3 way mixing valve questions

[Scotty2]

I'm using a Caleffi 5231 3 way mixing valve and having trouble understanding how these things work. This style has the numbered knob on top...e.g. 1 is coolest, 7 is hottest. It was set by plumber upon install at #3 on the dial (supposedly 117 degrees)...I've moved it to 4.5 (125 degrees)
The 1800 gallon water tank got to the high temperature end today...192 going to the water/water exchanger, yet after passing through another 2 loops it still reads 190 going to the mixing valve and 125 on the side of the valve going to the radiant floor.
If I want to pull the temperature down in the tank, can I do this by turning the Mixing Valve up? In effect dumping more heat to the slab? Can a slab get too hot?
Would I be better to turn up the temperature request for the slab to keep the water moving? or both?
Regards, Scotty

 

[maple1]

Your slab should be controlled by how warm you want the room it is in. If your storage is too warm, let the fire go out. Not clear on exactly your problem or situation though.

 

[velvetfoot]

I have that valve for tempering down the dhw coming out of my buffer tank.
Now, I don't have a wood boiler (pellet) so I'm sure I don't understand fully.
Your problem is that you have TOO MUCH heat in your 1800 gallon tank? I'd say, everybody should have that problem! If you want the tank temp to drop, just don't start a fire for a while!

You're using the mixing valve to put a certain temperature of water to the slab. That's the job of the valve, I'd say. The temperature of the top of the tank will probably stay high for a while, while the tank cools off from bottom to top (stratification). I'd say a good thing about a slab is that you can use lower temperature water, and can use your storage down to lower temperatures before you have to fire up the boiler again. I don't know if you can overheat the slab-the living space would get hotter at a minimum.

Sorry, I guess I don't understand your problem and hopefully someone else with a similar setup will chime in.

 

[Bob Rohr]

I'm using a Caleffi 5231 3 way mixing valve and having trouble understanding how these things work. This style has the numbered knob on top...e.g. 1 is coolest, 7 is hottest. It was set by plumber upon install at #3 on the dial (supposedly 117 degrees)...I've moved it to 4.5 (125 degrees)
The 1800 gallon water tank got to the high temperature end today...192 going to the water/water exchanger, yet after passing through another 2 loops it still reads 190 going to the mixing valve and 125 on the side of the valve going to the radiant floor.
If I want to pull the temperature down in the tank, can I do this by turning the Mixing Valve up? In effect dumping more heat to the slab? Can a slab get too hot?
Would I be better to turn up the temperature request for the slap to keep the water moving? or both?
Regards, Scotty

The mixing valve needs a supply of hot on the H port, and cooler water at the C side. Pump away from the M mix port.

There needs to be a 25° difference between the hot inlet and the mix temperature to be accurate.

In other words to mix down to 120, you want at least 140 on the H port.

Got a drawing of how it is installed and being used? is it mixing down a radiant zone from that192° tank?

 

[Scotty2]

The mixing valve needs a supply of hot on the H port, and cooler water at the C side. Pump away from the M mix port.

There needs to be a 25° difference between the hot inlet and the mix temperature to be accurate.

In other words to mix down to 120, you want at least 140 on the H port.

Got a drawing of how it is installed and being used? is it mixing down a radiant zone from that192° tank?

I'll attempt a poorly drawn sketch...
25 degree difference? So if I usually have trouble keeping the water at 140 or above, I should lower the mixing valve temp until I get the 25 degree differential? Otherwise I need to keep the water temp higher so I can raise the MV?

 

[jebatty]

A few comments. A flat plate heat exchanger is normally plumbed the opposite of your drawing, that is, if the top fitting from the tank is supply and bottom fitting is return, then the opposite top fitting is system return and opposite bottom fitting is system supply. This will give you the hottest exchange temperature.

A 10F temperature drop across a HE is "normal." Less than a 10F differential (close approach) is possible by sizing the HE appropriately. But losing 20F from the tank to the mixing valve suggests an improperly sized HE, poor insulation on your supply/return lines, or both.

