Sizing A Boiler For Mouat System

Harry_6

Gentlemen, I'm in the process of sizing a new boiler for a turn-of-the-century Mouat system equipped house. Now normally I would probably survey the attached radiation and multiply by .8, since the radiation should only be about 80% filled with steam. But since it's a vapor system the piping is large and so might call for a larger pickup multiplier than the usual 1.33 (Yes, it's all insulated). So perhaps I should just pretend that the load is the existing attached EDR (using 1.33)? Interested in your input (especially those old guys in the Cleveland area, where Mouat was king!). FYI, the house is around 9,000 square feet area, with a 719 EDR 1980's boiler (in Northern Ohio), which strikes me as way too small, but the installer was clueless - adding a condensate pump, unnecessary vents, etc., so I'm going back to the original. Haven't surveyed the EDR yet. Also, anyone see a problem with using vacuum vents on the mains to convert it to vapor/vacuum?

Paul S_3

33% pickup factor is fine....expecially if its all insulated...some guys would say go smaller

Jamie Hall

Use the EDR of the radiators. The square foot rating of the boiler includes the pickup, and I wouldn't adjust it. The pickup may be a bit too high, but not enough to worry about, in my opinion.

Being a Mouat -- or any of the other vapour systems -- doesn't change the ability of the radiation to condense steam (which is measured by the EDR) and therefore doesn't change the required boiler size!

There should be no harm to using vapour/vacuum vents.

Fred

I'm betting when you survey your radiation, you will find that the total EDR of your radiators to be in the 1300 to 1400 sq. ft range. I'm in Ohio (Dayton) and I have a 5000 sq. ft. house with about 720 sq. ft. of radiator EDR. How well did that boiler heat the house? Was the house comfortable or did you shut a number of radiators/rooms off?

Harry_6

My thinking with questioning the use of radiator EDR is that the actual size of the radiator isn't germane, as the orifice limits the steam condensed within it to a portion of the whole. For a given orifice I could have a radiator 100 sections long (pretty heavy, that) but it would still only condense the same amount of steam. My assumption is that the orifices, when correctly adjusted, would limit the amount of steam to perhaps 80% of the radiator surface area anyway.

Fred

You are fortunate to have @gerry gill right there in the Cleveland area. If I were you, I'd consult with him and see if he will maybe even configure and install your replacement boiler. He's one of the best!

Jamie Hall

Harry said:

My thinking with questioning the use of radiator EDR is that the actual size of the radiator isn't germane, as the orifice limits the steam condensed within it to a portion of the whole. For a given orifice I could have a radiator 100 sections long (pretty heavy, that) but it would still only condense the same amount of steam. My assumption is that the orifices, when correctly adjusted, would limit the amount of steam to perhaps 80% of the radiator surface area anyway.

Unless you know, positively, that the orifice for a particular radiator was undersized, it would be prudent to size for the radiator. Now I agree if you were to check all the orifices at the system running pressure, you might get closer...

jumper

I lean toward under sizing so no pick up. When boiler runs continuously home is toasty. Any room that isn't can have a little electric supplement when needed.

Jamie Hall

jumper said:

I lean toward under sizing so no pick up. When boiler runs continuously home is toasty. Any room that isn't can have a little electric supplement when needed.

Um. Well, maybe. As a building superintendent, however, I can assure you that if you installed a boiler in my building, and undersized it (no pickup) and it failed to keep my rooms up to the needed temperature on a cold night, you would be paying for the electric boost -- which isn't cheap -- and you would be installing a new boiler, big enough to do the job, on your dime. Just sayin'

jumper

Jamie,would you not be angry with issues from over sizing as well? I figure that hundred year old 9000 square foot house started with more radiation than it needs today.

Jamie Hall

jumper said:

Jamie,would you not be angry with issues from over sizing as well? I figure that hundred year old 9000 square foot house started with more radiation than it needs today.

No, and no. It still -- as it did when it was installed -- can just barely keep up on the coldest windy days with the installed radiation. And the boiler can keep up with the radiation -- but not by much. On a truly truly cold day (got down to -20 this winter) it will run as much as 50 minutes out of an hour, and shut off on pressure (10 ounces) perhaps once. I can live with that... Otherwise, Cedric never shuts off on pressure unless I ask it to recover from a big setback -- which I would never do.

Now that is partly "cheating" -- when the system was installed, it was not intended that the house be open all year (the dependency buildings still aren't), so the effect of the storm windows, general tightening up, and insulation has been to lower the "design" temperature from somewhere around 20 to -10 -- a quite considerable difference.

Seems to me that the ideal, of course, is to size the radiation to the design heat loss of the structure, and size the boiler to the radiation -- plus a factor for miscellaneous losses. And I would rather use the term miscellaneous losses than the term pickup, as the latter implies that all of those losses go away when the system is up to temperature. But do they? There will always be some heat loss from the steam mains, even though they are well insulated (and a total of almost 200 feet of three inch main is significant), and some loss from the risers, which are rarely insulated (mine aren't, for example, at all, although all the basement level runouts are). Do they amount to a third of the net output of the boiler? Unlikely. But are they zero? Also unlikely.

To me, the most prudent approach would be to size the boiler itself to include the pickup factor, and then down fire it so that it just barely manages the radiation -- which is what @Charlie from wmass has helped me do. What nominal "oversize" are we at now? Somewhere between 10 and 15 percent, I would say. It's a balancing act, though, between a little more delay in the radiators getting up to full output (that is not a function of venting, at least in this case) and the boiler shutting off on pressure.

PMJ

Harry,

I only just saw your post - I have a Mouat vapor system. About 1000 EDR in 3500sqft (or so the county says) 1926 house. Piping is large and Insulated. 1950's 460,000 input boiler that is downfired 25-30% as best as I can tell clocking the meter. Way more than needed - absolute max firing rate 50% of total time the way I run it which is plenty good down to -20F. Uninsulated brick with triple track storms. I like the extra EDR, love the huge pipes (big pipes make a lot of things a lot easier). Love the big boiler too - a real beauty now in her 60th year.

The system goes into natural vacuum every burner off cycle and have quite a few posts on the wall about that if you are interested.