Steam gas fired boiler sizing question

Steamhead (in transit) · December 2008

those risers aren't insulated? They usually were- with asbestos. Maybe the asbestos was removed as far as they could get to, but is still in the walls.

But if your assumptions are correct, you'd be looking for a boiler with a DOE rating of 241,000 BTU/hour.

All the boilers you've cited are atmospheric gas units. These lose some efficiency to excess combustion air and losses from the base of the boiler. Note that some excess air is needed to insure clean and safe combustion, but atmospheric gas boilers generally run with way too much excess air.

A more-efficient choice is a wet-base boiler with a power gas burner. These are basically oil-fired units with powered gas burners fitted to them. The "wet" base makes more heat-transfer surface available to the flame, and the powered burner mixes gas and air more efficiently so there's less excess air.

It's important that the wet-base boiler/powered gas burner combination be certified and supported by the boiler manufacturer. Not all manufacturers offer this combination from the factory, but Slant/Fin does. For your application, I'd use the Slant/Fin Intrepid TR-50 with a Midco, Carlin or HeatWise gas burner. It has a DOE (Gross) rating of 255,000 BTU/hour and a Square Feet rating of 796, and has iron push nipples instead of gaskets.

Note that Carlin gas burners have been hard to get lately due to high demand, but the HeatWise and Midco units are also good choices. Here's a Midco on a Slant/Fin TR-30 steamer we recently installed. Note the 3-inch header piping- the boiler's front section is actually tapped for this size. Nice!

If they give you the deer-in-the-headlights look when you mention this, have your contractor or supplier call Slant/Fin and ask to speak to Bob Flanagan. He'll see that you're fixed up.

<A HREF="http://www.heatinghelp.com/getListed.cfm?id=367&Step=30">To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"</A>

Brandon_7 · December 2008

Gas fired steam boiler sizing

Need some advice on boiler sizing and selection. Have 3 questions:

Question 1 - In short, what is the right pickup factor to use? Dan H's 1.5 factor appears to be for old systems that have extensive piping (which I don't believe I have) or intermittent operation. What qualifies as intermittent operation?

I have 669 Sq Ft of radiation in a one-pipe parallel flow steam system (2200 sqft house plus heated basement and garage). All radiators are installed, i.e. there are no extra unused pipes in the walls. The risers in the walls are not insulated however. The system is used with a thermostat that has a clock to turn the system down at night and during working hours in the day. Does this qualify as intermittent operation?

According to the Lost Art of Steam Heating, page 75, you take the 669 Sq Ft, multiply by 1.5 and multiply by 240 to get MBtu/hr. Then you select the boiler using the DOE heating capacity rating. The 1.5 factor is recommended for systems where unused system pipe may exist or there is intermittent operation (e.g. a thermostat with a clock).

This would call for a boiler with a gross rating of 240.847 MBtu/hr.

The boiler in service now is a 30 year-old Hydrotherm that has a gross rating of 192 MBtu/hr. It suffered from short cycling (bad main and radiator vents, uninsulated near boiler piping - repairing both fixed the short cycling). It also has water hammer due to an improperly piped Hartford loop (16 inches long) and incorrect header (bullheaded tees). Generally the only time the farthest radiators get heat is when the unit turns on in the morning to recover the house back to normal temp. The boiler generally fires about 30 min or more in this phase. The radiators occasionally breathe in and out of the vents, which I attribute to the near-boiler piping being wrong although perhaps due to undersizing of the boiler?

Weil Mclain makes an EG-75 that has 240 MBtu/hr. Based on Dan H's pickup factor of 1.5, and the installed radiation of 669, the EG-75 would be right for my house.

I am feeling like the situation Dan describes in the 2nd/3rd pages of the book ("How and undersize steam boiler can still heat a building"). I don't want to oversize the boiler - does using a clock thermostat qualify as intermittent operation and thus call for the 1.5 factor?

The next smallest Weil Mclain is the EG 65 that has a NET I=B=R of 664 SqFt. If I used the 1.333 pickup factor, I probably would get this unit, since I could probably downsize a bathroom radiator or two and drop 10 sq feet.(the bathroom radiator already overheats). Although, if I really thought a bit further ahead, I thought of adding some living space (and maybe 60 sqft of radiation) in the attic. If I needed to have an attached load in the mean time, I could put an extra radiator to help heat the garage or basement. So maybe I should cut it so close and just go with the larger boiler. Any suggestions here?

Which pickup factor is right for me?

Question 2 - anybody have some recommendations on boiler manufacturers?

Question 3 - Weil Mclain has the elastomeric seals on the boiler sections. Every other boiler has either metal push nipples (assume steel) or cast iron. From a gut engineering sense, the cast iron nipples make the most sense - they expand/contract at same rate. Anybody have experience with the elastomeric seals (how long have these been in service?

Here are some comparable makes with sizing:

Burham --> IN9 that will produce 231 MBtu/hr gross.

Peerless --> 63-06 - 238 MBtu/hr gross.

Slant/Fin -> GXH 275 - 223 MBtu/hr gross.

Utica --> PEG262C - 212 MBtu/hr gross, or the PEG 300C with 243 MBtu/hr gross.

Dunkirk --> PVSB-9D - 245 MBtu/hr gross

Many thanks. BTW I love the steam heat - grew up in houses with forced air and didn't know that you could have nice quiet heat like this. Dan's books are also great - well-written and great detail. I have been more knowledgeable than most of the contractors I have interviewed about this job thanks to the books.

Brandon_7 · December 2008

Steamhead - yes I'm positive the risers are not insulated. Here's why:

1) The basement has a finished ceiling. The asbestos insulation runs up to the ceiling. Above the plaster and lath I see from other nearby holes in the ceiling that the pipe on the other side is free of insulation.

2) In one area, a riser to the 2nd floor is visible at least four feet up by looking directly beneath it and you can see up the wall cavity. It is in a very narrow wall cavity that is not accessible. This pipe is bare.

3) I am renovating another second floor bathroom and you can look down a wall cavity (different than number 2) and also see bare pipe down at least 4/5 feet.

So with 2 of 6 second floor risers not insulated and the fact that insulation terminates in the basement at the ceiling on all of them, I conclude none are insulated.

Based on this and the intermittent operation - you think the 1.5 pickup factor is the way to go? The existing boiler has 600 Sq Ft I=B=R. So it's probably undersized by 20%

Thanks for the reference to the slant fin. It makes a lot of sense. I referred to this when speaking to an installer this morning and he'd never heard of it. I gave him the data and contact info. Let's see what happens.

Steam gas fired boiler sizing question

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