down-firing an atmospheric burner

nicholas bonham-carter

often when i read of other systems with 2-stage firing burners, i get envious!

i would be interested to hear of other peoples' experiences with the thus necessary down-firing of medium sized atmospheric burners [peerless 211a-1,050,000 btu].

i would like to fire my boiler in stages, controlled with a second vaporstat. as i would plan it, the burner would fire at 100% on the first call for heat, and then when the pressure got up to 2 ounces [venting complete], it would drop to perhaps 75% to maintain the low pressure. i could either use the peerless "mod-u-pac" option or simply fill in the missing parts myself [second smaller gas valve, and piping].

my boiler is pretty well sized, and i use no setback; but on a windy design temp day, i can sometimes get to the 12 ounce cut-out, and will frequently get to 10 ounces before the thermostat is satisfied.

there seems to be some divergence of opinion here as to the advisability of modulating the gas flow on an atmospheric burner, because the combustion air is not being controlled as well. it seems to me that it would still be an improvement as the flame, and flue gas movement continues uninterrupted, instead of being cut off and on, without this feature.

has anyone had any experiences one way or another?--nbc

Dave in QCA

Many mafacturers have this option

NIcholas, many manufacturer's, Weil-Mclain, Peerless, Burnham, etc., have the option of hi-lo firing on the medium sized atmospheric boilers.  While I have not seen an effiency rating specifically for firing at a lower rate, it stands to reason that they would not offer the option if it was not beneficial.



In my case, the boiler is way oversized for the connected load, as a result of the separate building at the back being converted to forced air and it was oversized even when it was connected.  I have lowered my firing rate from 1,050,000 to 750,000 by adjusting the pressure regulator adjacent to my gas valve.  My stock gas valve is a nonregulating diaphram valve, and so a regulator is required.  750,000 is as low as I can go with this method.  I have done additional testing by gradually pinching down the gas shutoff valve for the main burners to see what firing rate is capable of completely heating the system smoothly.  I have arrived at a firing rate of 600,000, and if it fires at that rate continously for 45 minutes, pressure will begin to creap up over 2oz, and will eventually get to the cutout point on the vaporstat, which is 8 oz.  However, given that I do not do setback, the boiler will never continuosly fire long enough to cutout on pressure. 



To provide a proper means of controlling the boiler at a 600,000 firing rate, with the option of kicking it up to high fire, I have purchased a 2-stage honeywell valve.  This is a regulating valve, which does not require a separate regulator.  Both the Hi and Lo Fire settings are adjustable over a broad range.



Here is the catalog info http://customer.honeywell.com/honeywell/ProductInfo.aspx/V8944N1020

and the product data sheet http://customer.honeywell.com/techlit/pdf/PackedLit/65-0214.pdf



It is my understanding the Peerless offers an option of 3 stage operation, as you already know. 

Hope this information is usefull to you.

BRIANJ

TWO STAGE VALVES

Hi Nick,

My boiler is 1/4 the size of yours. It's a Peerless 63-06 which generates 238K BTU. I read a lot of Gerry Gil's information on two stage set-up and went with the Maxitrol SR1 and an L608 Vaporstat. It was all installed by Scott Perry, a great steam man. I've found the best settings for my application is to let the pressure build to 4 oz. and then trigger the low-fire. The boiler then backs down to one oz. where it stays until the tstat is satisfied. I have to say that the heating has never been more comfortable. It seems so even and there are no bursts of heat that quickly satisfy the tstat. We also removed the low water cut-off probe which was shutting down the boiler every 10 minutes for 60 - 90 seconds. We replaced it with an older M&M low water cut-off.

I haven't done any stats yet this year for HDD but last year we were saving 20% in NG usage. I figure we'll have a three year payoff and then it's all savings.

Brian Reynolds