High Boiler BTU

MDNLansing

Here is the situation. I have an American Standard 6 section boiler installed in 1955. On the stamp plate the input BTU is stated as 360,000 BTU/H. This was of course with the original valve train. I have the old install docs and manuals and it looks like there were regulators, a mechanical pilot valve, an electronic gas valve and some kind of air intake regulator originally installed. All of this is of course gone now, and there is now just a single Robertshaw-Grayson 7000ERHC-S7C electric valve on a 1" gas line. This valve is specd for 725,000 BTU/H.



Tonight i measured the gas usage at the gas company meter and calculated an actual usage of 455,400 BTU/H when the boiler is firing.



So, is the extra 95,000 BTU/H a problem on the boiler? It has the original cast iron burners and baffle tubes and they work fine. All the orifices have gas flowing and the flame is a nice tight blue cone with little to no yellow. The valve was installed in 1994 so it doesn't appear to have been hurting anything, However, I'd like some input on this. Tonight I replaced the gas shut off with a new ball valve so I can reduce the flow to get it back inline with 360,000 BTU if needed. BTW, I checked the flow before and after replacing the shut off and it was identical. The new valve didn't increase the flow. So, is the extra energy just wasted here? Would the over firing have any negative effect on steam generation or turbulence? Should I turn it down?



OR



Is the 360,000 BTU/H rating based solely on the original valve train and has nothing to do with what the burners and water chest can handle?

Mike Kusiak_2

Overfiring

It sounds like your boiler is being overfired, comparing  the original specs to the measured gas useage.



The original specs were based on the burners themselves, their configuration and the number and size of the cast iron sections of the boiler. The original firing rate was determined by the gas orifices in the burners and the manifold gas pressure, not the gas train valves, etc.



If the orifices were not changed, it seems that the manifold pressure may have been set too high in the Robertshaw replacement valve, which is resulting in the overfiring condition. Typically, setting the manifold pressure to 3.5 inches WC results in the nominal BTU input rating being met. The only real way to tell is to have someone with a manometer check the manifold pressure and see if it has been set too high.



It is not a good idea to throttle the input valve to reduce the firing because you are reducing the gas pressure before the electric gas valve's pressure regulator, which will result in unreliable operation. The valve may not open and close properly and the flame size will vary will gas line pressure changes.



The only proper way to set the manifold pressure is by the regulator adjustment on the gas valve, and this should only be done by someone who knows what they are doing and with the proper equipment.



You might post your question in the Gas Heating section as Tim McElwain has extensive experience with older gas boilers and can give you all the info you need to know.

MDNLansing

How it's built

I get that the boiler plate stating 360,000 BTU input was based on the OEM equipment that came with the boiler, mainly the gas valve train. But my real question is, are the boilers built to withstand those BTU factors specifically, or can they take more? Is there something specific about the design that allow them to only withstand the manufacturers stated rate, or can they handle more than that? I realize not all boilers are the same and no one can know for sure in my case, but in general, is the overall design limited to these numbers?

nicholas bonham-carter

Boiler rating

Is there any rating on the boiler for sq.ft. of steam EDR?

The BTU ratings may have more to do with the water content of the boiler (steam volume capacity) than the limit of fire it can withstand.--NBC

MDNLansing

Steam

Stamping says 810 sq/ft of steam

MDNLansing

Regulator

I don't have much experience with electric gas valves other than replacing a few of them one for one with OEM units. I've never had to adjust the regulators to size the output. However, I am assuming the output of these valves is in fact adjustable? Since the specs say it's good up to 725,000 BTU, do you adjust them to match the rating of whatever your feeding? If this is the case, then someone specifically dialed in this higher BTU rate on purpose and it's been that way since 1994 when it is was installed, so I don't think it's hurting anything. But, how does this affect efficiency? Would the boiler run better at the stated rate or 360,000?

SWEI

Clocking the meter

just to be certain -- did you apply the conversion factor (on your bill) from CCF to Therms?  Ours is 0.86 x the meter reading.

MDNLansing

No I didn't

The calculations I found on here said to take the time the meter records one CUF of gas and divide that number by 3600 to calculate CUF per hour. Then multiply by the BTU therm conversion of 1050. I actually used 1015 which is the them conversion at my elevation according to a website I found that allows you to enter your zip code to get a precise value.



I wasn't aware the deviated them conversion was listed on the bill. I will check and see if their rate is in line with my 1015 vs 1050 difference.

Mike Kusiak_2

Optimum efficiency

A cast iron atmospheric steam boiler is usually designed to operate at or near its maximum efficiency at its nameplate input rate. If you are overfiring, the combustion efficiency is reduced and you may be generating additional CO because there is not enough excess air available to burn all the fuel cleanly. The stack temperature will also be higher and more heat will be lost up the chimney.



Usually replacement gas valves are set at the factory for a manifold pressure of 3.5 inches of water, which is pretty much the standard for atmospheric gas burners. If your burner was designed for a lower pressure and the gas valve was not reset, then that may be the cause of overfiring.



Another possibility is that someone felt that the boiler was not generating enough steam at its rated input, and deliberately increased the gas pressure. This is definitely not a good idea.



How does your boiler steam output seem to match your radiation? Do you cycle on pressure at  the end of a thermostat cycle, or can the thermostat be satisfied before the pressuretrol limit is reached?



In any case, if you actually are overfiring by as much as you suspect, you are definitely wasting fuel over what you would be using at rated input.

MDNLansing

Runs Great

Overall, the system runs great. But, I've done a lot of tweaking to get it there. I am now able to recover 2 degrees while staying under 1 psi. From a cold start, a 2 degree swing takes about 30 mins total, stays below 1 psi, and shuts off with the tstat.



I have an Eddy low pressure vacuum system. Runs in excess of 45 mins puts heat the air return lines and it's enough to shut the vacuum vents and start building pressure. In those runs, the pressuretrol will shut down at 2 psi and fade back to cut in at .5 psi in about 15 mins.



I don't do any setbacks over 2 degrees, so my system pretty much always runs at a few oz's and shuts down by tstat.



It's run for 20 years like this so I don't think it's hurting anything. And, the house is mostly uninsulated and 110 years old so it's drafty enough of never really have any CO issues. I do have detectors in several parts of the house, including near the boiler, and they never go off. I am however going to call someone to do a combustion analysis and look at the valve. As long as the exhaust is OK, I wouldn't mind having them turn it down and seeing if I can still maintain the low pressure cycles. I don't want system performance to change any, but if i can keep it the same, and save some fuel by turning it down, win win.