see evidence of a lot of water spraying out, then you will need the Gortons for sure. Keep the pressure down below 1.5 psi, (for basic functionality), or 5 ounces , (for comfort, and economy).
A good low pressure 0-3 psi gauge, graduated in ounces.—NBC

I didn't see much water (just steam condensing I think), but I would feel more comfortable with the Gorton #2, which I will order along with a 0-3 psi gauge. I will also reduce the pressure once I get to the bottom of these issues. Thank you for the quick response NBC.

davelasers

Mark N said:

It seems like the short cycling starts after the convectors are hot. Not sure how to set cycle rate on a Nest. With a Honeywell thermostat the usual setting is 1 cph for steam. Convectors are lower mass than radiators and heat up quicker. With a thermostat that you can set the cph you could try 2cph or even 3cph. This would most likely end the short cycling when the convectors are completely hot. More main venting is always good to a point. Need to know the length of the main and what size pipe.

Correct, shortly after the main valve closes. We really like the Nest even though it is not possible to set the CPH.

Would I expect everything to work well if I had a different Thermostat? If the temperature isn't reached, how would the Thermostat know to only cycle 1 or 2 times per hour? I was under the impression that Thermostats are just "on" until they've reached their set point.

In my case, the pressure shuts of the system well before the general temperature rises. I don't understand how the Thermostat would be able to overcome the short-cycling unless it had a signal back that the boiler was shut-off. Can you describe how this would work with a Honeywell Thermostat?

Finally, you were quick to respond, thank you! Please see my previous reply in regards to my main run and pipe size.

Jamie Hall

Yes, do use the Gortons. I love the Big Mouth vents -- but if there is any concern about water...

On cycling after the convectors are hot. That indicates that the boiler is oversized -- or over fired. If you have a really competent tech., you could investigate lowering the firing rate of the boiler -- but you still may get some cycling after all the convectors are hot. That's normal. It's very hard to get just exactly the right firing rate, especially as it changes with room conditions...

On the Nest. Do NOT allow it to set the temperature back more than 3 degrees, and preferably only once in a day. Particularly disable any occupied/unoccupied room temperature functions. All that fainciness is incompatible with anything except forced air heat.

There might be a cycles per hour setting on it, and if so you should try two, as has been suggested, since convectors heat and cool much faster than cast iron radiators. Otherwise, you could use a Honeywell VisionPro, which is programmable (but remember -- no setback more than three degrees!) -- or put the old Honeywell you took out back on. It works better than any of the new fancy ones, unless you need programming.

BobC

I'm sure someone will tell us what the EDR ratings of those convectors is.

90 ft of 2-1/2 pipe is about 4.7cuft and the Gorton #2 is rated at 1.1 cuft. That may not be enough but will be a lot better than what you have now.

That IN6 can be downfired some, where are you located? You could make it a 2 stage firing setup but that costs a fair amount of money. Best to have a competent tech look at it.

Bob

davelasers

Jamie Hall said:

On cycling after the convectors are hot. That indicates that the boiler is oversized -- or over fired. If you have a really competent tech., you could investigate lowering the firing rate of the boiler -- but you still may get some cycling after all the convectors are hot. That's normal. It's very hard to get just exactly the right firing rate, especially as it changes with room conditions...

This is my thought and concern. I am trying to schedule a visit with a competent tech, but given the cold season and time of year it has been hard to get them on-site.

Jamie Hall said:

On the Nest. Do NOT allow it to set the temperature back more than 3 degrees, and preferably only once in a day. Particularly disable any occupied/unoccupied room temperature functions. All that fainciness is incompatible with anything except forced air heat.

No cycles per hour. I enabled the "True Radiant" mode which should limit the number of cycles. But, do I understand you correctly? I shouldn't let the Nest reduce the temperature more than 3 degrees while away? Why not? I had hoped this would save money since there's 8-10 hours a day when heat is not needed.

davelasers

BobC said:

I'm sure someone will tell us what the EDR ratings of those convectors is.

90 ft of 2-1/2 pipe is about 4.7cuft and the Gorton #2 is rated at 1.1 cuft. That may not be enough but will be a lot better than what you have now.

That IN6 can be downfired some, where are you located? You could make it a 2 stage firing setup but that costs a fair amount of money. Best to have a competent tech look at it.

Bob

I hope that the Gorton #2 would be a big improvement. There was a Vent-rite #75 in the main vent when I moved in and I purchased the Hoffman #45 because Vent-rite listed that as an equivalent replacement. Looks like the Gorton is > 10 times more powerful so I think it should make a significant difference in regards to main venting.