Your diagram shows 115F supply to the pex loops and 110F return. At 1 gpm for each of the 4 loops, or 4 gpm total, that equates to a transfer of 10,000 Btuh, really low in my opinion. You must have a need for very little heat from you slab. And 1 gpm per 500 foot loop is very high flow for 1/2" pex. I have 1/2" pex with six loops, each 250-300 feet, my mixed supply is 100F, and return from the loops is 70F, floor sensor set at constant 61F. My total flow is 2.5 gpm, and a 30F temperature drop, for 37,500 Btuh heat transfer to the slab.

That 20F temperature drop to the mixing valve at 4 gpm is 40,000 Btuh from your tank, but only 10,000 Btuh to your slab, for a loss of 30,000 Btuh to the netherlands. I tend to think that you probably don't have a mixing valve problem, but rather a system design problem.

 

[Bob Rohr]

A few comments. A flat plate heat exchanger is normally plumbed the opposite of your drawing, that is, if the top fitting from the tank is supply and bottom fitting is return, then the opposite top fitting is system return and opposite bottom fitting is system supply. This will give you the hottest exchange temperature.

A 10F temperature drop across a HE is "normal." Less than a 10F differential (close approach) is possible by sizing the HE appropriately. But losing 20F from the tank to the mixing valve suggests an improperly sized HE, poor insulation on your supply/return lines, or both.

Your diagram shows 115F supply to the pex loops and 110F return. At 1 gpm for each of the 4 loops, or 4 gpm total, that equates to a transfer of 10,000 Btuh, really low in my opinion. You must have a need for very little heat from you slab. And 1 gpm per 500 foot loop is very high flow for 1/2" pex. I have 1/2" pex with six loops, each 250-300 feet, my mixed supply is 100F, and return from the loops is 70F, floor sensor set at constant 61F. My total flow is 2.5 gpm, and a 30F temperature drop, for 37,500 Btuh heat transfer to the slab.

That 20F temperature drop to the mixing valve at 4 gpm is 40,000 Btuh from your tank, but only 10,000 Btuh to your slab, for a loss of 30,000 Btuh to the netherlands. I tend to think that you probably don't have a mixing valve problem, but rather a system design problem.

What happens if you loose that differential the valve will "hunt" unable to keep a stable output. It never closes down it looses accuracy.

To make that system work you need multiple pumps.

P1 heat from tank to HX
P2 heat from HX to primary loop
P3 circulates primary loop, this must be in place to supply H port of mix valve wit hot supply
P4 is the loop pump, it needs to be sized to overcome the pressure drop of the valve and attached piping. Over 3-5 gpm will require a fairly
large pump. Thermostat mix valves have a lot of pressure drop.

 

[Scotty2]

Here is a better sketch...I hope.
I took the temp readings this eve...outdoor temp at 32 and fire had been out a couple of hours.
HE is plumbed as you suggested...5"x12" 50 plate...300,000-500,000 btu? (wrapped in rock wool...didn't open it up to see label)
There are 4 pumps, with the largest (Taco 0011 1/8) as the loop pump (to the warehouse)
There is a 20-25 degree spread between H and M on Mixing Valve
There is 122 into HE, 110 to primary loop, and 120 to MV (how is that possible?), then 100 to main loop (warehouse)
Pex in warehouse is showing 100 so minimal temp drop in 125' from shed to warehouse. Temp back to primary loop is 85.

Takes hours to raise warehouse temp from 50 to 60. Am I pumping too large a volume of water through 1/2" lines...e.g. not enough time to pull the btu from the water?
Or does it appear that I need to insulate the beejeeus out of the Quonset walls?.
no insulation under the pex...only around the slab/foundo seam.
water table is low

Ahh, what was the name of the recommended book on Radiant available on Amazon?

Regards, Scotty

 

[maple1]

Random:

I don't know how accurate your diagram is, but I see some flows going in the wrong direction (including at the HE - the flows should be against each other), and P4 is in the wrong place.

If your slab isn't insulated underneath, that is killing your heat.

If your quonset isn't insulated, that is killing your heat.

122 out of storage? That is not very warm, mine would be about depleted at that point.

