Why not size to heat loss?

Patrick_North

<span style="font-size:12pt">I’m in the midst of calculating heatloss and tallying EDR for my church and church rectory (I’m not a pro) in the hopes that we can get away with a smaller replacement boiler when the time comes. This is one pipe steam.  I’m finding the overall radiation is much larger than heatloss, though evenly so, room by room. In other words, each room’s radiator exceeds that room’s heatloss by about the same degree. </span>

<span style="font-size:12pt">While it would be great to eliminate and rearrange radiators to match heatloss nicely, this could be tricky.</span>

<span style="font-size:12pt">This seems like an ideal application of Dave “Boilerpro” Bunnell’s thoughts on sizing steam boilers; (to oversimplify) size to heatloss, vent mains completely, then vent rads slowly. However, I’ve read enough that seems to contradict this method (without addressing it directly) to give one pause.</span>

<span style="font-size:12pt">Any thoughts on how to proceed? Our choice of local contractors is a bit less than “Wall Grade.”</span>

<span style="font-size:12pt">Thanks,</span>

<span style="font-size:12pt">Patrick</span>

Steamhead

How much more

percentage-wise, than the rooms' heat losses? We'd want to know this to compare it to similar projects.



We don't often see undersized steam boilers, but the ones I've run into always had distribution problems.



Is the system big enough that you could use low-high-low firing?

Patrick_North

Can't say, yet...

I hope to complete my data gathering this weekend and can report back then. A complicating factor here- the buildings have essentially no insulation. Ideally we'll be able to square away all of the big insulation projects first, but it's possible that we'll be forced to replace the boiler first. And no, hi-lo is almost certainly out- we're talking residential sized boilers (two separate buildings). 

With insulation (I haven't run any of these numbers) I wouldn't be surprised to find the radiation oversized by 100%. In any case, oversized enough to be irksome!

Were the distribution problems you've seen in cases like this remedied with tweaks to venting or are we talking hopeless case?

Thanks,

Patrick

Mike Kusiak_2

Useage of buildings?

Something to consider in sizing the boilers may be how the buildings are used. It seems like you have two different situations here. The church which is used on only certain days and the parish house which may be occupied continuously.



In the case of the church, you may want to optimize the system for optimum recovery from setback. I am involved with a church project in which the building is normally kept at 50F, and only on Sunday or special occasions raised to 70F. Here the idea is to get the building up to temperature most efficiently. We have been experimenting with the firing rate of the existing boiler so that the building recovers in a reasonable amount of time, yet does not cycle on pressure during the several hour recovery. This information will be useful in choosing the size and firing rate of the eventual replacement boiler. In this case, accurate measurement of the actual EDR is difficult, as all the radiation is contained in various cabinets, which either limit or increase air flow and affect EDR dependent on the specific location.



http://www.heatinghelp.com/forum-thread/133219/Replacement-for-a-Smith-Mills-2000L

Jamie Hall

Matching heat loss...

Several considerations here.



First, Mike has a very good point.  In the sanctuary space, you are really not interested in matching heat loss -- you are interested (if your church is anything like mine!) in recovering from a very very deep setback in a reasonable length of time -- like a few hours.  You will need a lot more heat output in there than the heat loss would suggest to do that.  You just won't be using it all that often.



Second, a significant undersize of steam delivery relative to radiation is going to cause distrubution problems, even if the mains are vented like mad and insulated like mad as well.



Third, is this cast iron radiation?  Then, in my humble opinion, oversized radiation (and a boiler to match) just isn't a problem.  You get the radiators nice and warm -- evenly!! -- and then just let the system coast while the radiators do their thing.  With proper controls, all that will happen is that the boiler won't be firing all that often.  If your controls are correct (this may take some fiddling) your boiler will supply enough heat to match the heat loss.



Keep firmly in mind while you are doing all this that a system with a certain heat loss on a design day of, say, 0 Fahrenheit and a delta T inside to outside of 70 will be, automatically, twice as big as needed (100% oversized) on a day when the outside temp. is 35 and the delta T is 35!  And yet it works, and most folks don't complain...