Bypass 3-way thermostatic mixing valve HELP

NO2nice

This is my second radiant setup and I decided to go simpler than my last house and use 3way thermostatic valves instead of the expensive electronic mixing valves. There was an existing HW baseboard loop that we cut out and converted to radiant heat. I piped everything based on the attached drawing, the only difference is the primary circulator is on the supply side not return. I attached photos of the installation. After I initially bled the system it worked great, the boiler bypass valve was at 135 and the radiant heat was a solid 110. For some reason it does not work anymore. The primary loop works and I can control the temperature but the secondary loop only goes to 80 degrees and the hot water stops at the first of the close spaced Tees when I put my hand there one tee is cold and the other is hot. I am scratching my head and haven’t had enough time to really troubleshoot this. I was hoping someone with experience can see some error I made or maybe I have trapped air somehow? The TACO circulator is making a slight noise sometimes so I wonder if there is something going on. If I shut the return valves on my manifolds and open the manifold purge valve the hot water from the boiler goes right in but as soon as I shut that valve its like the return gets stopped. I am getting flow through all of the radiant loops so water is circulating. What am I missing here? My boiler is very large so I probably could have just went with 1 mixing valve but I wanted to be safe.

sallaberry

You don’t have an outdoor air sensor right? With newer boilers depending on outdoor air is warm enough the boiler will turndown. I’ve never done a mixing valve other than on domestic hot water. I usually do an injection system if I have multi temps.

EBEBRATT-Ed

The return from the radiant loops goes back to a tee splits one side enters the mixing valve the other side goes to the return. How can water return to the mixing valve when it goes into bypass the water will take the path of least resistance and go to the return.

NO2nice

EBEBRATT-Ed said:

The return from the radiant loops goes back to a tee splits one side enters the mixing valve the other side goes to the return. How can water return to the mixing valve when it goes into bypass the water will take the path of least resistance and go to the return.

The theory behind this is that the primary loop keeps the return water at 135 and then the secondary loop pulls hot water from the primary loop as needed. It worked fine for a few days and now it seems to be short cycling. I wonder if the mixing valves are clogged up, i might have to take it apart tonight when I get home.

Solid_Fuel_Man

Is this a radiant slab? I assume not as you said you removed a baseboard loop and put in radiant. Do you have plates etc?

NO2nice

I installed new tile floor and poured about 2-3 inches of concrete ontop of the pex loop so there is a decent slab. I was playing around with it again last night and I was able to get my secondary loop up to 120 degrees if I turned my primary mixing valve all the way down to about 80degrees. I dont like returning water to my boiler that cold. I wonder if my primary TACO circulator is too large or the AM-1 mixing valve on the secondary side is not flowing enough water? I'm running out of ideas.

hot_rod

The boiler circulator should really be pumping away from the mixed port to blend the flows best. What size is the Honeywell? What Cv? Quite a bit of pressure drop in those thermostatic valves.

NO2nice

The Honeywell AM-1 is a 3/4" with a 3.9Cv. the Taco is a 3/4" 2.3Cv mixing valve. I was worried about the placement of the primary circulator pump. If i have to move it then I have to do it but I just want to exhaust my other options. Is this a good, reliable method to control temps and guard the boiler? I wanted to go with an electronic valve but everything was getting very expensive.

thanks for the help!!!

HVACNUT

You're pumping into the mixing valve on the primary loop. No good. The circulator needs to be as the drawing shows.

NO2nice

Thank you, i guess I will have to relocate the primary pump....i was trying to avoid the extra work but not my luck.......yay!

NO2nice

I moved the primary circulator to the return leg as it is in the diagram but still no luck. Has anyone had bad experiences with the AM-1 mixing valve? the TACO valve seems to be working perfectly.

Bill_17

Please step back and take a better photo of the entire mixing & diverting valve setup and post it for all to see. It is hard to tell if it matches the line drawing. Label pipes as required, thanks.

Solid_Fuel_Man

I've used many AM-1 valves for heating. No issues. Like all other thermostatic mixing valves.

NO2nice

I dont have my laptop with me so this is the best I can do on my phone. I tried showing the hot in red, mixed in purple and return in blue. The hot water seems to stop at the first of the closely spaced Tees when you feel it with your hand. There are 6 radiant loops that operate off of this setup and each loop is close to 300'. Is there just too much cold water being returned?

BillyO

why a mixing valve on primary?

NO2nice

> @BillyO said:
> why a mixing valve on primary?