I'm near Boston, MA

Leon82

It will have to run longer to recover from say 6 degree setback

If there is a short cycle problem it will continue cycle until the tstat is satisfied. Which will be a longer heating time.

nicholas bonham-carter

The theory behind the pros and cons of setbacks is related to the fuel use for raising the temperature of the house after the setback. A lot of mass is being heated, and this uses a lot of fuel.
A lower constant temperature (65 degrees) can be more comfortable than a series of highs (70 degrees), and lows.
Naturally if you are away for a few days, there could be some savings in setting down the temperature.
Just as in driving on the highway, where a constant speed will give greater economy, a constant temperature is better.
You will certainly need more than 1 Gorton 2.
The big mouth is twice the capacity of the Gorton 2, at the same price. --NBC

Fred

As has been said, the boiler is most likely short cycling because it is over sized and those convectors can't condense the steam as rapidly as the boiler makes it. That will show up when all the convectors are full (hot all the way across). Putting big vents on those mains will help a lot, in that it will reduce the amount of time the boiler is running just to push air out of the system to deliver steam to those convectors. It sounds like your system is pretty well tuned. Put bigger vents on, minimize the use of set-backs and I'm sure you will see an improvements. I stopped using set backs a few years ago, for this reason. The boiler has to run for a long time to bring the house back up to temp and short cycling was a problem. I only see a short cycle once or twice on a cycle on days when the outside temp is down to or below design day. I have the Big Mouths on my mains also. They are an excellent Main vent and they were a big help in resolving my issue.

davelasers

Looks like I'm out of luck with regards to the main vent, I don't have the clearance to fit a Gorton #2 in my main vent by about an inch. Looks like my best bet is to get a Gorton #1 (or maybe a couple of these with a Stainless Steel tee).
Additionally, I could put Gorton #D on all the convectors. If I put 11 of these on the system, I should be able to vent 0.33*11 = 3.63 CFM. Would this be a good idea?

Alternatively, I thought I saw the that Gorton #2 vents out of the side? Could I install it with a right angle connection?

Before I invest more money and time, how can I determine if it's

1. My Thermostat? I understand the comments that Nest is not preferred for Steam, but it doesn't seem to be the reason for constant cycling.
   
2. Air Vents. Since the system seems to be heating and venting properly (albeit slowly), it seems like replacing the vents will only make the system hear and cycle quicker.
   
3. Oversized boiler. This seems to be a theme in Holohan's books. I guess maybe my boiler can be set up a 2-stage system or maybe lower the firing rate, but this will be expensive and/or marginal improvement.
   

I apologize for my naivety but I'm still confused. If the t-stat hasn't reached the set temp, it doesn't seem like that would be the cause of the short-cycling. In Holohan's books, he blames most short-cycling on improper air venting, but it's described as having no vents (since plugs are cheaper). Since my vents are closing and the convectors are hot, is it reasonable to assume that bigger vents would make a difference? Could air be stuck in the system and that's causing the pressure to rise? That only leaves oversized boiler, which makes sense to me. If the system is closed (with steam) and continues to generate steam faster than it is condensed, then pressure will rise.

The part that's confusing is the pressure drops close to the rate at which it rises. To me it seems the only difference in time is the time it takes for the exhaust flue to close and open..

davelasers

Looks like I'm out of luck with regards to the main vent, I don't have the clearance to fit a Gorton #2 in my main vent by about an inch. Looks like my best bet is to get a Gorton #1 (or maybe a couple of these with a Stainless Steel tee).
Additionally, I could put Gorton #D on all the convectors. If I put 12 of these on the system, I should be able to vent 0.33*11 = 3.63 CFM. Would this be a good idea?

Alternatively, I thought I saw the that Gorton #2 vents out of the side? Could I install it with a right angle connection?

Before I invest more money and time, how can I determine if it's

1. My Thermostat? I understand the comments that Nest is not preferred for Steam, but it doesn't seem to be the reason for constant cycling.
   
2. Air Vents. Since the system seems to be heating and venting properly (albeit slowly), it seems like replacing the vents will only make the system hear and cycle quicker.
   
3. Oversized boiler. This seems to be a theme in Holohan's books. I guess maybe my boiler can be set up a 2-stage system or maybe lower the firing rate, but this will be expensive and/or marginal improvement.
   

I apologize for my naivety but I'm still confused. If the t-stat hasn't reached the set temp, it doesn't seem like that would be the cause of the short-cycling. In Holohan's books, he blames most short-cycling on improper air venting, but it's described as having no vents (since plugs are cheaper). Since my vents are closing and the convectors are hot, is it reasonable to assume that bigger vents would make a difference? Could air be stuck in the system and that's causing the pressure to rise? That only leaves oversized boiler, which makes sense to me. If the system is closed (with steam) and continues to generate steam faster than it is condensed, then pressure will rise. The part that's confusing is the pressure drops close to the rate at which it rises.

Jamie Hall

OK. You're getting there. The first, and probably most important thing for you at this point is that there are two kinds of short cycling.

The first kind occurs early during a cycle, before all the radiation -- in you case convectors -- are hot. That is caused by inadequate venting. In your situation, I would suggest using BigMouths -- they are slightly shorter than Gortons -- and there is no reason why they can't be put on a T out to one side of the main, if there isn't enough head room. Additional venting on the convectors won't help, and may lead to more or less bad imbalance. Don't do it.

