Short Cycle - Oversized Boiler?

Bus Tarbox

I have two heated floors in a single-family residence. I calculate the 8 radiators at total EDR 230. I use a 1.33 factor to get about Gross EDR of 310, which gives me a Gross MBH of 74. All piping, except near the boiler on the wet side are asbestos covered. The boiler is a Burnham American V34. I'd guess about 20 years old. Just bought the house, so don't know for sure. Boiler is rated at either 172 or 186 Gross MBH. Steam MBH of 129 / 139.5. When we moved in the pressuretrol was set at about cut-in 6 with a differential of 3. The inspector lowered it somewhat.
I've replaced the one main vent, and the eight radiator vents. The immediate result was that the two radiators that had no heat are now heating. I cleaned the boiler water, then added Squick. I replaced the pressure gage that didn't seem to be working. I dropped the pressuretrol down to 0.5 cut-in, and sequentially cut the differential from 3 to 2 to 1. The new pressure gage and the pressuretrol correspond nicely, so I'd guess the pressuretrol is doing its job.
[An aside, I think: - There is a control on the boiler that seems to keep the water at its setting of 140 degree if the burner has power, but the thermostat is not calling for heat. I have a gas hot water heater, so that control is not currently, at least, serving the purpose of heating watwer for the potable system.]

Jeez, I hope this isn't too long. Here is the problem:
The house heats fine, but the cycle between pressuretrol 0.5 cut-in to 1.5 cut-out is 3 minutes of burner on, to 2 minutes of boiler off. If the burner has been off for several hours because of a setback thermostat, it takes about 30 minutes to get to cut-in pressure (I assume coming up from 140 degrees. The sight glass is not steady as a rock, but fluctuations are only around a half inch up and down from midpoint. There is no noticeable water loss over time. I have done nothing at the burner level - e.g. same nozzle as when we moved in.

From the specs and symptoms, does it appear I have an oversized boiler? Is the short cycle destroying my burner? If so, and a different size nozzle doesn't significantly help, must I go to higher pressure and/or differential? New furnace? (I've read THE book several times.)
One last thing. At the end of the heating cycle, when the thermostat is no longer calling for heat, a couple of my radiator vents sound like tea kettles whistling enough to wake us. Is that air being sucked back into the vents? Any solutions to that?
Thank you,
Mat Tarbox, New London CT.

chris smith_2

over sized

bus,
with the 140f aquastat i would guess your boiler was sized to have a tankless coil lots of techs allow 40,000btu's for this, hire a good contractor he may down fire and change or remove your aqua stat

chris smith

paradise porter maine

Steamhead

30 minutes from 140 to pressure cutout

isn't that bad. But Chris is right, get a good oil tech in there to tell you for sure. If he concludes he can down-fire the burner slightly, you will save some oil.

The V-34 is a nice boiler. Here's a shot of one I worked on recently.

To Learn More About This Contractor, Click Here to Visit Their Ad in "Find A Contractor"

Steamhead

30 minutes from 140 to pressure cutout

isn't that bad. But Chris is right, get a good oil tech in there to tell you for sure. If he concludes he can down-fire the burner slightly, you will save some oil.

The V-34 is a nice boiler. Here's a shot of one I worked on recently. This one's on a Hoffman Vapor system with later-style Differential Loop.

To Learn More About This Contractor, Click Here to Visit Their Ad in "Find A Contractor"

Steamhead

30 minutes from 140 to pressure cutout

isn't that bad. But Chris is right, get a good oil tech in there to tell you for sure. If he concludes he can down-fire the burner slightly, you will save some oil.

The V-34 is a nice boiler. Here's a shot of one I worked on recently. This one's on a Hoffman Vapor system with later-style Differential Loop.

To Learn More About This Contractor, Click Here to Visit Their Ad in "Find A Contractor"

Steamhead

30 minutes from 140 to pressure cutout

isn't that bad. But Chris is right, get a good oil tech in there to tell you for sure. If he concludes he can down-fire the burner slightly, you will save some oil.

The V-34 is a nice boiler. Here's a shot of one I worked on recently. This one's on a Hoffman Vapor system with later-style Differential Loop.

To Learn More About This Contractor, Click Here to Visit Their Ad in "Find A Contractor"

Steamhead

30 minutes from 140 to pressure cutout

isn't that bad. But Chris is right, get a good oil tech in there to tell you for sure. If he concludes he can down-fire the burner slightly, you will save some oil.

The V-34 is a nice boiler. Here's a shot of one I worked on recently. This one's on a Hoffman Vapor system with later-style Differential Loop.

