I have an open loop radiant system installed in my home with two 40 gallon hot water heaters in series. There are no valves to ensure water flow through the radiant PEX during the summer. I would like to convert my domestic hot water and radiant system to the following: 1) 1 hot water heater for domestic hot water (we only have 1 on during the summer anyway) 2) 1 hot water heater for the radiant system in a closed loop configuration Is this as simple as re-plumbing the two water heaters? Do I need any other components? Do I need to place any additives in the system? Freezing is not a concern but I currently have a heated towel rack that is off/dry because of corrosion fears with the open loop system.

I should mention that this system was installed ~ 2004/5 (i.e. uses brass fittings + PEX, not an "old" radiant system).

If you go from two to one heaters, you will have 1/2 the space heating capacity.

House square footage?
Alternately to dedicating one hot water heater you could consider a TACO X block. It will likley plumb in to your system nicely with minimal effort and reasonable cost.
This will isolate your heat system form potable with an flat plate heat exchanger and pick up some additional out door reset function.
Good plan to loose the open loop system,
Dan

We have designed hundreds of systems in the US and Canada used water heaters and Taco XPB's to drive radiant floors as Dan describes. We have many condensing water heaters providing 100% space and DHW in super-insulated or passive-houses and additions. Brilliant!

A dedicated atmospheric gas water heater for radiant floor heating must be protected from the corrosive affects of a condensing stack with a mixing control, built into the XPB.

Thanks folks. The radiant system has 2 zones; a 28x40ft concrete slab and ~a 900sq.ft. house addition (the rest of the house is on a ducted system).

The TACO X block looks like a good solution if I'm installing from scratch. However, I already have the pumps, controls, etc. for this existing 2 zone system... it looks like I'd need to buy 2 X-blocks to retrofit my current install. Is this going to be my most cost effective option?

If the PEX used in the original system isn't an oxygen-barrier type you may still have some corrosion issues (particularly for iron rather than bronze pumps), but they would be reduced by an order of magnitude or more over an open system.

1 x block,
swap pumps if non barrier pipe to non ferrous pumps (stainless or bronze). same with flanges.
potable H20 expansion tank, reduce pressure to 15 psi
X block has illustration fora 2 pump system on there PDF link http://media.blueridgecompany.com/documents/X-Pump-Block.pdf
You will want to add a Temperature/Pressure Gauge 0-60 psir for monitoring purpose and possibly a auto fill to keep system topped off.
Dan

if you already own 2 pumps, and they are nonferrous, you can reuse them on either side of a heat exchanger with an injection mixing control and do the same thing more cheaply. more work though.

The pumps and manifolds are bronze; I'll check the PEX when I get home (visually it's milky-white/clear).

Rob, if I'm using an injection mixing control then I'd have to split the hot water heaters (1 for radiant, 1 for domestic), right? Otherwise the water from the return outlet is just going to go back into my water heaters (domestic supply)?

Summary so far (please correct me if I'm wrong):
1) Keep both water heaters in the same circuit (run in parallel might be a plus?). Get TACO XPB and insert it prior to the existing circulators. There is currently a H2O expansion tank between the water heaters and existing circulators. Do I need to add another one between the XPB and exiting circulators as well (i.e. water heater->tank->XPB->tank->circulator)? Should I place any additives in the new closed loop system's water (anti-corrosion?). My understanding is that I do NOT want to replace the water in the closed loop periodically?
2) Split the two heaters; one for radiant, one for domestic. Lose some capacity from both systems. Install a mixing valve to prevent ultra-hot water from hitting the downstream components. Is the mixing valve required if I turn the heat down on the water heater?

Thanks for the great advice!

no, with correct piping you can draw from both and return to both heaters. run in paralell, with a reverse/return configuration if necessary.

it is very unlikely if you have two heaters here that one of them is well sized for your heating load. maybe..

Posted By NRT.Rob on 20 Aug 2012 03:33 PM
no, with correct piping you can draw from both and return to both heaters. run in paralell, with a reverse/return configuration if necessary.

it is very unlikely if you have two heaters here that one of them is well sized for your heating load. maybe..

Gotcha.

Posted By NRT.Rob on 20 Aug 2012 03:33 PM
if you already own 2 pumps, and they are nonferrous, you can reuse them on either side of a heat exchanger with an injection mixing control and do the same thing more cheaply. more work though.

Would a mixing valve work in this application or am I going to have to get a mixing control (Taco RMB, etc.)? It seems that the RMB and XPB are fairly idiot-proof where the other options require very installation specific pipe sizing/spacing, balancing valves, etc. Is the XPB an RMB + heat exchanger block? It looks like I do NOT need an expansion tank on the supply side of the XPB (only need 1 in the closed loop + a blowoff valve)?

an injection mixing control would be cheaper than a motorized mixing valve and not too much more than a fixed temp valve.

it sounds like you need some design help, you're kind of starting from zero here. but in concept I would recommend variable speed injection mixing through a heat exchanger, like the XPB does.

Posted By NRT.Rob on 21 Aug 2012 09:20 AM
it sounds like you need some design help, you're kind of starting from zero here.

Yeah, I was hoping for a "cut here, solder there" kind of solution but it looks like it's not going to be that easy (to do it the right way at least).

Not so,
The X block is a cut in solution, you have bronze pumps, you are interrupting the supply and return to the pumps with the x block.
Then you will add make up water, expansion tank, air scrubber, on the heat side side.
Dan

there is a bit more to it than that. I'm not saying a full design needs to be done here but at least an examination of what is installed and connected needs to be done. design by forum doesn't sound like it's going to cut it.

If the system has been heating this 2,000 square foot area fine already and we assume it has, or at least I did not read of an issue, I could be wrong then all the x block is doing is injecting Via 2 pumps and a heat ex changer with out door reset all built in to one unit.
If one follows the directions provided by the factory it is a straight forward injection of parts.
For a home owner DUI the x block presents a cleaner solution than the build your own system, granted the cost is a bit more, but a lot of complex issues are solved in one bite with directions.
But it is just an option of many.
Good news is its one less open loop system,
Dan

To clarify, the current open loop system is heating the 2 zones without any problems. The only other change would be to change the water heater configuration (two in series to two in parallel). I'd be adding a shut off to the one water heater because we only need it when the heating system is active.

Here are some pictures of my existing system.  The parts are Taco SR 501 relays and 008-BF6 cartridge circulators (bronze) + expansion tank & air eliminator.  When looking at the first picture, there are 6 vertical pipes.  From left to right, they are:

1) Domestic hot water
2) Radiant supply to circulator (garage zone)
3) Cold water in from street
4) Radiant supply to circulator (house addition).  The output from the circulator splits into a copper line and a PEX line.  Any idea why this was done?  Is the addition's hot water being supplied through the circulator ?!?
5) Cold water into water heater (1-way valve installed).  I had to install this because when the pumps were operating we got hot water pushed back into our domestic cold water pipes.
6) Radiant water return to water heater (enters bottom of water heater)

Question: what is salvageable from this system?  Should I consider new relays or can these tie into (signal) the XPB? 

OK, trying to post the images again...