Separate circulating temperatures for zones on same radiant manifold

RascalOrnery

Hey All, I have a 5 zone manifold supplying 4 zones under wood flooring and 1 zone under tile flooring.

The degree difference between the two desired is between 5 and 10 degrees, currently I have them all the same, the tile is a tinge warm and the wood a tinge cold so I'm not seeing a drastic need for major difference between the two zones. It should be noted that these is supplemental heat only (for warm toesies) and is not controlled by a thermostat.

What is the best way to allow for individual mixing for the tile floor? I don't see anyway to do this without having a separate circulator for the tile zone, which is OK. But beyond that I'm scratching my head a little, this is the best I could come up with.

Basically since the whole system is thermostatically consistent through a mixing valve, a proportional valve to throttle flow will in turn control temperature. My thought was a slight choke to the supply from the main circulator and in return force the zone circulator to take on some of the return, but is it possible the main circulator will just overpower the zone circulator? Where do I go from here?

Maybe I'm really overthinking this and I just just the flow meter to reduce the flow to the zone via the manifold??

Jamie Hall

If you want to have different circulating temperatures in the two zones, you really need to have two separate mixing valves — and, correspondingly, two pumps pulling from the mix valve outlets. However, if there is a balance valve on the supply to the tile zone, you could try running the whole thing warmer and just throttling the flow to the wood floor zone. That may or may not work… depends as much as anything on how the wood floor piping is laid out.

hot_rod

If required SWT are within 8-10° of one another, no need to add a second mix station. Trying to adjust loop temperature by chocking flow may cause the end of the loop to run too cool.

Residential radiant loops design around a 10- 15° temperature droop across the loop.

Here is a look at how that plays out.

RascalOrnery

I'm a little confused, I think we all agree that choking the flow may help some areas to have the proper floor temp but at the price of some being too low.

What's the normal strategy to determining flow rate? I would imagine there's a lot of variables but I would think circuit lengths would have rough start points for flow rates to achieve somewhat uniform outputs, like the longer the run the faster the flow, but by how much and how little for short runs?

Why would the setup I sketched fail?

DCContrarian

"What's the normal strategy to determining flow rate?"

First you determine how much heat you want out of a zone. Then you decide what temperature drop you want between the sent water and returning water. The heat output (in BTU/hr) is equal to the temperature drop (in F) times the flow rate (in GPM) times 500.

From your description it sounds like you'd rather target a specific floor temperature rather than a heat output. If that's the case, the heat output (in BTU/hr) is equal to the difference between the room temperature and the floor temperature times the square footage of the floor, times 2.

RascalOrnery

Sure, that makes sense, but when it comes to a given floor surface temperature remaining nearly uniform through the majority of the loop, I feel like there has to be some starter flow rates in relation to loop lengths per different types of emitters to try to achieve that.
Having a little hard of a time wrapping my head around it, but would it be accurate to say, if the end of the loop is slightly cold, increase flow, if the end of the loop is near the same temperature as the beginning but the outputs suffers, increase the supply temp.

Still, does anyone think my diagram would most assuredly not work or cause other issues?

hot_rod

the loop will never be the same at the end as the beginning, it gives up heat along it’s entire length.

The design criteria for radiant floor loops is 10- 15 degree difference from supply to return.

Typical flow rate for 1/2” Pex loops is .50 - .65 gpm per loop.

Jamie Hall

As noted, the end of the loop will always have a lower temperature than the beginning if there is any flow at all. The more flow, the less the difference.

This is why, to maintain a reasonably uniform floor temperature, some type of looping back is needed, so that any part of the floor is served by piping relatively near the beginning of each loop and also by piping similarly near the end of the same loop. Either that, or relatively short loops. For reasonably uniform floor temperatures piping layout is critical.

Now if the loops are laid out in that manner, yes, you can reduce the overall average floor temperature by simply reducing the flow through the loop. This will give you a larger delta T, but since supply and return everywhere are close to each other this won't be a problem — the average temperature will simply be less and the floor cooler.

RascalOrnery

Thanks guys, I did run my first loops near all the wall edges and such to at least send the hottest water to the areas of most heat loss.