Downfiring oversized boiler??

Nick_37

First of all thanks to all the experts that have helped me over the years. The information in this site is invaluable. To the task at hand. I've expereinced some intermittant short cycling and started to investigate. We have a 3,000 sq ft home across four floors including the basement. I started to calculate the the total EDR because I always thought the boiler was quite large for the space. Its a Pemco with 225,000 BTUs output. My EDR calculations came in around 140,00 BTUs. I installed a vaporstat and the boiler runs at less the 1psi. One of our HVAC contractors suggested that I downfire the boiler but I wanted thoughts from this formum before going down that road. I know that could create other problems. The system is properly vented with Gorton #2 on the two mains and Gortons on the rads everywhere else.

Jamie Hall

If your EDR calculations do NOT include a pickup factor, and the output you quote is net (it's so much easier to just compare the EDR rating on the boiler with the total EDR of the system...), then you really aren't that badly oversize -- about 10% or so. It may well be possible to downfire the boiler, depending on whether it's gas or oil, and what burner is in it.

The key question is practical. If you are recovering from a setback, or otherwise have a very long run, does the boiler cycle on pressure? What are the cycle timings (how long off vs. how long on)?

JUGHNE

Did he say how he would downfire the boiler?

---Reducing the gas pressure to the manifold within limits advised by the manufacturer.
---or
---Reducing the burner orifice sizes again as suggested by boiler manufacturer.

It is a common topic on this site.

Nick_37

I included a pick up factor. I think it cycles on pressure. The boiler cycle is 20 mins from standing start to to full heat. It will then cycle a few times immediately starting and stopping before it finally stops. Sorry what is the setback? Is that for the thermostat?

nicholas bonham-carter

The Gorton 2 is good for 20 feet of 2 inch pipe, so during the venting phase as the boiler is just starting to make steam, I would look on your low pressure gauge, and see whether you are letting the air escape at less than 2 ounces, or paying for more fuel to force it out. The Big Mouth vent is a wonderful device, with at least 2 times the Gorton’s capacity, and was invented by one of our genius members on this site.—NBC

Nick_37

HVAC chap said to remove some of the burners.

JUGHNE

There should be some opinions here on that!

Nick_37

Jamie - I see it happening after recovering from a set back.

ethicalpaul

> @Nick_37 said:
> HVAC chap said to remove some of the burners.

I’m very looking forward to this discussion. I want to disable one or two of my burners to see what a non-oversized BTU generating system looks like. But only if I can find a tech to do a combustion test on it. Any takers in northern NJ?

Jamie Hall

Nick_37 said:

Jamie - I see it happening after recovering from a set back.

OK. Setbacks will almost always -- not quite always, but almost -- do that, since they put the heaviest stress on the system. Twenty minutes to the first shut off does suggest that you really and truly are oversized -- but not all that much. What is critical is the on time vs. off time once the boiler starts to cycle on pressure. That will tell you remarkably closely just exactly how many BTUh your radiation can absorb in relation to how many BTUh your boiler really is putting out as net output. So what you need to do is to measure -- over several cycles starting after the first time the boiler quits on pressure -- how long, minutes and seconds, the boiler fires, followed by how long, minutes and seconds, the boiler is off, followed... etc. for at least three or four cycles. From that you can calculate very easily just how much overfired you are and then, keeping in mind that you still need a pickup factor, how much you might be able to downfire.

Ironman

Nick_37 said:

HVAC chap said to remove some of the burners.

Realize that if you do it, that baby is totally yours. No warranty, no responsibility for the manufacturer, probably no insurance coverage should something happen and its against code to modify a gas train without the approval of the manufacturer.

Fred

It is not recommended that burner tubes be removed from the burner as a means of "Down firing". Most manufacturers will tell you that can create an uneven heat pattern across the bottom of the cast iron block and cause premature failure. Removing burner tubes can certainly affect combustion too. Some of the Pros can better weigh in on that issue. The best way to down fire a gas boiler is probably a two stage gas valve.

Also, what type Low water cut off do you have on that boiler. If it is a Cyclegard, they will shut the boiler down at certain intervals to check the water level.

mikeg2015

If you pull burners you have to block off airflow. Even then you’ll still end up with uneven combustion, and uneven heading of the block which could cause some thermal expansion issues. That being said, Peerless uses only 3 blocks but has I think 6 burner sizes on their 63 series boilers. BUT it was designed specifically for that. I suspect the baffles are carefully configured to make it work.

I’d look at system balance. I suspect you need to triple your venting and then possibly slow down the radiator venting. The goal is to CONSUME all of the steam as it’s made. If you fill radiators unevenly but venting them too fast, you actually consume less steam as the section heat unevenly and don’t get to full temp.

