radiant heat question?

tony_16

well i came across another radiant heat job and another question to go with it.i have a boiler with nine zones four zones are hydro air zones and one is domestic hot water and the other four are for radiant heat.i used four 3 way mixing valves for the radiant heat. the biggest zone is the kitchen zone which consists of 5 loops of 1/2 pex tubing each loop is 250 feet in lenght.so i set up a manifold about 30 feet from the boiler and im supplying the manifold with 1" feed and return and i used a 1" mixing valve i had installed a taco 007 for the zone than i was worried it will not push enough water so i changed it and used a taco 0010 just to make sure. so heres the question someone brought it up to me but couldnt answer it.with those five loops on that manifold being fed from the mixing valve the valve might not be rated for what i want it to do as far as the gpm and the cv rating[what ever cv means] of the valve.the cv rating is 3.2 and the gpm is 14 so will this work? and what about the pump size? are there any charts for on the job pump sizing?i was originaly gonna use one big 1-1/2 three way mixing valve for all four zones and use zone valves for each zone which i guess you still have to make sure the mixing valve can handle it.and should i have pipe over a bigger feed and return to the manifold instead of 1". im very greatful for any input thanks very much.

kevin

well here are a few thoughts..

What is the heat loss for each zone. Thats a start to what size the piping needs to be. 1" sounds plenty big to me. The pump choice doesn't. 250' loops need more like a 008 or grundfos 15-42. the 0010 won't cut it either.if my thumbnail figuring is right.Did you do a complete heat loss figuring in the pressure drop w/ the mixing valve? Get the highest rated valve (Cv)# possible, sparco is quite high. good luck kpc

Julie

Observations

Some observations about the installation you described:

14 gpm in 1" pipe is moving excess of 4 fps, which is usually the recommended upper limit for water velocity in smaller pipes (because of water noise). The pressure drop created by 14 gpm in the 1" piping is also outside the recommended range. 1-1/4" pipe would be more appropriate.

A valve Cv rating tells how many gpm can go through the valve and produce a 1 psi pressure drop. A valve rating of 3.2 means that 3.2 gpm create a 1 psi (= 2.31 feet) pressure drop. Pressure drop increases approximately as the square of the flow rate. So a valve with a Cv of 3.2 passing 14 gpm would have a pressure drop of (14/3.2)^2 x 2.31 = 44 feet. A Taco 0010 can pump 14 gpm at a maximum of about 9 feet. Therefore the installation will not work as intended. I agree with Kevin that you should look for a valve with a larger Cv. Without knowing the pressure drop of the other elements in the loop, however, it is hard to know what Cv you need. I would try for at least a Cv of 14 (pressure drop of 2.31 feet).

Good luck with this project.

KCA_2

OOOPS!


It seems as though your question may be getting read wrong. I see it as one zone with (5) 250-300' loops working off of a temporing valve. The temporing valve has a Cv of 3.2 (that's a factor of feet of head with regard to velocity. in other words, at every 3.2 gallons through the valve, it costs you 1 foot of head). The 14GPM would be its max rating.

Anyway.. You have (5) 1/2" loops in a zone with its' own Temporing valve. The pump is located on the Mix side of the valve? I am guessing so.
The feet of head associated with 300 feet of 1/2" PEX at 140deg @ 100% water & 1GPM is +-10'. Now.. The piping, Temporing valve etc. also have head associated with them at 1GPM so lets say 14' total. A 008 will pump between 4 & 5 GPM at 14'.

Now that's 1 GPM per loop although head isn't cumlative. But the 1GPM / loop comes into play with the heat generated by it. In other words 1GPM at 20deg Delta T = 10MBTU. So with (5) loops the system would produce 50MBTU. All of that may not end up in the room but none-the-less..

Or if you would rather, 1GPM through (5) loops at lees that 1/4GPM / loop at about 10MBTU for the room. about 1.2'/head for the tubing and 4 for the other items and now a 006 would probably be fine.

A single T-valve would have worked but since there was an expense of a few of them maybe "Injection" with indoor-outdoor would have been a better choice. Actually, it's very often a better choice.

The 1" piping is adequate for 10GPM so a 3/4" manifold would have been OK. 1/2" if you're going for the 10MBTU's for the room.

Hope this helps..:-)
KCA

Julie

Valve Cv

The definition of a valve Cv is "the amount water at 60 F in gallons per minute which will flow through a valve in the open position with a pressure drop through the valve of 1 pound per square inch." So a valve with a Cv of 3.2 will create a 1 psi pressure drop when 3.2 gpm is flowing through it. And for water at 60 F, 1 psi=2.31 feet w.c.

As for the 14 gpm total, I may have misunderstood Tony's system. Tony, do you mean 14 gpm for the manifold in question? or does the 14 gpm reference something about the valve itself?

Julie

Julie

Valve Cv

The definition of a valve Cv is "the amount water at 60 F in gallons per minute which will flow through a valve in the open position with a pressure drop through the valve of 1 pound per square inch." So a valve with a Cv of 3.2 will create a 1 psi pressure drop when 3.2 gpm is flowing through it. And for water at 60 F, 1 psi=2.31 feet w.c.

As for the 14 gpm total, I may have misunderstood Tony's system. Tony, do you mean 14 gpm for the manifold in question? or does the 14 gpm reference something about the valve itself?

Julie

tony_16

hi julie ok i refering to the mixing valve itself which is has a cv rating of 3.2 and then is says that the flow/gpm as they wrote is 14.so on the 1" three way valve comming out of the mix side i put a taco 0010 and then piped it to the manifold where there is 5 loops of pex tubing. i was gonna use one big 1 1/2 mixing valve and out of the mix side use zone valves for each zone but with all that cv stuff i change my mind and used four seperate mixing valves with single loops all except the kitchen which has the 5 loops of pex tubing working off of a 1" mixing valve. onece again i thank you for any input. tony

Julie

Mixing valve ratings

O.K., it sounds like I goofed and the 14 gpm was a factory marking, rather than the gpm you intended for the loop (I thought 14 gpm sounded awfully high for number of zones and size of pipe, but since this was one of my first posts, I didn't want to be too critical).

Kevin's comments posted above are really good advice. Calculate the heat loss of the space and figure out the actual gpm requirements. Verify that the tempering valve Cv won't create excessive pressure loss. Then calculate the pressure drop of the PEX, the other piping in the loop and the tempering valve, and select the appropriate pump.

You had asked in your original message about charts for pump sizing. Taco and Grundfos can both provide you with pump charts showing flow in gpm versus total head in feet for different models. Is this what you were looking for?