To Learn More About This Contractor, Click Here to Visit Their Ad in "Find A Contractor"

Steamhead

30 minutes from 140 to pressure cutout

isn't that bad. But Chris is right, get a good oil tech in there to tell you for sure. If he concludes he can down-fire the burner slightly, you will save some oil.

The V-34 is a nice boiler. Here's a shot of one I worked on recently. This one's on a Hoffman Vapor system with later-style Differential Loop.

To Learn More About This Contractor, Click Here to Visit Their Ad in "Find A Contractor"

Steamhead

30 minutes from 140 degrees to 1-1/2 pounds

isn't that bad- but Chris is right, get a good oil tech to look at it. If he concludes that it can be down-fired slightly, you'll save some oil.

The V-34 is a nice boiler. Here's a shot of one I worked on recently. This one is on a Hoffman Controlled-Heat Vapor System, with later-style Differential Loop.

The whistling vents on shutdown are caused by condensing steam forming a vacuum, which pulls air in.

To Learn More About This Contractor, Click Here to Visit Their Ad in "Find A Contractor"

Cosmo

Have you tried this?

Simpy checking to make sure the heat anticipator setting is correct on your thermostat? I you are using a T-87 from Honeywell, simply check amp draw of R and W at the thermostat location using a meter. Some electronic thermostats use a selection switch for gas heat/ etc. Just a thought...



Casmo
Dependable P.H.C. Inc.

J.C.A.

Bus,

1 more suggestion. If the burner doesn't already have one , get a oil tech to put on a pre-purge AND post-purge primary control with an oil valve .

Almost all the Unburned products of combustion(I hate the word soot) occur on the start and stop on a properly tuned burner . This will provide a positive draft and correct motor speed to deliver the oil,and a positive(electrical) shutoff of the oil after the cycle .This is my opoinion , but I'm sure you'll not find too many folks here that don't agree . Chris

Bus Tarbox

Clearing some clutter

Hoping this is the right way to add to my post about Short Cycling:
First, I'm impressed with the suggestions and explanations so soon after my original post. Amazing forum. But I may have cluttered up my original post with too much info.

Here is the crux:
The thermostat calls for heat. I watch the pressure build on the gage at the boiler. It goes from .5 to 1.5 in 3 minutes, then cuts out as expected because of the pressuretrol setting. In 2 minutes the pressure drops to .5 again and the burner kicks back in, etc. If instead I uses a differential of 2, the cycle is almost the same, - something like 3.5 minutes on, 2.5 minutes off. What should be the cycle time for a well-functioning system?
Thanks again,
Bus

Ron Beck

Oversized boiler

You mentioned you have 230 sq ft of connected load. The V34 depending on the firing rate is rated at 538 or 581 sq ft of steam. The boiler is over twice the size required. You do not add anything to the connected load calc if mains are insulated as we already added 33% for piping loss. With the boiler this much oversized there is not to much you can do. You cannot reduce the firing rate far enough to properly fire the system due to producing a gross amount of flue gas condensation.

Bus Tarbox

Twice the size - now what?

I've been thinking about the responses and I guess here is where I am in my thinking.

As Chris Smith said, the aquistat is an indication that when the boiler was originally sized, it included the hot water load. As Ron Beck says, there isn't too much I can do with the oversize. I guess I've done all the things - venting, cleaning, setting pressuretrol, etc. - that should be done. JCA's response means (I think) that since it is oversized, I should consider pre- and post-purges to at least reduce the "Unburned products of combustion." I think that would help the efficiency in several ways - all having to do with reducing "soot."

I think my limited options are to find a good oil tech here in Southeastern CT to reduce the firing rate if possible without producing a gross amount of flue gas condensation, and have the pre- and post-purging primary control thing considered. After having that done, I guess it is still best to keep the steam pressure at 0.5 to 1.5, since it is doing a good job of heating at that range.

Finally, I can't get over the fact that there are people out there who are real engineers AND artists, and that you take the time to help novices like me. Thanks to all of you that contributed. I hope I understood you correctly.
Bus

Noel Kelly

I think that you can downfire this enough to lengthen the cycle rate.
The specs say that the V34 is rated at 134/146 Net MBH. This translates into firing rates of 1.5/1.65 (they gave the option of two firing rates). Since you are obviously oversized the solution is to downfire. While most manufacturers recommend not going less than 15% or so I would suggest that you go down to a firing rate of 1 gph (33%). However, because of the size of the combustion chamber it might be a good idea to install a Quickie style combustion chamber - this will give you a more ideal combustion environment. It is my guess that with this lower firing rate you will still achieve a stack temp of at least 350 degrees gross which should be sufficient. If the boiler is vented into an inside chimney, all the better. This firing rate will bring you closer to your design and should result in longer cycles.
I would also be inclined to leave the aquastat in place as the amount of oil it uses during the off-season is more than compensated for by the long term benefits to the boiler. Also, be sure to check those coil gaskets for signs of seepage and corrosion.
Needless to say, this should all be done by a qualified technician.
Noel Kelly.

