Secondary Water Circulation Control

[Stephen Leedham]

Hi Everyone,
thought I'd pick your brains on some ideas I have for controlling a DHW secondary circulation pump circuit that I'll be installing as part of my new build. The DHW supply will consist of an Air-Water Heat Pump source linked to an unvented cylinder (at mains pressure). Never had the need to do secondary recirculation before, but understand the basic control using timer and temperature sensor on the return flowpipe. However, 2 scenarios exist which, as much as I looked, cannot find any posts that point to an efficient solution. I've got ideas, but don't know whether they have hidden (or unthought of) issues, or whether there is a more elegant solution to cover them. So, I am looking for feedback on these:

Scenario 1 - 'The 3am Bathroom Call'. Outside of the timer/pump being on, how to minimize draw off time (and associated wasted water)? I'm thinking to install a simple flow sensor in the outflow of the cylinder which would switch on the pump (assuming temperature in return flow is below limit). My thinking is that it would pull some of the 'cold' water past the outlet tap at the same time that the tap is emptying into the sink and hence reduce the time/water waste before hot water reaches the tap. Not the perfect solution, but with the pump pulling between 1 - 2 l/s there should be a noticeable improvement, shouldn't there?

Scenario 2 - 'The Family Sunday Morning Church Outing'. Simply put, how to control the system when the hot water cylinder is emptied of hot water e.g. multiple baths/showers at the same time. If the water temperature in the cylinder is below the limit, then the return flow temperature will never reach the pump's switch off point. So the system will continually recirculate tepid water. Therefore I'm thinking to add a second cylinder thermostat (or branch off existing?) that would switch off the recirc pump until the desired temp is reached.

Hope my explanations are understandable enough and look forward to your wise words...

Stephen

 

[flameport]

Forget the timer and the temperature sensor.
Fit a PIR in each bathroom which activates the circulator.

 

[Stephen Leedham]

I'd thought about that, but for 4 bathrooms, a kitchen and a utility room, it's a lot of PIRs (and wiring). I'm trying to keep it all contained in the 1 location i.e. where the cylinder is

 

[ianmcd]

Just use wireless PIRs , not rocket science, very common these days

 

[flameport]

it's a lot of PIRs (and wiring).

It's 6 PIRs and a single cable that links them all, with the end of the cable at the pump location.

Not exactly difficult, expensive or time consuming when

installing as part of my new build.