Radiant Zone Off a Gravity Boiler

ARsales1

Folks,
I just received a phone call from one of my most excellent contractors asking me if he could do a radiant job of 20,000 BTU's and pipe it off of a gravity boiler that the homeowners are extremely happy with. he only needs to inject .57 GPM off of the boiler loop. my concern is how we might disrupt the gravity flow of the existing part of the home. Will it have an effect? If we pipe and valve it correctly we're only going to move a bit more that 1/2 GPM through the boiler on the worst/coldest day of the year.

Firecontrol933

Closely spaced tee's on the supply line leading out of the boiler? I would stay away from the boiler itself because it's any one's guess what kind of sediment might be in there that a circulator could stir up and inject into your radiant system.

Just thinking out loud.

Dave H_2

what about using the RMB or the X-Block? I was thinking the same thing with the closely spaced tees, just afraid of inducing flow in the existing system

Steamhead

How about some sort of side-arm heat exchanger circulating by gravity on the boiler side and pumped on the radiant side?

Danny Scully

@Steamhead, that's the ticket. Everhot has 2" tappings.

Ironman

I would recommend either method mentioned using variable speed injection mixing and Tekmar's 356. The injection bridge would require minimal flow while the radiant loop circ maintained a separate flow rate.

I have one MAJOR question: how would you prevent gravity flow to the rest of the house when only the radiant was calling? Or, how would you get the boiler to fire if only the radiant needed heat without heating the rest of the house?

Danny Scully

Strap-on aquastat?

Ironman

Danny Scully said:

Strap-on aquastat?

Ok, but how do you bring the boiler on for radiant only? Any time you fire the boiler, you're gonna induce gravity flow in the house. Most any radiant, especially in slab, will require more run time than the C.I. rads in the house. So, how do you stop the gravity portion from heating when only the radiant calls?

The simple answer is you can't unless you create two separate zones. That means zone with two circulators or one circ and at least two zone valves. I don't see zone valves being a good option due to the sized required for the large gravity pipes. Not to mention what effect they may have on a gravity flow system. Either way, it wouldn't be gravity anymore if you have to add a circ(s).

4Johnpipe

What is in place for domestic hot water? Perhaps an upgrade to a new water heater suitable for space heating like the HTP product line of water heaters...