Multiple ODRs?

LukeWarm

I am having a radiant system installed in my house. About ½ the area is tube in concrete slab, and the rest is staple up in joist bays – therefore there are 2 different supply temperatures. The boiler is a Triangle Tube that has 2 separate outdoor reset curves. The manual says that the boiler will supply water temperature to satisfy the highest ODR curve when there are 2 zones calling for heat. The contractor wants to put in a thermostatic mixing valve to prevent the slab from receiving the higher temperature water when this happens. Seems to me that this is basically eliminating the ODR on the slab side as it will always get the design temperature water when the joisted area is always calling for heat unless ODR curve of the higher temperature lzone falls below the mixing valve setpoint. Is this acceptable or should I ask for an ODR mixing valve?

STEVEusaPA

That's one way...

The contractor has a good solution, ODR is better, if it doesn't conflict with the main control (shouldn't)...but their are better ones, that require some repiping, and different controls. 

How is it piped? Are both radiant zones using zone valves?

What are the design temps for each zone?

What's the floor covering on the staple up?

Zman

ODR

Tough one,

I love the TT boilers. This is their biggest weakness.



 Tekmar has great stuff that will always run the boiler at the lowest temp, it will mix down your slab when needed. They are not cheap.



The thermostatic valve will give you a good max protection but will not give you ODR when both zones are calling.



A non thermostatic will allow one zone to lag the other and have a reset curve. You would have to always run the boiler at the the higher temp costing you some efficiency.



You could use an odr mixer and keep the boiler the way it is. This solution may take a little longer to get dialed in but would work well. It would be a bit "messy" from a style point of view but would be inexpensive and work well.

Carl

SWEI

It's possible

Depending on zone temp requirements and sizing, you might be able to run one zone after the other (hot side of one fed from cold side of the other.)  Either directly in series (less likely) or with sequential closely-spaced tees (more likely.)

Jean-David Beyer

The manual says that the boiler will supply water temperature to satisfy the highest ODR curve when there are 2 zones calling for heat.

On my W-M Ultra, the decision is made based on the priority of the thermostats calling for heat. The one with the highest priority wins, and the reset curve for that priority applies.



In my system, the higher priority thermostat is the one that runs my radiant slab, the lower temperature one. The lower priority thermostat is the one that runs my small baseboard zone, the higher temperature one. (The highest priority is for the indirect water heater, but when that runs, all heat to the heating zones is cut off.)



So if both call for heat, the LOWER temperature ODR curve is the one that is applied. That means the radiant slab gets the heat it wants, and the baseboard gets some heat, but not as much as it might desire.  But I do not put overly hot water into the slab. If only the lower priority thermostat calls for heat, the baseboard gets its reset curve, and all the heat it wants.

Rich_49

How large

is the difference in water temps between the slab and staple up ?  I believe too many people get caught up in the overheating game . If you run one temp and zone aggressively or in your case zone the slab will be zoned differently than the staple up you do not HAVE TO use 2 SWTs or slab sensors .  Surface temp of your emitter determines temp in room , thermostat satisfies when setpoint is reached , zone shuts down , no more flow to floor , no more temp climb in emitter . Your slab may even be a bit more responsive but if zoned properly it will not overshoot .

   In very large houses however where there are several thousand BTUs to be had you should certainly run multiple temps .  If you do decide to use mixing , let the boiler control higher temp as single boiler setpoint and use a mixing device such as Taco I Series with ODR . Very easy to implement and your already slow responding slab won't be starved and have to play constant catch up .

LukeWarm

More info

Each temperature area is split into 3 zones (valved). Slabs should be at 85° and the joist area at 120°. There is a mix of joist trak in living/dining/BR (hardwood) and simple staple up in bath/kitchen (tile).



Jean David - interesting, but wouldn't help in my case, the living quarters are the joisted area, slab is only basement and workshop so higher priority would be higher temp anyway.



SWEI - I like that idea, could still have issues depending on number of zones for each heat range were active. Would I still need some kind of mixing valve to prevent too hot water to the slabs?



Sounds like the Taco I series w/ outdoor reset may be the easiest to implement and get correct temps to both areas, but I agree with Zman in that it isn't a very elegant solution. Seems like I'm paying for redundant systems to do the same thing. I like simple.

SWEI

More options

Mixing device would be best -- let the boiler run the higher curve, though I'd probably try running them in series first, just to see how close that got me.  With a mixing valve on closely-spaced tees that take off after the high heat zone, you will maximize boiler ∆T.



I'm not a fan of zone valves for most applications, but if you use the thermostats as high limit controllers with a properly tuned curve (or curves in this case) it can work.  Best practice IMO is to only use them where an external source of heat (solar gain, wood stove, kitchen heat, etc.) is likely to affect system balance.  I'd remove at least one stat from each temp zone if at all possible.