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# Three port thermostatic mixiing valves

Member Posts: 42
Hi

When you look up the Cv of a three port TMV, is the pressure drop and flow based on flow from one inlet to the outlet or both inlets to the outlet, or does it not really matter. I am thinking about the circuit where a TMV is used for boiler protection - the biggest pressure drop is likely to be across the TMV, but I can't get my head around if it makes a difference if one or both ports are open?

Also, we use metrics in NZ but most Cv's I have seen are based on imperial measures - are there conversion factors to convert Cv's or do you just have to develop your own?

Thanks.

Regards,

Santilal

## Comments

• Member Posts: 15,066
The metric flow factor unit is Kv

Kv would be cubic meters per hour (m3/hr.) of a 16• c water with a 1 bar pressure drop.

Cv is GPM of 60•F water with a 1 psi pressure drop

Kv = 0.862 x Cv

The thermostatic valves built for boiler protection have larger flow rates and are usually a fixed temperature cartridge.

The pressure drop either Cv or Kv is the flow thru the valve to the outlet.

Keep in mind a valve with a Cv of 3 can flow more then 3 gpm, but you will have additional pressure drop.

Here is a little spreadsheet we use to calculate flow, pressure drop, and gpm. I'll see if we have one in metric units if you would like.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 42
Thanks

Hi

Thanks for that. From your answer it appears that the Cv is not dependent on either or both hot and cold ports being open or closed.

Cheers.

Santilal
• Member Posts: 15,066
correct

what comes out of the mixed port is the blend of the H &C port. If it needs more hot, that port opens as the cold closes down. The wax cartridge inside move the spool up and down to vary then opening into the valve from the H & C
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
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