eleft(retired)

in your area

Bus,
this might help

http://Bonafidecomfort.com.... copy and paste

southeastern CT. contractor


al

David Efflandt

In same boat

I just calculated EDR of my 6 single-pipe radiators at 158 sq ft and that is probably overkill since some radiators are covered and could be reduced in size. Although, this lets them retain heat longer after boiler goes off. My 2 yr old Bryant BS2 gas boiler is rated for 283 sq ft net. Smallest one in that line rated 196 sq ft net should have still been plenty.

It is a 2-story 3-bedroom Dutch Colonial built 1910 in Elgin, IL that I moved into last summer. It has insulation blown into its barn style roof (which technically makes it 1.5 story) and plenty of thermal mass (drywall over plaster). So fortunately the boiler does not run very often. Pressuretrol is all the way down, but I have not timed its cycling.

David Efflandt

In same boat

I just calculated EDR of my 6 single-pipe radiators at 158 sq ft and that is probably overkill since some radiators are covered and could be reduced in size. Although, this lets them retain heat longer after boiler goes off. My 2 yr old Bryant BS2 gas boiler is rated for 283 sq ft net. Smallest one in that line rated 196 sq ft net should have still been plenty.

It is a 2-story 3-bedroom Dutch Colonial built 1910 in Elgin, IL that I moved into last summer. It has insulation blown into its barn style roof (which technically makes it 1.5 story) and plenty of thermal mass (drywall over plaster). So fortunately the boiler controlled by Honeywell CT3500 Smart Response thermostat maintains even temp without running very often and ramps up gradually from setback. Pressuretrol is all the way down, but I have not timed its cycling.

David Efflandt

In same boat

I just calculated EDR of my 6 single-pipe radiators at 158 sq ft and that is probably overkill since some radiators are covered and could be reduced in size. Although, this lets them retain heat longer after boiler goes off. My 2 yr old Bryant BS2 gas boiler is rated for 283 sq ft net. Smallest one in that line rated 196 sq ft net should have still been plenty.

It is a 2-story 3-bedroom Dutch Colonial built 1910 in Elgin, IL that I moved into last summer. It has insulation blown into its barn style roof (which technically makes it 1.5 story) and plenty of thermal mass (drywall over plaster). So the Honeywell CT3500 Smart Response thermostat maintains even temp without running the boiler very often and ramps up gradually from setback. Pressuretrol is all the way down, but I have not timed its cycling.

David Efflandt

In same boat

I just calculated EDR of my 6 single-pipe radiators at 158 sq ft and that is probably overkill since some radiators are covered and could be reduced in size. Although, this lets them retain heat longer after boiler goes off. My 2 yr old Bryant BS2 gas boiler is rated for 283 sq ft net. Smallest one in that line rated 196 sq ft net would have been more than enough.

It is a 2-story 3-bedroom Dutch Colonial built 1910 in Elgin, IL that I moved into last summer. It has insulation blown into its barn style roof (which technically makes it 1.5 story) and plenty of thermal mass (drywall over plaster). So the Honeywell CT3500 Smart Response thermostat maintains even temp without running the boiler very often and ramps up gradually from setback. Pressuretrol is all the way down, but I have not timed its cycling.

David Efflandt

In same boat

I just calculated EDR of my 6 single-pipe radiators at 158 sq ft and that is probably overkill since some radiators are covered and could be reduced in size. Although, this lets them provide heat longer after boiler goes off. My 2 yr old Bryant BS2 gas boiler is rated for 283 sq ft net. Smallest one in that line rated 196 sq ft net would have been more than enough.

It is a 2-story 3-bedroom Dutch Colonial built 1910 in Elgin, IL that I moved into last summer. It has insulation blown into its barn style roof (which technically makes it 1.5 story) and plenty of thermal mass (drywall over plaster). So the Honeywell CT3500 Smart Response thermostat maintains even temp without running the boiler very often and ramps up gradually from setback. Pressuretrol is all the way down, but I have not timed its cycling.

David Efflandt

In same boat

I just calculated EDR of my 6 single-pipe radiators at 158 sq ft and that is probably overkill since some radiators are covered and could be reduced in size. Although, this lets them provide heat longer after boiler goes off. My 2 yr old Bryant BS2 gas boiler is rated for 283 sq ft net. Smallest one in that line rated 196 sq ft net should have been more than enough.

