Boiler size for Hot Water Loop?

vaporvac

Hello all,



I'm thinking of adding a HWloop off of the boiler for radiant heat in my kitchen.  It currently has no radiators, depending on a kickspace forced air unit  which we installed during the kitchen redo.  We had removed the kitchen radiator, but plan to reinstall it in the foyer where a similar sized one had been removed.

I'm also toying with the notion of radiant flooring in a 1st Floor solarium.  There are two large radiators, but with the expanse of windows (even with good interior storms), and the fact that it sits over a well-insulated, but unheated garage, it is really unusable in the winter.

There is also a rather large space we would like to install radiant flooring on a concrete base, but we would not be using that on a regular basis or at the same time as the solarium or kitchen.  We might also consider using glycol there if that makes a difference to the set-up.

I'd also like an indirect HW heater.

The question is : how much bigger should one size the boiler to allow for this and is it worth it?  Is there some chart to which I can refer?  Are Indirects as efficient in practice as they would appear in theory?

I have  a Trane 2-pipe VaporVacuum System with gas boiler and will be replacing the leaking boiler when the weather  warms up a bit.  I'm just trying to figure everything out first so I don't regret not have included some change when I had the chance. I've posted prior so feel free to research the history of my system.

Thanks everyone for the immeasurable help you've provided.



Colleen

Brad White

Break out the HW BTU load

Colleen,

The question you raise is, (as I read it), will the HW draw quench or defeat the steam production? It will not and this is why:



As an obvious statement, the total BTU output of the boiler has to at least match the heat loss of the house and being a steam system, the steam generation capacity has to fill the radiators.



If your house has an EDR of, oh, 400 SF EDR but a third of that load (24 MBH) will be HW, you still have 300 SF of steam radiation to fill. The boiler can still generate, potentially, enough steam to fill 400 SF without the HW side drawing from that.  No problem there.



 In our example, your 24 MBH if a 20 degree temperature drop (say 210 to 190 when steaming or between steaming cycles) will still be returning 2.4 gpm at a temperature very close to what condensate will be returning from the steam side (at whatever pressure). The boiler will not see a difference and will generate the sensible heat required to bring the water back to boiling, then add the latent heat to again make steam.



If HW only is needed,  you are firing to sub-steam temperatures, so no effect of course because the boiler is not steaming.



An indirect is for practical purposes, 98% efficient, the losses being the jacket losses of about one degree per hour, not even.  (2 inches of urethane foam around a 200F tank in a 60 degree ambient basement would lose 11.6 BTUH per SF of tank surface. A 45 gallon tank with, say 25 SF of outer surface, would lose 300 BTUH per hour, less than one degree per hour standby losses.)  Exchanger coefficients are high in single-wall designs plus you have parasitic pump losses.

vaporvac

Is there a Chart?

Thanks for that answer, Brad.  It helped alot.   Is there a way  to size the boiler to account for the hw loop.  I'm not sure what boiler I'll go with or whether I will go with a multi-stage (for a later post), but I would like to ensure that it is large enough.  I thought there might be a chart or something.  Any thoughts, anyone?  thanks.



Colleen

Brad White

No chart that I know of, Colleen

Just the boiler selection tables you would use to select any boiler.

This is not to say that others may not have what you seek though!

It is all BTUs in the end.



Brad