Need help. Cant get my return side cold enough for radiant mixing valve

[lemahoney08]

I just hooked up a new wood burner. My outdoor water temp is 160 +-5. This then runs through a heat exchanger in the furnace as well as through slab radiant heat in basement floor. I have pump at burner running 24x7 so if the upstairs thermostat calls for heat the exchanger is ready. The radiant pump is wired into a baseboard thermostat so the basement doesnt overheat. I was trying to bring the slab up to about 100deg water slowly so was trying to start with about 80deg mixed water going in. However my problem is that the return from the heat exchanger is way hotter than that and transferring heat backwards to the return side of my radiant loop. So my mixing valve cold side is 150deg. Not sure how to get my mixing valve cold side temps down to where I could put 100 deg mixed water to my radiant pump. Did I miss something? In the picture the top pex to coper line is hot supply and the bottom is return. The copper lines going up are to my heat exchanger and return.

 

[maple1]

I can't get my head around what is going on there in that pic. Can you add arrows & words to it to tell what is going to & coming from where?


EDIT : But there must be something plumbed wrong here, because as soon as the floor circ starts, it should be sending cold water of whatever temp the cold floor is, into the mixing valve right off the bat.

EDIT AGAIN: If I'm seeing right, sure seems like it's plumbed wrong - there is only one place for the slab return water to go, after it leaves the circ? That is into the cold mixer port? Which isn't right. It should have two possible places to go - to that port, or back to the boiler/HX return. Also the circ should be pulling through the mixer. Those two pex pipes coming through the wall, together with the two coppers going up, have me confused. Along with the red & blue ball valve handles - usually red is for hot (supply) & blue is for cold (return).

 

[maple1]

OK, last reply before hearing back on some things.

I think I see what is going on here. You have a parallel setup, and the only HX is the one in your furnace.

And the mixed mixer port is the one on the bottom, and the circ is pulling through that. And the ball valve handles do show hot & cold. First look I was seeing flow going the other way, from what i was reading. The parallel setup is confusing.

The primary problem (I think) is that you have your hot supply hooked to the same 'in' port on the mixer, as the floor return is also sending (trying to send) water to. The floor return, should be T'd, before it sees the mixer. With one leg of the T going into the cold input side of the mixer, and the other leg going to boiler return. How you want to fix that or work it into the parallel setup, is another thing. I wouldn't have done anything parallel, I likely would have just started by plumbing the boiler supply through the furnace HX. One loop. And then running the floor supply & return via close T's, off the furnace HX return line. Primary/secondary.

And if this is an (unpressurized open) OWB you have, I would also likely run all the floor stuff off a flat plate HX, so that stuff is pressurized & closed. But that is getting a bit off course of the current problem.

 

[TCaldwell]

Maple is correct, the supply into your valve is from the boiler supply, the return into your valve is from the radiant return ( usually a t from the boiler return) and the third port is the mix supply. Typically most are labeled, A port, boiler supply, B port , return and AB is the mix supply. Check the valve instructions, most can be piped as a diverting application as well.

 

[maple1]

I quickly did this. It may work. Still not sure which port is hot supply & which is cold supply. But if hot supply port is on the right & cold supply port is on the left, shift the cold floor return to the left of the mixer using a T. The blue line.

 

[E Yoder]

I see the problem being that it's piped in parallel, with the pump pushing in through the underground piping putting pressure into the mixing valve. The only flow through the mixing valve should be the floor pump pulling through it. The orientation of the tees as it's piped now appears to be causing some ghost flow issues too.
It needs to be repiped as a secondary loop using close tees off the return line before it exits out through the wall. Currently it tees off both the supply and return. Research primary/secondary piping.

 

[peakbagger]

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[maple1]

I see the problem being that it's piped in parallel, with the pump pushing in through the underground piping putting pressure into the mixing valve. The only flow through the mixing valve should be the floor pump pulling through it. The orientation of the tees as it's piped now appears to be causing some ghost flow issues too.
It needs to be repiped as a secondary loop using close tees off the return line before it exits out through the wall. Currently it tees off both the supply and return. Research primary/secondary piping.

Yes, I was thinking that also but wasn't sure on the impact. I threw the above quick pic in as a suggestion on what MIGHT work that would be (relatively) easy to try. And I guess I wasn't too far off track with the primary/secondary thing, since someone else has suggested it now too.

Time to dig out the pipe cutters & soldering gear.