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ink 02-23-2010 10:19 PM

Help w/ mixing valve (I think)
I installed a hydronic radiant floor in my kitchen, but I hired out the task of installing the manifold into the existing hydronic heat (the rest of the house is heated by baseboard and radiator heat). The contractor installed a mixing valve that has the supply from the boiler and a branch off of the return from the kitchen as it's inputs, and the output from the mixing valve feeds the manifold. I've temporarily taken the radiant floor out of the loop by connection a 3ft section of PEX across the manifold.

Here's a description of the problem:
It seems as if the mixing valve is feeding hot water through into the return from the manifold. With the zone valve open, the return branch going into the mixing valve is hot, as is the zone valve. It seems that hot water is flowing from the supply from the boiler, through the mixing valve, out the other mixing valve supply, and then straight out through the zone valve.With the zone valve closed, the return pipe from the manifold will get hotter in the direction of the manifold. Here, it seems that the hot water can no longer escape through the zone valve and is instead going up the return from the manifold as far as the pressure will let it.

In both cases, I can't get water to move through the manifold. It it sits stagnant.

I realize how convoluted this must sound, but any help is appreciated!


beenthere 02-24-2010 04:25 AM

Mixing valve needs to feed a circulator.

If the circulator is feeding the mixing valve, it won't work.

ink 02-24-2010 10:13 AM

1 Attachment(s)
Thanks for that. I've attached a jpg of the current layout. So perhaps I need to add a circulating pump at Ref B? The HVAC guy installed the mixing valve in a place where the output feeds the baseboard heat in a small room that is on it's own zone. I didn't draw in the return or the zone valve for that zone. I'm a little annoyed he did this, but is it problematic?

beenthere 02-24-2010 02:21 PM

1 Attachment(s)
Ain't gonna work the way its piped.

Here is a drawing I did for someone else.
The orange piping areas with circ and mixing valve is how you would want yours.

ink 02-24-2010 08:35 PM

Ok, I think I can do that. A couple of questions about it, though.

1. I'll have a zone valve instead of the backflow valve. Can I hook up the circulating pump and the zone valve to both be triggered by the thermostat?

2. Do I need to pull the baseboard radiators off that supply, or will it be ok even with the pump.

Thanks again!


beenthere 02-24-2010 10:33 PM

The baseboard will need to be on its own zone. or use the boilers circ, and be independent of the radiant loop.

If you use a Taco circ with its own built in relay, you can have the zone valve activate the circ.

ink 02-25-2010 01:30 PM

Excellent, thanks!

I'm trying to determine the correct circulator to get, now. To do so, I understand I'll need to determine the gpm and head loss. I have roughly 50ft of copper (supply and return to the manifold) and 400ft of PEX in this run. Everything is 1/2in. Using the chart on Taco's site to determine the equivalent length of various elbows and valves, I'm able to determine I have about 465ft (once I isolate the baseboard to a seperate zone). At about 120-140 deg, what am I looking at here for head loss? Any circulator recommendations?

Thanks again!

beenthere 02-25-2010 01:57 PM

What is the TEL of the pex separately, and of the copper pipe.
It makes a difference.

ink 02-25-2010 03:53 PM

Do the turns in the PEX have an Estimated Length, or can I just use the physical length of the PEX runs for the TEL of the PEX. THe PEX is 400ft in physical length divided evenly over two runs. The remaining 65 ft is the copper.


beenthere 02-25-2010 04:53 PM

So your pex length/run is not 400'
EG: 1-200 foot run, and 1 250 foot run. Only the 250' run length is used for head. But the total GPM for both loops are used for circ GPM.
Head is not accumulative. So what is the length of the longest loop.

The bends for pex are allowed for in most friction/head loss formulas.
So you don't need to add for them.

ink 02-25-2010 10:42 PM

The longer pex run is at or slightly under 230ft.

beenthere 02-25-2010 10:57 PM

At 1.5GPM, you'll be close to 15' of head total. A 008 should handle it

ink 02-26-2010 11:14 AM

First, I more closely measured the copper and came up with 60 feet TEL from the mixing valve to the zone valve. I don't know that the 5 feet will make a significant difference.

I took a look at the spec sheet for the 008 you recommended. It does seem to fit the bill. The connections listed in the spec sheet have 3/4" as the smallest. Can I just use an adapter to bring it down to 1/2" to fit in my system?

beenthere 02-26-2010 06:13 PM

Yes, you can use reducing male adapters.

ink 02-28-2010 08:23 PM

Thanks again.

While I'm ordering additional parts, I should ask this..

I have 5 heat zones, now, all using the same model of taco zone valve. They're wired to two transformers. There is currently no ZVC. Is this something worth investing in before I add the circulator and new zone valve to the mix, or should I just stick with how it is.


PS... cast iron circulator?

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