Gas flue back pressure at start-up in Massachusetts

Steve Garson_2

I have a Smith model 28A boiler that has just been converted from oil to a Powerflame modulated gas burner.  The boiler specs are for no draft control.  Massachusetts gas code specified the need for a spill switch, which means we must have a draft control.  The problem is that after a long shut off period, especially since we now use a very low burn rate after steam is generated, that when the burner comes on, occasionally the spill switch is activated which shuts down the system.

I checked with the chimney company and they said the chimney and draft is fine.  The problem is simply that the chimney is cold and the flue is long.  Has anyone ever heard of getting an exception to the use of the spill switch?  Any suggestions?  The problem is very intermittent, but that is a big problem, anyway.

Thanks,

Steve Garson

Boston, MA

icesailor

Flue Gas Backpressure/Gas Connection:

Unless there is a 2" gas ball valve above the main burner and pilot connection, that is an illegal connection in Massachusetts. You need a separate shut off on the pilot feed but there must be a way of shutting them both off at once and not individually.  Also, is it legal to connect the gas pilot line to the fitting supplied by the manufacturer that is a compression fitting and not a flare fitting like Massachusetts gas code requires? I recently found a Power Flame gas burner with a leaking compression fitting. I changed it to flare.

As far as the spillover switch, I doubt the board will give you approval to remove it. You probably have an issue with negative pressure in the building caused by exhaust fans making the flue gases go in reverse in the chimney.

Your chimney person doesn't understand draft and its complexities. In my opinion.

JStar

RE

"the chimney and draft is fine"



"Fine" can mean a lot of things to a lot of different people. I would get exact numbers on the draft. Then do some depressurization tests to see if it is affected by exhaust fans or other appliances.

icesailor

Draft

Steve,

Where I work, the wind blows regularly with enough force to unravel a crow bar. The chimney "experts" may say the draft is fine. It may be. And it may be too fine. Like too high. They said that it is cold and very long. I do mostly oil. It is extremely difficult to set up burners for draft. One day, there is no wind. Later that afternoon, it is gusting 40 MPH.

When they set up the burner, how much draft did they set it up for? Is it set up for up to .003 WC of draft and the damper is all the way open on a calm day? A windy day? If the damper is all the way open and showing .003 or more, you need a second damper. We oil guys often use two dampers. You need to use a Model "M" double acting damper. If the AHJ will let you add a second damper, it will probably solve your blow back problem.

You need one of those big Bacharach draft meters with the big scale that you can read from 10' away. If that jumps around on a windy day, you need to put positive air into the boiler room. Like a Tjernlund enforcer wired in with the burner.

There is a creative solution to your problem.

VictoriaEnergy

move building to a different state, failing that try...

What can you say about a regulation that will make an appliance spillage susceptible just so you can add the switch to say: "Ah-ha, see it spills; therefore the additional regulation is a success."



A lot of fan assisted appliances start the cycle really as a category 3 vent (under positive pressure) and only transition to a cat 1 once the flue warms up.  So the reg is that is only really relevant for true cat 1 appliance shouldn't be applied.....or so you'd think...  It's an argument that is a bit delicate to present since the inspector could say fine ditch the draft regulator and spill switch, but then install a vent system certified to cat 3 (Oh $#it!!)



 Is the vent a masonry chimney or Bvent?  Is it in the middle of the building , or does it run up an exterior wall?



Perhaps the simplest things to try would be select a setting, or get a card for the burner control, that will give you a much longer pre-purge cycle to help warm up the vent a bit before the burner fires.



Please post your successful outcome. 

Jim Davis_3

Spill switfch

Sounds like your boiler room is depressurizing during the off cycle because that is the only way the flue is going to get cold.  What are you using for combustion air?  Might want to locate the spill switch on the bottom of the barometric instead.

Steve Garson_2

Combustion air

Combustion air comes via a large fan. My boiler technician is suggesting a draft inducer fan in the connection pipe, though he is a bit concerned that it night flow out through the pipe that vents the water heater.



We had none of this problem with the oil burner. But now we run a low burn after steam pressure build.



Steve

Steve Garson_2

The photo is sideways

Rotate 90 degrees clockwise in your head.

icesailor

Spill switch location:

I thought of that today.

How does that spill switch work? Temperature? Below would be better if it is temperature I would think. Is that upper position the recommended location for that application?

icesailor

Fans:

Is this fan connected to the outside?

You need to have the boiler guy come with a big old Bacharach draft meter. See how bad it jumps around when it fires off.

It's like multi-meters for electrical. Everyone wants a digital. No one wants an analog. But if you are checking amp drays, a digital won't show a spike. An analog amp.clamp will.

A digital combustion analyzer won't show spikes well. But an analog draft meter will.

Like this one:

http://www.bacharach-inc.com/draft-gauges.htm

Jim Davis_3

Spill Switch, Draft Gauge

Spill switches work off of temperature, which might be another problem.  If you are making steam the spill switch has to be 20 degrees higher than the steam temperature.  If the spill switch is only 180-200 degrees it would trip just from the boiler heat in the off cycle. 



I use a Dwyer 460 Air meter.  Has a little white ball than is sensitive to .001" and is instant response.

Tim McElwain

Steve I just specified a job

with the HB Smith 28A -S/W-08 with a Power Flame Dual Fuel CR2-GO-20A for 2,500,000 BTU's. we installed a Tjernlund PAI-6 fan powered ceiling mounted air intake system with a fan proving kit. This was to overcome some air issues and pressurization issues that had existed all the way back when this was just an oil burner in this boiler.



The job has been installed and up an running for two months without any problem. I feel the Tjernlund unit was a must with this unit. I noticed when we cut down to low fire we had to do some re-balancing on the draft regulator also. This in no way affected draft on the high end. I tend with these jobs to locate the spill switch on the bottom of the barometric and bend it up slightly. They just seem to work better with no issues as far as safety.

VictoriaEnergy

Retraction

I take back what I said about not needing a spill switch.  The initial post didn't mention it was common venting with a water heater.



Spill switch is a must here for sure.



Can you post info on the vent system; size, height, interior/exterior, vent connector rise, vent connector run, BTUs of boiler at max and min, hwh too?