Short cycling and hissing radiator air vents - oversized boiler?

lmoore8

I've come to the conclusion that my boiler is oversized. I haven't looked at any calculations to figure this out, but if someone can point me in the direction of a way to figure it out, I can investigate that.

I recently overhauled my venting, and I think it's sufficient. I have two Gorton No 2s for the main venting, and Gorton No 6s and No Cs on the radiators. I also installed a 0-5 psi gauge and a vaporstat (set at 16 oz with a 12 oz subtractive differential). I'm reasonably confident that the vaporstat is calibrated correctly because the 0-5 psi gauge matches it when it shuts off.

From cold, it takes about 25 minutes to start showing some pressure, and the radiators are starting to get warm at that point. Another 5 or 10 minutes and the pressure is up to 16 oz, and the vaporstat shuts it off. It takes about one minute for the pressure to get back down below 4 oz, and then 2 and a half minutes before it gets back up to 16 oz. That seems to repeat until the thermostat is satisfied.

I also have a few of the Gorton radiator vents that are hissing when the system is running. My association is that a hissing sound comes from too high of a pressure for the rate that it can vent. I wouldn't think that 1 pound of pressure would be too much though. I could decrease the pressure, but then it would just short cycle even faster. Also, when I put something in front of the vent to try to tell what it's doing, it seems like the air coming out is quite wet, and I've heard a sputtering sound a few times.

From what I gather, if the boiler is oversized, there's not really much I can do about the short cycling, since increasing the pressure setting isn't a good option. So unless I can have the boiler downsized somehow, I just have to live with that. But what about the hissing radiator vents? Is that also just something I have to live with? Any tips or ideas?

acwagner

Run the calculations first. They are straightforward, and good information to have. You need to sum up the heating capacity of all your radiators. This is typically given as an EDR rating in square feet, but you can also convert to BTU/HR.

If you post a photo of a typical radiator someone here can help walk you through the calculation. They vary based on the type of radiators you have.

Then, compare that value to the output rating of your boiler. Usually there's a plaque on the outside of the boiler with this information, but you can also look it up online by the brand and model number of the boiler you have. There's a few numbers generally listed, but, again, someone here can help decipher if you post it.

Does the vaporstat shut it down on a regular call for heat, or are you recovering from a temperature setback?

Big Ed_4

To figure out the proper boiler size need , we start with figuring out the surface area of the radiators in the system . Which is called the EDR of the radiators . I use Dan's book EDR Every Damn Radiator . Most systems I have come across the boiler is over sizes because the calculation had not been done .. With venting you want to size the radiator vents to the size of the radiator . Resitance is used to control the incoming steam , the goal is to heat up each radiator at the same time .. You mentioned wet steam ? Check water line and pipe insulation ..

lmoore8

Thanks for the input. I'm putting pictures of two radiators below. I'll start counting sections and taking some measurements as well. I'm googling how to calculate the EDR.

The vaporstat shutting it down is recovering from a setback. We haven't lived in the house during heating season yet, so I'm not sure how it behaves regularly. Honestly, convincing my wife that we should just keep it at a regular temperature instead of cooling it down at night or while we're at work will be challenging.

acwagner

Those are Corto style radiators by American Radiator. I have the same type. The EDR ratings are listed in this PDF:

https://heatinghelp.com/assets/documents/American-Radiators.pdf

Measure the height from the floor to the top in inches, number of columns, and number of sections. The ratings in the tables are EDR per section.

How big of a setback are you doing?

Jamie Hall

Actually the minute off/two and a half minutes on isn't that bad a cycle timing for coming out of a setback. A bit bigger than you might have -- but not by too much. Which is the good news. Setbacks -- unless they are very small -- are likely to do that with steam. The other good news is that it really isn't hurting your efficiency much. How long -- total -- does the system take to recover from your setback, and how much setback are you using?

lmoore8

When I turned it on today, it was 61 degrees at the thermostat and I set it for 64 degrees. I didn't time it, but I would guess that increase (and actually two degrees more, because it ended up at 66) was somewhere around an hour. My thermostat said I had 45 minutes of use, which I'm guessing is actual burn time.

Thanks for the chart. All of the radiators are the same style, so that's very helpful. Taking measurements and using that book, I come up with an EDR of 205 sq ft (across 8 radiators). Some Googling says that 240 BTUH per sq ft is a good conversion to use? If so, that gets me just over 49,100 BTUH.

Looking at the boiler, the panel list "steam square feet" as 360 and "steam BTU/H" as 86,300.

So looking at either of these measurements, it seems that the boiler is 175% of the size it needs to be. Is that big deal? And is there anything that can be done about it?

Jamie Hall

The numbers to compare are the 360 square feet rating on the boiler and the 205 square feet on the radiators. Which add up to the boiler being half again as big as it should be.

Which is a big deal. It's going to short cycle.

Unfortunately, I doubt that the boiler can be downfired to be even reasonably close.

ChrisJ

When your radiators heat, does the top get hot first or the bottom? And do the sections get hot in the middle or does the steam seem take a short cut and run over and shut the vent before pushing much air out?

This is important, especially with an oversized boiler.

lmoore8

I'll check next time they're running, but I think the tops do get hot before the bottoms. I haven't noticed any radiator that didn't get hot in the middle though. They seem to heat all the way across, starting at the pipe and working towards the vent.

ChrisJ

lmoore8 said:

I'll check next time they're running, but I think the tops do get hot before the bottoms. I haven't noticed any radiator that didn't get hot in the middle though. They seem to heat all the way across, starting at the pipe and working towards the vent.

Mine heat along the top, and because of how I vent them it does tend to go across the top faster than the sections heat. But over time, the whole radiators do heat if the system runs long enough.

I've found if I go too fast it short cuts across the bottom and shuts the vent. My neighbors did the same thing when I went a little too fast with venting and though the system heated fine, it did cause the system pressure to build faster which was a bad thing. I was using Gorton 6's on all of his and went up to C's and the overall output actually dropped because of the shortcutting.

Went back to 6's and things went back to normal. But I feel it depends on a lot of things like radiator size, system pressure etc so it's not an easy thing to predict.

acwagner

A three degree setback isn't huge. The least intrusive approach to trying to keep the pressure down during recovery is to use the thermostat to slowly recover from setback over a period of time.

Is your thermostat programmable? If it is, try setting such that it raises 1 degree every 45 minutes or hour something like that. It'll give the radiators time to release their heat to the room. That will also help with overshooting the set temperature.