Yes, the 120 at P4 makes no sense - so your temp measuring is likely off, maybe throughout?

 

[Scotty2]

Random:

I don't know how accurate your diagram is, but I see some flows going in the wrong direction (including at the HE - the flows should be against each other), and P4 is in the wrong place.

If your slab isn't insulated underneath, that is killing your heat.

If your quonset isn't insulated, that is killing your heat.

122 out of storage? That is not very warm, mine would be about depleted at that point.

Yes, the 120 at P4 makes no sense - so your temp measuring is likely off, maybe throughout?

Where should the P4 be in relation to the diagram? I have it pumping into the MV currently. Should it be between the MV and the loop to the warehouse?
In the HE I show the hottest inbound from tank opposing the coolest return from primary loop...that's wrong?
The primary loop p3...is that pumping the correct direction? (assuming the sketch is correct?)
I will always regret taking the advice of ___ over at _____ mfg. With his comprehensive website at _____.com and his awshucks attitude, and his onsite visit, he convinced me to not insulate under the slab and use 500' runs of 1/2". Not something I can change, however I will be investigating how to best insulate a Quonset hut this coming summer.
I found the name of Holohans 2 books...I'm ordering today...a lot of winter reading coming up.
(I thought it politically correct to omit the names above as they're not given an opportunity to explain themselves)
Scotty

 

[maple1]

Where should the P4 be in relation to the diagram? I have it pumping into the MV currently. Should it be between the MV and the loop to the warehouse?
In the HE I show the hottest inbound from tank opposing the coolest return from primary loop...that's wrong?
The primary loop p3...is that pumping the correct direction? (assuming the sketch is correct?)
I will always regret taking the advice of Fred over at Seton mfg. With his comprehensive website at Rohor.com and his awshucks attitude, and his onsite visit, he convinced me to not insulated under the slab and use 500' runs of 1/2". Not something I can change, however I will be investigating how to best insulate a Quonset hut this coming summer.
I found the name of Holohans 2 books...I'm ordering today...a lot of winter reading coming up.
Scotty

P4, I would put on the other side of the MV so it can pull both legs through the MV. The way it is there, the flow out of P4 is against the mix leg flow you want coming into the MV.

The HE flows should be against each other, generally speaking, not going same direction. Not sure the magnitude of impact though.

Yes, I think P3 is going the wrong way - but vs. P2. If you reverse P2, that would fix that.

That tank supply temp of 122 is awfully low? Also should see a dT of 20° or so across the HE - so seems like there's some flow imbalance there somewhere, but then again not sure how accurate your temp readings are.

You're likely pretty hamstrung by your slab piping & lack of insulation too, but not much that can be done about that at this point - I suspect your mixing valve isn't up to the flows that the 3500 ft. of 1/2" pex is needing - maybe.

 

[Scotty2]

P4, I would put on the other side of the MV so it can pull both legs through the MV. The way it is there, the flow out of P4 is against the mix leg flow you want coming into the MV.

The HE flows should be against each other, generally speaking, not going same direction. Not sure the magnitude of impact though.

Yes, I think P3 is going the wrong way - but vs. P2. If you reverse P2, that would fix that.

That tank supply temp of 122 is awfully low? Also should see a dT of 20° or so across the HE - so seems like there's some flow imbalance there somewhere, but then again not sure how accurate your temp readings are.

You're likely pretty hamstrung by your slab piping & lack of insulation too, but not much that can be done about that at this point - I suspect your mixing valve isn't up to the flows that the 3500 ft. of 1/2" pex is needing - maybe.

I went back out in the daylight, opened up some of the insulation so I could get a better view.
The P4 pump is where you suggest...thus pulling both H and C through the MV.
The P2 is reversed compared to the schematic I loaded last night
Mixing Valve is Caleffi 5231...shows recommended flow rates between 4.4 and 40 for temperature stability.
As to temp readings...I'll use my 'keychain' thermal temperature tester to compare
Thanks for all the input everyone...I'll start saving up to insulate the Quonset better this summer. Oh yeah! I have lots of stuff directly on the floor. Would I be better off to put it on pallets so it's not sucking the heat out of the slab?