Which live for secondary hot water pump?

[pedr0]

Hi all,

Seeking your opinions to clear up disagreement between plumber and electrician regarding which live to use for a secondary pump.

I've got a system boiler with hot water cylinder, underfloor heating and radiators so three zone valves.

The secondary pump keeps hot water flowing round a circuit so that hot water comes out of the taps around the house very quickly. Otherwise it takes ages (>2 minutes!) for the hot water to make it all the way down from the cylinder in the loft to the kitchen tap on the ground floor (it's just a 2 up 2 down - not a mansion, honest!)

Currently the live for that secondary pump comes off the switched live that connects the boiler to the zone valves.

But I've now been told it would be better to take the live from the brown wire that goes into the hot water zone valve (i.e. the one that comes from my Nest gateway to tell the hot water to zone valve to open).

I want the best compromise between having instant hot water when I need it, and minimising unnecessary heat loss from running hot water around the circuit.

Main question: Which wiring is best?

Q2: If the secondary pump is connected to the switched live of the boiler, then does that mean I'll only get instant hot water when the boiler is actually firing? I.e. in the summer when the boiler is idle most of the time, I'll be back to having to wait ages for the hot water.

Q3: If the secondary pump is connected to the control of the hot water zone valve, should it be connected to the input or the output of the cylinder stat? I'm guessing that it should be before the cylinder stat because otherwise the circuit would only flow when the cylinder is actually heating up.

Q4: If the secondary pump is connected to the input of the hot water cylinder stat, should I set my hot water programme to heat the hot water all the time I'm in the house, rather than say just an hour in the morning? If so, is that going to make my bills crazy high, or will it be negligible because the cylinder will keep most of its heat?

Thanks!

 

[bernardgreen]

A pipe thermostat on the return on the hot water circuit at the last tap would turn the secondary pump ON when the water in the pipe has cooled down. The thermostat could switch a permanent Live supply or via a timer so the secondary pump does not operate for times when hot water is not going to be used

 

[pedr0]

Thanks Bernard,

Very elegant solution. Would that need additional wiring between the stat on the last tap and the pump? If so, that might not be an option - all walls, floors and ceilings are now fully finished.

 

[dilalio]

We normally use a separate timer for the recirc pump that is set to be on at various occupancy and high demand times.

The complexity of the demand periods will dictate the timer required and the set up schedules.

If you're really intent on making it intuitive, then use PIRs to control the recirc pump.

Enjoy...
Dean

 

[Dan Robinson]

Prefer to use the autoAdapt Grunnies personally...


Hey Dean - finally invoicing that shower and Well Walk amongst others

 

[bernardgreen]

Would that need additional wiring between the stat on the last tap and the pump?

Yes it would. You could put the thermostat closer to the secondary pump, but as far from the pump as ease of wiring permits.

If new mains cables cannot be installed to the thermostat location but ELV wiring ( 12 volt miniature cable ) can be installed then a relay controlled by the thermostat and the relay contacts used to switch the mains to the pump is an option. ( requires a 12 volt power supply )

 

[lostinthelight]

Right or wrong, my seconary pump to the uv cylinder is on 15mm pipe, has its own power suply via a timer and thermostat which turns off the pump when HW return is up to temperature.
grundfos also do a comfort pump thats self learning.

 

[Dan Robinson]

Right or wrong, my seconary pump to the uv cylinder is on 15mm pipe, has its own power suply via a timer and thermostat which turns off the pump when HW return is up to temperature.
grundfos also do a comfort pump thats self learning.

That is the correct setup.

 

[SimonH2]

Another option is to use a modulating pump and a valve specifically designed for the purpose. The pump runs all the time, and the valve closes when up to a set temperature - restricting (or completely stopping) the flow, and the modulating pump throttles back to near enough idle as the valve shuts. I recall looking at a range from ESBE a while ago when thinking of doing just this with a thermal store.
Can't find them now, looks a bit like ESBE don't do them any more. I did find this similar product though from Caleffi - that was doing a search for "return line temperature control valve DHW circulation".
This is how some large hotels are done - a BFO tank of hot water, a BFO pump to send it round the building, and at the end of each branch there's one of these valves that automatically lets water round until it's hot. Some include options for manual or automatic sterilising - raise the feed temperature and circulate water till all the pipes are hot enough to kill any legionella.

 

[pedr0]

Thanks very much for the replies. I wish the plumber had put one of those self-learning grundfoss ones in - they seem pretty clever.

But for now I'm looking for an easy to fit solution (something that's cheap to buy and will not involve draining systems or much plumber/electrician labour time).

So I think a timer and possibly a thermostat on the return.

2 Questions:
1) Can the thermostat be put on the return close to the recirc pump (as opposed to on the last tap, which would require significant extra wiring)? @lostinsight - is your thermostat right next to the pump?

2) For the timer I might even go with a wifi plug. Can the pump be wired via a normal 3-pin plug that I could just plug into something like this: https://eu.dlink.com/uk/en/products/dsp-w115-mydlink-wifi-smart-plug

Thanks.

 

[lostinthelight]

Close to pump, as its monitoring the return if the temp is ok there it should be ok at the last tap?
My pump runs off a 3pin socket from junction box with plug in segmented timer
Have you got the return pipe insulated?

 

[Dan Robinson]

For the timer I might even go with a wifi plug. Can the pump be wired via a normal 3-pin plug that I could just plug into something like this:

A simple plug in timer plug door dah is fine..

 

[pedr0]

Have you got the return pipe insulated?

I don't think any of my pipes are insulated . I told the contractor I'd like the hot pipes insulated throughout the house right at the start of this project, and he rubbished the idea - Grrr.

Does that change things?

Edit: Currently all the power for the hot water and heating controls/pumps etc is controlled by a single switched fused spur which also feeds the boiler power (I presume this is a requirement or at least best practice). If we put a three pin plug on the recirc pump would that still have to be connected to the same fused spur, or could I use a vacant socket that is nearby but not on that fused spur?

 

[flameport]

Does that change things?

It means that whenever the pump is on, you are throwing the hot water away. Heat loss from an uninsulated hot water circuit will be massive.