Sizing a Radiant Radiator for Steam

chris_69

Old house with one-pipe steam. The house is over-radiated, so the boiler doesn't fire very often, even on the coldest days.

Luckily, those honking big old cast-iron radiators hold the heat nicely, evening out the heat between firing, and the result is OK heat.

There's a room under renovation -- In order to fit everything in, need to replace the existing 3-column monster with a radiant radiator recessed into the wall, like a Sunrad.

Now, if I did a heat loss and made the radiator the "right size" for the room according to the heat loss then it would really be too small because, since the rest of the house is over-radiated, the burner isn't going to fire often enough.

So, I probably ought to make the new radiator the same size as the old. But that still might not be right -- two radiators of the same E.D.R. aren't necessarily going to heat the same way if one of them is lightweight (heats up quickly and then loses the heat quickly when the boiler cuts out) and the other is heavy (holds the heat longer). -- the temp. in the room with the light radiator is going to jump around more than in the other room.

So, I guess I really have three questions:

1) Does a cast-iron radiant radiator act very different than an old 3-colum? (is it much lighter or much heavier or "faster" or "slower"?)

2) If it's recessed, you're supposed to add 10%. But if I go with a radiator that is "too big" - so as to match this room to the rest of the house - should I still add the 10%?

3) If the sunrad acts a lot "lighter" than the old 3-column, can I do anything to make up the difference? Like, say, install it with a hunk of cast iron touching it so the cast iron soaks up some of the heat and acts to even things out between firings?

Patrick Linhardt, \"Linhardt's Field Guide to Ste

interesting question

Hi Chris,

Some things to consider when mixing types of radiation on one pipe systems.

1) The mass of the radiator, or weight of the cast iron, has an effect. A lighter radiator heats up quicker, and also cools faster.

2) The amount of air the radiation can hold has an effect. The less air, the faster it will heat up. That's a good thing, since your proposed radiator will have significantly less air than a column radiator.

3) The size of the supply opening and supply valve. Will the new unit have a large enough opening? Some modern radiation may not. This could be a deal breaker.

4) The type of vent you use. This can be easily resolved with proper selection or an adjustable model.

5) Are you moving the location, or will the new radiator be the same distance from the main? There are limitations to the distance the radaitor can be located from the main. Pipe size and pitch can be the limiting factors.

In the Library section of this website, you can find a good read on this subject. Under the Steam Heating category, look for "Controlled Air Venting of one pipe steam systems". With one pipe, its all in the details.

Best regards, Pat

chris_69

Thanks! Also Q re: runouts

Thanks for your comments and for pointing me to the articles. I appreciate you taking the time.

The old radiator is right next to a riser supplied right off a tee in the riser. New location is about 3 feet away, so I don't think added pipe length is going to be a problem. For that 3 foot distance into 30 sq ft of radiator, would you run a plain piece of horizontal runout, or does it need properly swung joints and pitched runout to avoid having condensate slopping around in the runout pipe?

brucewo1b

Pitch

3 feet still needs pitch so as to not trap water wether there is enough in the pipe will decide if you need swing joint

B.S.

chris_69

How good is good enuff?

So, threading a piece of 3 foot long inch and a half into a tee that's already in the riser (so the 3' piece sticks out horizontally) and then forcing the other end up by about 3/4 of an inch to give it a little pitch -- would the Dead Men smile on that or tell me I was being half-arsed?

brucewo1b

Beware what might happen below

if you force it to much

B.S.

chris_69

Cheating on swing joints

I know, I'm not being cheap about a couple of elbows, it's just in an impossible place to work unless you have three elbow joints in your arm.

I wonder if a trained elephant would make a great assistant -- think of the grip he could get reaching that trunk up into an otherwise inaccessable space and wrapping it around a piece of pipe!