Pressure drop through a thermostatic mixing valve

[JDphxdg]

Hello all. I'm trying to understand the fluid dynamics of the shared piping and valving (mainly the mixing valve) between the two loops that make up an hot water re-circulation system; the closed pumped recirc loop and the open loop where water is released through a faucet, shower etc.

When all faucets are turned off, this can be looked at as a single closed loop. However, as faucets are turned on and flow increases, I would like to know how this increase in flow effects what the recirc pump "feels" in the pressure drop across the mixing valve.

Thank you in advance for your help.

 

[LittleInch]

Can you sketch your system please

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[PEDARRIN2]

There is a plumbing engineering forum.

The recirculating system works when there is no flow through a fixture, i.e. no flow into the system from the water supply and no flow out. So the pump is typically small, only fractional horsepower, only having to overcome pipe/fitting friction, but no height differences.

When a fixture is opened, the pressure from the water main, which typically ranges from 20-80 psig, will overwhelm any influence the recirculating pump would have. The pipe sizes in the supplies, cold and hot, are larger (3 or 4" vs 0.75") and the flows are typically larger than what the pump would be rated for. The pump might squeeze a bit of water flow through, but would likely be very low, maybe 0.1 gpm.

 

[LittleInch]

I'm still struggling to work out where this mixer valve is. Often the best way is to instal a small pump at the far end of the system, as though you had left a small tap on, to pump liquid back to the main tank. Then no main flow goes through it or past it.

If you sketch your system and post it it might be able to see whatever is your issue.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[PEDARRIN2]

Little Inch

This is a plumbing issue where the water heater heats the water up to ~140 F and there is a mixing valve which will blend cold water and hot water to provide ~120 F water to fixtures. Since, during times of no use, the water in the hot water piping will cool down, approaching ambient. The recirculating pump maintains a slight flow in the hot water system, basically enough to overcome this heat loss to maintain ~120 F in the piping. The pump is typically located near the water heater. A portion of the discharge from the pump flows through the mixing valve and a portion goes back through the water heater through the cold water feed. See the attached file for a representative diagram of what is typically installed.

 

[LittleInch]

Ok, I now understand your system - would be easier in future if you try and post it first time around...

The pump is normally sized solely for the re-circulation mode, i.e. relatively small DP and small flow (3-5% of max flow). When the taps are opened / showers used, the pressure at the pump inlet will fall whilst the upstream pressure doesn't change very much. Often these little pumps will stall and be unable to pump against the incoming pressure, but as they are so small it doesn't have an impact on the pump and then it just sits there and waits until the usage falls to such a level that the pressures come back to their no flow pressure that the pump is designed for. That's one reason why the NRVs are there to make sure the flow only goes one way.

I have noted on other posts that these re-circ systems may be needed for hotels and other systems with lots of users and some long distances from the water heater but domestically they just look like a good way to burn dollars for not much benefit....

Has this answered your question? - Basically the pump if it needed to flow would need to generate a much bigger DP, for which a small centrifugal is no use as the pressure range would be too large. If for some reason you want the recirc to be constant then you would need to use a PD pump which is much more able to handle a wide range of DP for the same flow rate.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way