It's a safe guard to keep the return water above 135 to avoid condensation. I am regretting doing it because the boiler is big and probably would have been fine

clammy

I doubt u need that mixing valve on the primary , being it’s a multi circulator system you should move that radiant loop pump from the return to the supply pumping away w the rest of your pumps . Your tee set up for the secondary circuit should have straight pipe before and afternoon your set of tees I would suggest at least 10 inches for a better mix w the primary loop . Looking at your radiant loop pump it is pumping from your mix port of your valve .from the price point a simple taco I series valve would serve you better plus it would cya on return temp protection in the grand scheme of things it’s usually nothing compared to the pex ,good quality manifold w flow indicator and a few temperature gauges and pump and cost of installation that taco valves cost should not break the bank . Peace and good luck clammy

NO2nice

Thanks for your review, at this point i already moved the primary circulator to the return and moving it back would just add insult to injury but the distance from the close spaced TEEs from the elbows is critical and I didnt pipe this out right. Thank you for the catch. I will have to reconfigure this tonight. Its just frustrating because its all done and compact...smh

learning experiences!

Bill_17

When operating for a while, what are the temperatures at the points I have circled on the attachment? And, what are the design flow rates in the primary and secondary (radiant) circuits?

hot_rod

A rule of thumb is if the low temperature load is around 10% or less of the boiler output, probably no need for boiler protection

NO2nice

When the system has been working for a couple hours the temps stabilize per the attached photo. For this project I was under the gun and didnt do any flow rate calculations. I know there will be enough heat for about 1400sqft with spray foam insulation on all walls, rafters and crawlspace. The boiler is 250MBTU so I may just eliminate one circulator and mixing valve and make this whole thing simpler.

Solid_Fuel_Man

If I'm reading that correctly you are returning 178 degree water to the boiler?

NO2nice

No the 178degree water goes to the hot input of the primary loop mixing valve and also goes to the close spaced tees.

hot_rod

250,000 for 1400 square feet, or is there more space it heats?

NO2nice

the boiler heats the rest of the house which is another 1000sqft 1st floor and about 1000sqft for the upstairs bedrooms. All baseboard heating.

hot_rod

Still sounds like a grossly oversized boiler, 3400 sq ft? Certainly should not need boiler protection with that size boiler

I’d worry more about condensation issues from short cycling?

NO2nice

The rest of the house is 1920s and 1950s construction so the boiler gets some usage but yes it was sized wrong by whomever installed it last. I did install a ss liner and when the boiler kicks on it takes some time to bring all the water up to temperature so I'm not too worried about short cycling.

At the end of the day instead of re piping the straight sections off of the closely spaced tees I should just eliminate the secondary loop and make this a 1 circulator, 1 mixing valve zone? Thoughts?

Bill_17

From the diagram you have 178F water coming from the source, but near the secondary (radiant) loop mixing valve it is down to 100F indicating that the flow through the closely spaced Tees is likely opposite the intended direction. Otherwise, it would be 178F at that point. This is limiting the overall supply temp to the radiant circuit.

Bill_17

I should have added, that the radiant loop mixing valve could also be the problem if it is not allowing much flow from the (hot) boiler to mix with the radiant return.

NO2nice

Bill_17 said:

From the diagram you have 178F water coming from the source, but near the secondary (radiant) loop mixing valve it is down to 100F indicating that the flow through the closely spaced Tees is likely opposite the intended direction. Otherwise, it would be 178F at that point. This is limiting the overall supply temp to the radiant circuit.

I am assuming the flow of return water through the primary mixing valve is too slow which is creating a high pressure zone and then the secondary loop is sucking cold return water back through the closely spaced Tees. I looked at the design recommendations for Tees and for 3/4" pipe I should be a minimum of 3" from the return loop 90deg elbow and 6" or more on the supply side Tee. i roughly marked it on the attached photo. Do you think extending these straight sections will get this working correctly?

NO2nice

Is it possible that the radiant mixing valve might just have a problem from the factory?

hot_rod

Kind of hard to follow the actual boiler piping. A basic sketch of the near boiler piping may help us clear up the problem

Bill_17

Anything is possible, or there could also be some debris in either of the mixing valves holding back the intended flow. The thermostat in a valve could be the wrong temperature range. A partially plugged strainer or check valve installed backwards, all will cause unintended consequences. Looking deeper, if you don't have enough return flow passing through the valve with the green top the supply water temperature to the secondary loop mixing valve will never get that hot. Generally speaking, I think most would agree that with the closely spaced Tees you probably have all the return temperature protection you need. But before you proceed, you should see what the experts have to say on this matter.

NO2nice

Just want to thank you for your time troubleshooting this setup. I drew up exactly how i piped it. I double checked the checkvalve direction also. The only thing i can think of is that those screens on the mixing valve inlets may be dirty. Please let me know what you think.

NO2nice

same sketch just jpeg format

Solid_Fuel_Man

I've piped several radiant zones like this.
Just pipe it like a zone, but the closely spaced Tees are the takeoffs to your conventionally piped mix valve and circulator. This works well if the radiant zone isnt huge and the boiler is oversized (most are) and yours is by a factor of at least 3.

NO2nice

This morning it was the first really cold day at 15deg outside (I dont have outside reset) so I turned down the primary loop mixing valve all the way and i was able to get up to130deg mixed water temp on the secondary valve when the return mixed temp on the primary was at 90-100deg. So I think I will follow your advice and just pipe the primary like a reg loop without a mixing valve.