It is a 2-story 3-bedroom Dutch Colonial built 1910 in Elgin, IL that I moved into last summer. It has insulation blown into its barn style roof (which technically makes it 1.5 story) and plenty of thermal mass (drywall over plaster). So the Honeywell CT3500 Smart Response thermostat maintains even temp without running the boiler very often and ramps up gradually from setback. Pressuretrol is all the way down, but I have not timed its cycling.

David Efflandt

In same boat

I just calculated EDR of my 6 single-pipe radiators at 158 sq ft and that is probably overkill since some radiators are covered and could be reduced in size. Although, this lets them provide heat longer after boiler goes off. My 2 yr old Bryant BS2 gas boiler is rated for 283 sq ft net. Smallest one in that line rated 196 sq ft net should have been more than enough.

It is a 2-story 3-bedroom Dutch Colonial built 1910 in Elgin, IL that I moved into last summer. It has insulation blown into its barn style roof (which technically makes it 1.5 story) and plenty of thermal mass (drywall over plaster). So the Honeywell CT3500 thermostat maintains even temp without running the boiler very often and Smart Response ramps it up gradually from setback. Pressuretrol is all the way down, but I have not timed its cycling.

David Efflandt

In same boat

I just calculated EDR of my 6 single-pipe radiators at 158 sq ft and that is probably overkill since some radiators are covered and could be reduced in size. Although, this lets them provide heat longer after boiler goes off. My 2 yr old Bryant BS2 gas boiler is rated for 283 sq ft net. Smallest one in that line rated 196 sq ft net should have been more than enough.

It is a 2-story 3-bedroom Dutch Colonial built 1910 in Elgin, IL that I moved into last summer. It has insulation blown into its barn style roof (which technically makes it 1.5 story) and plenty of thermal mass (drywall over plaster). So the Honeywell CT3500 thermostat (which has gravity/steam setting) maintains even temp without running the boiler very often and Smart Response ramps it up gradually from setback. Pressuretrol is all the way down, but I have not timed its cycling.

David Efflandt

In same boat

I just calculated EDR of my 6 single-pipe radiators at 158 sq ft and that is probably overkill since some radiators are covered and could be reduced in size. Although, this lets them provide heat longer after boiler goes off. My 2 yr old Bryant BS2 gas boiler is rated for 283 sq ft net. Smallest one in that line rated 196 sq ft net should have been more than enough.

It is a 2-story 3-bedroom Dutch Colonial built 1910 in Elgin, IL that I moved into last summer. It has insulation blown into its barn style roof (which technically makes it 1.5 story) and plenty of thermal mass (drywall over plaster). So the Honeywell CT3500 thermostat (which has gravity/steam setting) maintains even temp without running the boiler very often, and Smart Response ramps it up gradually from setback. Pressuretrol is all the way down, but I have not timed its cycling.

David Efflandt

In same boat

I just calculated EDR of my 6 single-pipe radiators at 158 sq ft and that is probably overkill since some radiators are covered and could be reduced in size. Although, this lets them provide heat longer after boiler goes off. My 2 yr old Bryant BS2 gas boiler is rated for 283 sq ft net. Smallest one in that line rated 196 sq ft net should have been more than enough.

It is a 2-story 3-bedroom Dutch Colonial built 1910 in Elgin, IL that I moved into last summer. It has insulation blown into its barn style roof (which technically makes it 1.5 story) and plenty of thermal mass (drywall over plaster). So the Honeywell CT3500 thermostat (which has gravity/steam setting) maintains even temp without running the boiler very often, and Smart Response ramps it up gradually from setback. Pressuretrol is all the way down, but I have not timed its cycling.

David Efflandt

In same boat

I just calculated EDR of my 6 single-pipe radiators at 158 sq ft and that is probably overkill since some radiators are covered (boxed in) and could be reduced in size. Although, this lets them provide heat longer after boiler goes off. My 2 yr old Bryant BS2 gas boiler is rated for 283 sq ft net. Smallest one in that line rated 196 sq ft net should have been more than enough.

It is a 2-story 3-bedroom Dutch Colonial built 1910 in Elgin, IL that I moved into last summer. It has insulation blown into its barn style roof (which technically makes it 1.5 story) and plenty of thermal mass (drywall over plaster). So the Honeywell CT3500 thermostat (which has gravity/steam setting) maintains even temp without running the boiler very often, and Smart Response ramps it up gradually from setback. Pressuretrol is all the way down, but I have not timed its cycling.