Simple steam heat questions

Bob W._3

If your steam boiler is not oversized enough, couldn't you do a condensate hot water loop? Just curious.

http://www.heatinghelp.com/heating_howcome6.cfm

Nick_21

I need education

In the remodel we are doing (2 storey addition + basement) I am wondering if we can extend the steam heat system we have, and which, with a few (mild) noises works very well. I have a lot of questions, and while I will have professionals to do the job I would like to understand some of the basic factors going in.

We have a Weil-McLain SGO-3 boiler (4 yrs old) which lists a heating capacity of 114K btu (gross) 354 sq ft EDR. We are going from 1420 sq ft to almost 2300 sq ft. The heat loss calculation I did gives me a value of 75-80K btu for old plus new (fudged because the new addition will have more insulation then the old part). We will be adding radiators for 4 new rooms. So how do I calculate this?

1) I have the heat loss value of 75-80K btu
2) An area calculation would suggest I need 2300 x 45 (btu/sq ft) = 103K btu
3) I have the gross output value of 114K btu
4) I have a net output value of 85K btu
5) The EDR suggests I can get a maximum output of 354 x 240 = 85K btu

Which is the appropriate calculation to determine the required output figure (I am assuming the heat loss calc)?
What is the appropriate figure for boiler/system capacity which I should compare to this to determine if I have sufficient capacity?

Mark_60

You may be better off sizing to your sq ft of steam. Steam boilers should be sized to the radiation. Calculate the sq ft of steam needed to satisfy your existing radiaton and figure out how much radiaton you need to add. Once you have the total sq ft compare this to the output of the boiler. If the boiler is capable of satisfying the total radiation load you will be ok.

Mike T., Swampeast MO

As you noticed, steam boilers frequently have an EDR rating.

EDR stands for equivalence of direct radiation and is measured in square feet.

Steam boilers are sized based on EDR (and piping), not heat loss. For instance your boiler would be nearly perfect for a system with 350 square feet of EDR in the radiators and "typical" piping.

First, find the EDR of your radiators. Ratings were quite standardized and you'll find such for common radiators at this site. Look under "Books and More" (Dan gives very useful "teasers"), "The Library" and "Hot Tech Topics".

Once you know the EDR of the existing radiation you'll have a very good idea of how much (if any) reserve is available in the boiler.

Watch out when it comes time to size the new radiation for the addition! You want to compute heat loss for every room--new AND old. Each square foot of EDR is good for 240 btu/hr with steam. Compute the actual heating ability of the rads in the old spaces and compare to your computed heat loss. You'll likely find that the rads are significantly oversized (have much greater output ability than the loss).

When you size the NEW radiation, size similarly to the relation of the old radiators to the associated heat loss. If you find 30% of general oversizing, increase the new radiators by 30% as well. If you size the new radiation strictly to the heat loss without considering the existing, there's a very good chance that the new areas will underheat significantly compared to the old. Do note however that many people prefer cooler sleeping rooms, so some undersizing in such areas is frequently preferred. Conversely, many people prefer warmer bathrooms, so some oversizing there is often desirable as well.

You may well find a significant difference in sizing of the old radiators based on ground floor/upper floor(s). Remove any insulation from the heat loss and you'll probably see why. While the upper floors (typically with bedrooms) may have been originally slightly undersized (rising air and the bedroom preference) insulation of the ceiling/roof creates the opposite situation.

If you have a one-pipe system and your radiators are sizable (they'll have small rods running across at top and bottom) you can technically re-size and re-engineer the entire system to the actual heat loss. If you don't have significant "reserve" available in your current boiler (sounds likely) you may well be able to use it if you can re-size all of the radiation. (Do expect "ghosts" from the old radiator feet than cannot be disguised however--call it "patina").

Nick_21

that's the next possibility

Thanks for your help. I will continue to work on sizing the radiators to the boiler output. A hot water loop is the fall-back if things get too complicated