Steam - underfiring power gas gun

steve_173

I have a Slant/Fin Intrepid TR50. The oil rating is 280,000BTU input for for a 746 sq ft output. I have some flexibility with the input due to the power gas gun. I recall something here about underfiring being more efficient. That is confirmed by the Intrepid tables where the lower firing rates are slightly more efficient. Is that due to longer run cycles? How much can one underfire? Is there any way to know approximately how much steam will be produced? What is the best way to tweak this without an endlessly iterative process? Will this cause problems of inadequate heat on very cold days?



The actual EDR is 724 (+18 when I get around to adding a removed radiator back...it could be a while so give suggestions both ways if that matters to save fuel in the meantime). That's mixed radiators/convectors. I know some feel the EDR numbers for convectors are too high. That should give me some wiggle room while still being sure the radiators get the steam they need. I'm thinking 270MBTU should be fine. Can I go lower? If everything is well insulated (it's not now...if you know someone good at this on Long Island, please email me), is the 1.33 pickup factor too much? What is a reasonable factor then as it greatly impacts the steam need?



I have an 8" oval chimney liner that has an insulating blanket. I don't want to underfire too low as to have the chimney become oversized.

Steamhead

You could have a pro try it

but with your EDR that close to the rating it probably wouldn't make that much difference. Does the boiler short-cycle now?



If you do want to try it, have a pro do it with a digital combustion analyzer, to make sure the combustion process remains within spec. They did use an analyzer when they installed it, right?

steve_173

Not sure

I hope so, but I'm not sure. They are coming back to adjust it. It's turned too low at the moment. A combustion analyzer will check if the gas and air input are correctly adjusted to each other. Will the size of the boiler and amount of steam it makes have anything to do with that?



Are you saying just run it at 280MBTU spec? I think it's set at about 200MBTU now based upon the air setting. It seems to be heating the house even with uninsulated mains. It's not been very cold so that isn't a true test. If the piping factor drops to 1.2 when insulated, it can probably run 245-250MTU input. Maybe thereabout is a good starting point. I mainly don’t want to go too low as to cause problems with venting or otherwise. Is lower input always a lower operating cost as long as there's enough steam or does it work against one at some point?



The pressure is somewhere about 1.5 cut in. That seems high. I'm going to try it close to a 0.5. Do you recommend swapping the gauge (0-15oz or 0-20oz?) and vaporstat (L408J1009?) to low pressure models? Slant/Fin could include those. Who needs a 30psi gauge and and 10 psi stat on these systems?

Fizz

Hope this helps

Just converted WM SGO-6 oil burner with Wayne 250.  My radiation in house is 518 sq ft, the boiler is oversized at 642 sq ft which includes allowance for piping.  According to Wayne conversiong chart, the gun should be fired at 140000 btu x the oil gun rating of gal per hr.  The SGO 6 is rated at 1.75 gph, which equates to 245000 btu firing rate per Wayne chart(1.75 x 140K btu's).  Initially my installer fired the gun at 155K's which was causing short cycling and lack of heat.  I recalculated my sq ft with boiler sizing, and came-up with a ratio of roughly 80%(518/642), which equates to roughly 198000 btu's.  My installer refired boiler to 189,000 btu's and the results are very positive.  No more short cycling and heat is very good.  The old oil burner was overfired and heat was too much, now very comfortable, and efficient!  I too have steam heat, a Richardson system, which is vapor vacuum, works on low pressure of less than 1psi.  Currently have a pressuretrol, thinking of going with vaporstat, but not sure if worth it.  Hope this helps.

steve_173

Is it linear?

That math works if it is linear and if there is no rate at which the boiler or system can have problems.



I assume both numbers should be 198,000?

Fizz

Linear equation?

Don't know if both need to be at 198K, think allowance is around 15%.

Fizz

Linear equation?

Don't know if both need to be at 198K, think allowance is around 15%.

steve_173

You lost me

Where are you getting 15% in your numbers? Are you saying you can fire at 15% below what you need due to the piping pickup factor? Usually for steam, I=B=R includes a 33% pickup factor. I'm guessing that is too high for well insulated mains. I don't know how much is more reasonable.

Fizz

Piping factor

Yes piping factor of 1/3 would be included in equation.