Oversized Boiler not enough EDR

RemlingerHTCL

Hello all, i recently took over maintenance at a school district that has 5 steam boilers and 4 hot water boilers ranging from various sizes and power burners to natural draft.

My current issue is with the elementary school which originally had two steam boilers for a big portion of the school and a hot water boiler for another portion.

They just did a project where they took all of the convectors and steam fan coils off of the steam system and installed unit ventilators with hot water coils and piped them to the hot water boiler. So now the only thing left is two Mammoth RTUs with Steam coils in them. they have motorized valves that modulate in the RTU. They also took our one of the steam boilers because it was condemned by the state fire marshal. Leaving the one steam boiler for the steam side. I might add that according to the past two building maintenance guys they've only ever ran on one boiler at a time anyway, the boiler is rated for 5513 sq. ft. of steam or 1323 MBH. My new fear is that now they have taken away all of the EDR and the steam boiler is way oversized. Also what is going to happen to the boiler now that it just has the two coils with motorized valves when they shut down will this create a vacuum between the boiler feed pumps? The bright side is that the boiler feed pumps are shot along with the reservoir so I currently need to replace and resize this portion of the system. so if we need to add a vacuum pump, vacuum breaker, motorized valve, etc. to make this thing function properly now is the time. i fear that the architect and engineer royally messed this system over by removing all of the radiation from the system. I appreciate any help on this. let me know if i need to add more info. Thanks

Jamie Hall

Oh aren't you going to have fun. Yes, the boiler will be oversize -- probably dramatically so -- so there may be some difficulty controlling pressure within a reasonable range (never mind where you would like to keep it). Second, you will indeed get a vacuum when the motorized valves close -- on the downstream side (you'll get a nasty pressure spike on the upstream side...) -- unless there are vacuum breakers on the returns and before any traps which may be present (there should be traps on the returns -- probably F&Ts, so you'll also need to be sure you have adequate venting, since an F&T shouldn't be asked to double as a vent).

You may not need to fiddle with much else (well, replace what's shot...)

It's going to be a bit of a nightmare.

Others -- particularly @Steamhead -- will have a lot more detailed comments I'm sure.

Steamhead

@RemlingerHTCL , what model boiler? What burner is on it? Does the burner fire at just one rate, or does it have step (lo-hi-lo) or modulated firing?

RemlingerHTCL

@steamhead, it is a Burnham v911 with a power flame burner model: c2-g-20b, it does have high/low fire its my understanding it can modulate. it does have a firing rate control already on it too, the operating control is set at about 6.5 psi and the firing rate control is set at about 4 I believe but don't hold me to that I can't remember exactly.

109A_5

Hello @RemlingerHTCL,
Are the hot water boilers large enough to handle the additional load of the unit ventilators or is it going to be 45 degrees F inside when it is 10 degrees F outside ?

As far as the Steam stuff, see the 2 pictures in this post for one easy method to eliminate the vacuum on the coil side.
https://forum.heatinghelp.com/discussion/192161/lets-get-the-water-back-in-the-boiler

As far as boiler pressure, that too can be managed, Vaporstat (or the like), Vaporstat and timer, duty cycle the boiler firing and limit the burn to only RTU calls.

Do you need 6.5 psi to get steam the the RTUs ? It may work very well at a lot less pressure.

JUGHNE

I hate to say abandon the steam and install another hot water boiler dedicated to the RTU's. Make that a glycol system....which I also hate.

EBEBRATT-Ed

Keep the steam boiler and adjust the controls so it cycles as little as possible. This is easier said than done. With fan coils you don't have to keep the steam pressure at the 2 psi point. I would set the operating control at 6 psi and set the modulating or lo-high fire control to send the burner back to low fire at 4 psi (as long as you can get enough heat out of the coils at that pressure) and back to high fire when the pressure is at 1 psi control.

Also have a GOOD burner technician with combustion test equipment set the burner up with a smaller flame as long as he can maintain good combustion.

Make sure you have the steam traps, venting aand vacuum breakers in good order.

Try that before trying more expensive solutions

RemlingerHTCL

@109A_5 I am unsure of the what the load will be for the added univents but that it also something I wondered about. This is all a mess that was basically thrown to me because the superintendent of the district really respects my knowledge and ability over other companies that have been there and I'm alumni. The thing is I'm a two man crew that's mostly resi now days. I've worked for companies in the past where i was THE boiler guy on commercial and resi. My thing is I want to fix these up right because they've had nothing but big companies come in and cobble the things and just kinda guess on a lot of stuff. My next question was do we really need that kind of pressure for the two coils that are probably less than 100 linear feet of pipe away from the boilers? the rise is maybe 15 feet as well. The main is 6" and the two rtu come off at 2".

@JUGHNE I think they would have a hard time abandoning the steamer being its only 4 years old and was $35k. not to mention converting the steam coils to hot water coils in the mammoth rtus would be money well wasted when they are budgeting to replace the mammoths in 2026.
Now when the time comes to 2026 and they ask my advice on what to do with that it may be something we explore.

RemlingerHTCL