I I have boiler that oversized by 50%, but have plenty of tons main venting and ended up downsizing vents on most of the radiators. Most are ventrites, Hoffman 40’s or smaller Maid O Mist vents. But i have 1 big mount, 1 Groton #2, 2 Groton #1, and 3 varivents, and 2 Groton D’s Scattered along the header. Overall works pretty darn well. Radiators heat up all at the same time and by about the same rate with exception ot deliberat room balancing.

Running pressure is <0.5 oz on a 9oz gauge... ( smallest gauge i could find). It’s possible.

Nick_37

The truth is that the AC guy actually removed the burners and told my wife while I was at work. They were working on our AC system at the time. Good intentions I'm sure but I replaced the burners immediately when I got home. I didn't want to wait and find out the result.

Gordy

You need to tell your AC guy to stick with the AC. As a matter of fact I’d find a new hvac specialist.......

ethicalpaul

Nick_37 said:

The truth is that the AC guy actually removed the burners and told my wife while I was at work. They were working on our AC system at the time. Good intentions I'm sure but I replaced the burners immediately when I got home. I didn't want to wait and find out the result.

Wow, OK that sounds crazy even to me. Did he do a pressure check after he did that, or do a combustion check? And did he plug the orifices or just pull the burners? Did you ensure they were all firing again?

JohnNY

Nick_37 said:

The truth is that the AC guy actually removed the burners and told my wife while I was at work. They were working on our AC system at the time.

Terrifying.

EzzyT

If you’re looking to down fire a boiler you want to stage fire the boiler and as others have said have the AC guys put back those burners

Koan

I have an over-sized boiler as well, 750 EDR on 400 EDR of radiator area. I have the same symptoms with pressure cycling on set-back. I looked in to down-firing the burners, and my research showed that unless the boiler burners are designed for a lower firing rate, changing to down-firing can have unintended and dangerous consequences. In your case , you may want to see if the boiler has an EDR rating and then calculate your radiators aggregate EDR to see how over-sized the boiler is. Using bigger venting on the mains might help, it did for me when I installed a Barnes and Jones Big Mouth 3BM main vent to replace a Gorton no 2. Steam gets through system more quickly now, but a setback of more than 2 deg F still gets me to cycle on pressure.

In the future, I am going to try a TECO controller to limit cycling along with an ASCO solenoid valve on the vent to close once the air is out of the system and the burners shut off.

My system is a 2 pipe Hoffman vapor, YMMV depending on your setup. Others here have had some success with this approach with over-sized boilers. For a warm system, re-firing into a vacuum means the water boils at a lower temp. and the steam goes to fill the void without having to push all the air out again. Also, the process controller limits cycling on pressure using pressure and temperature sensors.

Nick_37

The burners were plugged and not just removed. I did check that the burners where all firing properly when they were replaced.

PMJ

So much angst over boiler size.

The radiation installed with most of our systems was for a continuous coal fire and was well past what was required to heat the house - even on design day. There would be no way to control the continuous fire system if the radiators were ever completely full. So these systems were designed to never have the radiators full. Same with the boiler.The rating on the coal boiler was well past what was ever needed also as it was never intended(read that couldn't be) to be run at "maximum" fire. Both the radiation and the boiler ran somewhere in the middle of their ranges so that even in the bitter cold the system would be controllable. That is why there are so many big replacement boilers out there - they simply matched the rating of the coal one - the max output of which the original designers knew would never be used one time.

So now we have changed to intermittent fire and have a lot of big boilers out there that can ONLY run on maximum fire (hence the mismatch). But there is still no reason to fill the installed (oversize) radiation up with enough steam to build pressure or even close. So I say just don't do it. Limit the single max duration run time so that a pressure stop is literally quite impossible, setback or no. There simply is never a reason to fill up your radiators that much. When you approach the project this way it makes no difference whatsoever how much "too big" the boiler is. You simply match the max run time to whatever boiler you have and space out evenly the firings to the number of cycles per hour that you want.

I think we should stop giving so many people the impression that they own a permanently dysfunctional system simply because they have a lot more boiler than is needed. The expensive controls, at the end of the day, do exactly what I am saying, they limit burn times based on what is needed.

@nick_37,consider shifting your thinking to controlling your run time instead of messing around with the fire and related combustion worries. It is a far less costly and ultimately much more flexible approach to your pressure issues than trying to change anything about the boiler itself. I probably have twice the boiler I really need(because it likely replaced the coal one), but never have more than a fraction of an inch water column pressure. My boiler has been quite happy for 62 years. I'm not changing a thing about it now. I part ways with many here and view it's oversize as an asset, not a liability.

gennady

Some of the differences in boiler sizing really just burners quantity with the same CI block. Flue pipe collar size different as well. Not a big deal, but of course such boiler modification Not velcomed because it actually becoming different boiler than it’s label states.