Derating and/or 2-Stage Steam Boiler for Mouat

Harry_6

I have a job where someone installed a Utica JE 600S based on a radiation survey. Trouble is, it's a Mouat system, so the actual load is 80% of the boiler rating. So I have two problems: The boiler is too big for the load by at least 20%, and it really needs a 2-stage gas train to make controllable operation possible. I had another big boiler where the manufacturer blessed a 2-stage gas train, and I was able to use it with a Mouat with a stable 2 oz steam pressure indefinitely. Since it's a two module boiler, perhaps only firing one set of burners would do the trick, but I think the factory didn't like that idea because of potential thermal problems, with only heating half the iron. Thoughts?

Harry_6

I did the math, and it turns out to be 30% oversized.

Mad Dog_2

How long has the boiler been installed?  Still in Warranty? Depending on the situation, I have sometimes taken on the Full Responsibility for a situation like this because NO ONE wants to make a move and the Homeowner is stuck in a Horrible position where no one will step in Nx hwoo him..not the Rep , The Manufacturers, The original installer, et al. They are abandoned to suffer with a bad install & noisy system that costs them a small fortune to run 🏃‍♂️. 

I got tired of the smoke & mirrors and evasiveness of all involved. I got tired of hearing : " But you'll void the Warranty...this will become YOUR Waterloo...You'll be married to this job!!!.  It already was and The Homeowner kept calling ME to help him.  Tired of the Namby-Pamby nonsense,  I just set it to fix it.  To this day, those problem Headache jobs are humming along, and they got my name on em...The Buck Stops Here!  Fine with that.  Mad Dog 🐕 

PMJ

@Harry_6 ,

What is the EDR of the Mouat system the boiler is connected to? I have a 463,000 BTU input connected to 1000 EDR.

I wouldn't dream of running my Mouat system without what amounts to PWM (pulse width modulation).
Unfortunately that requires a custom control. Even my boiler is too big to be allowed to run flat out to tstat satisfaction without building pressure and overshooting. So I break the burns up in shorter even pulses. Works like a charm.

Harry_6

Boiler EDR is 1454, system load EDR is 1021. I have recently been thinking about your approach of pulsing, but was put off by the short-cycling.

PMJ

Harry_6 said:

Boiler EDR is 1454, system load EDR is 1021. I have recently been thinking about your approach of pulsing, but was put off by the short-cycling.

Obviously more of a mismatch than mine. As far as I am concerned as long as the pressure doesn't start building until radiators start filling I think PWM is the way to go. So easy and inexpensive to try it's a why not.

Also depends on your definition of short cycle. The bad ones are when the boiler is a couple minutes on and off pressurizing the whole system waiting for tstat satisfaction. My burns are relatively short but I never have any pressure and the whole system goes in vacuum in-between. My dead cold start to partial filled radiators where I want them is 23 minutes. My ongoing warm system burns are about 8 minutes with
the shortest off wait times of about 17minutes +/-. Been doing it like that for 20+ years and love it.

See the attached chart. I end burns after a remote sensor sees steam (TTS - time to steam) and start the next one after that sensor has cooled some (TTC - time to cool). That way the cycles adjust to conditions and I always burn to the same partial fill level no matter how long the boiler has been off. Wait times more than 20 minutes had a tstat satisfaction in there. The system was warm and the conditions moderate so burns anywhere 8-12 minutes.

Harry_6

Intriguing. It's giving me ideas. I have always used remote vaporstat and/or remote temperature sensor to control Mouat, but sometimes that's still not enough.

PMJ

Looks like we have the same size system.

Mouat is great for PWM and natural vacuum. Many many years experience with it now so I'm no longer guessing. Happy to share any time.

EBEBRATT-Ed

You can probably do a combustion test and dial the gas back a little bit.

Harry_6

I haven't checked with the manufacturer yet as to whether they would allow a two-stage gas train. It says nothing about it in the literature, so I'm going to assume not. I haven't looked yet to see what the minimum allowable manifold pressure is. I did a similar sized Burnham a few years back, and they did offer two-stage gas as an option.

Harry_6

Do you have the vaporstat at the end of the line, along with the temperature sensor?

PMJ

Harry_6 said:

Do you have the vaporstat at the end of the line, along with the temperature sensor?

If this question was for me - no, just the temperature sensor. The vaporstat for the standard control is still on the boiler and functional but currently I run at either vacuum or a max header pressure of 2-3 inches of water at the end of burns - well below vaporstat range.