However, as I understand it, your short cycling occurs after the radiation is satisfied. That is caused by the boiler making more steam than the radiation can condense, and so you have to shut off the boiler to allow the radiation to catch up. That can only be fixed by making the boiler make steam less rapidly -- smaller boiler or down firing. However, that short cycling doesn't have much impact on efficiency, so it is one of the last things to worry about.

The pressure will drop at least as fast as it rises, as the steam condenses in the radiation. That's the nature of the system.

The Nest -- and any thermostat which creates a setback of more than a few degrees -- will waste more fuel than it saves in the process of bringing the temperature back up. It's not just a problem with the Nest (although for some of us, the Nest has other problems -- I, for one, am not really keen on Google knowing when I am home). There has been a lot of debate on the Wall about setbacks for daily use, but the general consensus seems to be more than a few degrees is wasteful. Note that this is not true for forced air, which is what the Nest and others like was designed for.

davelasers

Jamie Hall said:

OK. You're getting there. The first, and probably most important thing for you at this point is that there are two kinds of short cycling.

The first kind occurs early during a cycle, before all the radiation -- in you case convectors -- are hot. That is caused by inadequate venting. In your situation, I would suggest using BigMouths -- they are slightly shorter than Gortons -- and there is no reason why they can't be put on a T out to one side of the main, if there isn't enough head room. Additional venting on the convectors won't help, and may lead to more or less bad imbalance. Don't do it.

However, as I understand it, your short cycling occurs after the radiation is satisfied. That is caused by the boiler making more steam than the radiation can condense, and so you have to shut off the boiler to allow the radiation to catch up. That can only be fixed by making the boiler make steam less rapidly -- smaller boiler or down firing. However, that short cycling doesn't have much impact on efficiency, so it is one of the last things to worry about.

The pressure will drop at least as fast as it rises, as the steam condenses in the radiation. That's the nature of the system.

The Nest -- and any thermostat which creates a setback of more than a few degrees -- will waste more fuel than it saves in the process of bringing the temperature back up. It's not just a problem with the Nest (although for some of us, the Nest has other problems -- I, for one, am not really keen on Google knowing when I am home). There has been a lot of debate on the Wall about setbacks for daily use, but the general consensus seems to be more than a few degrees is wasteful. Note that this is not true for forced air, which is what the Nest and others like was designed for.

Got it! It makes sense that there's multiple types of short-cycling but I'm trying to tackle both. I would agree that in my case that the short-cycling is happening only after "radiation is satisfied".

Fred

@davelasers , The Big Mouth is only 4 and 3/4's tall. One Big Mouth is equal to two Gorton #2's. It is currently in stock at Amazon. You can also order the Barnes and Jones Vari-Vent from Andrew at Barnes and Jones. His number is 1-781-963-8000, Extension 206. It is even shorter and wide open is still, about 1.5 times a Gorton 2.

BobC

If you don't have good main venting you are paying the gas company to compress the air in your lines, you have to decide if you want to pay to have that air compressed or pay to heat yur space.

Gorton #1's are about 1/3 the capacity of a #2 and 1/6 that of the big mouth. Find away to make the bigmouth fit, you can mount the tailpiece first and then the vent so it's easier to mount than the G@ and it might be an inch shorter.

Bob

acwagner

@davelasers

You might consider adding a timer in series with the thermostat to control the boiler burns to prevent it from building too much pressure during recovery from setback. I use temperature setbacks for comfort, and my system would short cycle on pressure during the end of the recovery period (with several annoying side-effects like hissing vents). I set the timer to have the boiler run for 18 minutes, then idle for 12 minutes, and repeat this process until the thermostat is satisfied. I came up with the time intervals by timing my system and some trial and error. My goal was for the boiler to run long enough to satisfy a normal call to maintain temperature with one continuous burn. In most cases, the boiler shuts down before the thermostat stops the call for heat. But, during the idle period, my cast iron radiators are still giving off heat. Usually around 5 minutes into the idle period the thermostat is satisfied. During a temperature recovery, the idle time between burns is long enough to dissipate enough heat such that the each burn doesn't build enough pressure to start short cycling. Another benefit to this setup is it prevents wild temperatures swings after a normal call for heat.

I have a regular 7-day programmable thermostat. I'm not sure how a timer setup would work with a smart thermostat like yours. I'm guessing it wouldn't work. You may need to downgrade your thermostat if you want to pursue this setup.

Jon2018

Just came across this post not sure if the Tuttle & Bailey info is still needed but I have some info

gfrbrookline

I live in Brookline, I would suggest you have New England Steamworks check out your system. Ryan worked wonders on mine.

From my experience ditch the nest it does not understand steam. I tried it and went back to the honeywell vision pro it was much better. Since you have convectors set it to 2 or 3 cpr. I think the nest is a great thermostat but is intended for a wood framed building and forced hot air or hot water radiators. It works great in my summer house with hot water radiators.

Setbacks with convectors may be different then my cast iron radiators but if your building is brick like mine heating and cooling the mass of the building makes set back costly on the heating bill.

As for your main venting put 2 or 3 big mouths on and you should see a huge improvement in balance and cycling and run time on the boiler.