Secondary HW return circuits - how to control the pump?

[john37]

Hello

In a large house with 7 showers and 3 kitchen sinks I think we need two secondary circuits for hot water, with the two 15mm returns connected back to a single re-circulating pump - probably one of the Grundfos Comfort pumps.

How can I:

1. Switch off each return circuit separately when it gets up to temperature? Maybe use a Danfoss MTCV in each return?

2. Switch off the pump when both circuits get up to temperature and are switched off?

3. Prevent the water from one return being drawn up into the other return when they are both open? Use a NRV in each return?

Grateful for any advice in designing the secondary circuits and pump control.

 

[glenntaf]

Secondary return circuits aren't usually thermostatically controlled. You just need to wire the pump to a time switch a have it coming on when there is likely to be a demand for hot water, and make sure the return pipework has sufficient insulation to cut down on heat loss. A check valve should be installed, near to the cylinder where the secondary return is connected in. This will stop any cooler water being drawn out of the cylinder and back up the return. Also if you are running the return pipework to several areas you may want to put a lock shield valve where it tee's into the hot water pipe so you can throttle and balance it up later.

Hope this helps

Glenn

 

[lame plumber]

get a plumber in to appraise the situation and advise you what needs doing, most of your points are completely off key.

 

[snowhead]

2. Switch off the pump when both circuits get up to temperature and are switched off?

Serious misunderstanding of the point of secondary return circuits.

 

[solutions]

Serious misunderstanding of the point of secondary return circuits.

I dont understand snowhead, how is the op misunderstanding the point of a secondary return? What is wrong with wanting to turn the pump off when the circuit is up to temperature?

 

[josh]

Have done many multiple loops of one pump and never had any issues with it going back up the other circuit with just the one NRV fitted after the bronze pump, i do always use lockshields/gate valves for balancing purposes and have to be aware when you shut the hot of to one part of a building unless you shut the lockshield of to the loop to the hot will still be live back feeding up the return unless you did use a check valve, if that makes sense.

i have never used a pipe stat to switch the pump on and of when up to temperature as only meant to have a max of 10c differential between the hot outlet and secondary return and have always thought would just make the pump cycle on and of a lot. If you wanted to make it stupidly complicated could be done with solenoid valves but i really don't see the point.

 

[john37]

Thanks snowhead. Maybe I didn't express it clearly. My understanding is that the point of a secondary return is to circulate hot water in order to reduce the delay before hot water arrives at the taps or outlets, which are on short "spurs". I tried to insert a diagram to make it clear but couldn't upload it. This Danfoss article shows the idea though:

heating.danfoss.com/pcmpdf/vbd3a302_sep2014_lowres.pdf

When all the outlets/taps of any "circuit" are closed the pump only needs to circulate water through that circuit if its temperature (measured on the return leg) is below some target temperature. Otherwise, it seems to me, hot water is circulating through that circuit needlessly.

 

[Dotty]

If you use a smart pump, which you'll need to do any way, it'll build an internal database according to usage and temperature, and react accordingly.

 

[chris watkins]

Use the Grundfos Comfort Auto-adapt it will turn itself on & off as it learns how you use hot water. (or other smart pump as above)

Unless you really are using the fittings on each secondary at different times then it isn't worth controlling them independently all the hot water pipework will have to be insulated so the losses will be very small.

 

[solutions]

As above the comfort auto adapt is the way to go. Bit pricey mind.
Make sure you get the Grundfoss pump kit with the unions included had ti order some unions in special from Bes when I got the wrong pump kit without the unions included and parts center gave me irons when I ordered the unions separately, despite stating it was for secondary return. Idiots.