Need Help Sizing Boiler

John_201

I have completed the Slant-Fin calculations and actually measured everything. It came up with a heat loss of 48500. When in doubt on some items I used the more conservative parameter to come up with a "worst case" number.

Now, what size boiler do I need without getting one that is too large? It is for a single loop in-concrete system. Thanks

kpc_10

oil or gas?

if you sized it to -10F... Whaen you look at the boiler outputs there will be 2 #'s...go w/ he lower(IBR) number. 50K should do the trick....is that all that will be on the system? kpc

John_201

It is gas. I went with -10F. That is rare here in Central Indiana but does happen every few years. The in-concrete radiant is the only thing on the system. No DHW. Thanks

Uni R_2

I doubt you'd even need 35 MBH at design

I certainly hope you're getting a modulating condensing boiler.

Brad White_185

Boiler size-

I agree with UniR that your actual heat loss will be less in practice, plus with the storage mass of concrete, you could calculate the heat loss for the 99.6% column per ASHRAE.

For example, in Indianapolis this is -3F. and the "Median of Extreme Lows" is -8F. Your actual location may vary.

(See the attached PDF, I also have data for Evansville Regional, Fort Wayne/Baer, Terre Haute/Hulman, and South Bend if you like.)

That said, the concrete thermal storage could get you through some really cold hours and catch up on the other side without you feeling much if any drop.

Regardless, finding Modulating Condensing (Mod-Con) boilers that small is not easy. Mod-Cons are all you should be considering at this point, I think most would advise you.

The Lochinvar Knight line has a 50 MBH input wall-mounted model and I heard (rumor so far) that there may be a 60 MBH Triangle Tube Prestige in the works. The venerable HTP Munchkin also has a 50 MBH offering in both a wall-hung and stand-mount version. There may be others but I cannot think of any. The next increment I know of is the 80 MBH range, then the low 90's and over 100 MBH.

Part of me says to set that boiler up for some domestic hot water production at least as a back-up to your other means. This will absorb any "overages" in capacity when the weather is much above design cold days plus acts as some insurance. Nothing wrong with "simple heating only" though!

Nice project and comfortable approach you are taking! Insulate that slab well on all five sides, leave the top open

Brad