What type of pipe stat do I need to control secondary circulation pump

[pedr0]

Hi,

I've got a hot water cylinder and a secondary circulation pump to make sure the water is warm at the taps. At the moment it's just on a mechanical timer, meaning that it's pumping way more than it needs to and more heat than necessary is being lost through the pipes.

I've been advised to put a pipe stat on the return pipe to cut off the pump when the return gets warm.

Two questions:
1) Is this the best configuration to strike the balance between instant warm water at the taps and energy efficiency?
2) Do I need a high limit pipestat or a low limit?

Thanks.

 

[stem]

It's normally done like yours is, with the pump running continually.

I think there could be accuracy issues with fitting a pipe stat, especially if other areas of the pipe cool were to cool more quickly, yet the area where the stat is remains warm enough to keep the pump off. If then you were to use water, further up the loop parts of the pipe could become warmer by the natural flow from from the cylinder, leading to different temperatures around the 'ring'.

The only way to maintain a constant, even temperature is to have the circulation as it is now.

Also, the pipe stat will come 'on' and start the pump in order to stop the pipe going cold, meaning that as the pipe will always be warm it will still be loosing heat anyway, so I can't see the potential savings being all that much at all. The best way to save money would be to insulate the pipe really well.

 

[ianmcd]

why was the secondary pump installed ? do you have many outlets or is it because you have a few rooms a fair distance from the cylinder ?

 

[pedr0]

Thanks both.

@stem I was thinking more like having the stat turn the pump off when it reaches an OK temperature - maybe 40 degrees or so. At least the taps will always be reasonably warm, if not quite at the temp of the water inside the cylinder.

@ianmcd It's not a mansion but if the recirculation pump is off it can take literally 3 or 4 minutes for the taps to get warm! Unfortunately I asked the contractor to insulate the pipes, but he said it was unnecessary and would make it difficult to route the pipes. (I'm now far wiser and more fluent in his language = "It'll cost me more in labour and I can't be bothered mate").

 

[ianmcd]

No problem , the reason I asked is a few of us on here have had success with fitting PIR sensors to bring the pump on when someone walks into say a bathroom or en suite and its not running all the time, dont know your set up so not sure if something similar could be adapted

 

[stem]

I was thinking more like having the stat turn the pump off when it reaches an OK temperature - maybe 40 degrees or so. At least the taps will always be reasonably warm, if not quite at the temp of the water inside the cylinder.

That was sort of my point. Probably I didn't explain it very well. In the above scenario, the pump goes off at 40 degrees, then when the pipe drops to say 38 degrees the pump starts up again, the cooler water at 38 degrees will be put back in the hot water cylinder dropping its temperature and causing it to reheat. So the heat lost in the pipe will still have to be put back into it. OK it won't be loosing quite as much as if it were kept 20 degrees hotter around the entire ring, but every time the pump started up, the hot water at cylinder temperature will be entering the beginning of the ring anyway, so whilst the far end with the thermostat was 40, the beginning and a fair way around the loop would still be higher.

The PIR sensor (or sensors) is a good solution as it will anticipate water use and will allow the water in the ring to circulate briefly before the a tap is opened, this will help prevent creating a temperature differential across various parts of the ring, which would occur with a pipe stat if water happened to be drawn off whilst the pipe stat wasn't actually operating the pump. In addition the PIR won't keep the pipe warm at all when you are absent, but with a a pipe thermostat it will be turning 'on' and 'off' still keeping the pipe warm whilst you are out for the evening.

I would also imagine that if the pipe stat were 'off' and you wanted some hot water say for washing pots, if it came out at 40 degrees you would end up running water and waiting for hotter water to arrive from the cylinder, which sort of defeats the purpose of the ring in the first place.

Hope that makes a bit more sense. Having said that it's not a big expense or job to fit a pipe stat, so you could try it and then if it doesn't work and you remove it it's no big loss.

 

[pedr0]

OK. Got it now. Thanks.

I've got an alarm system that I might be able to use the PIRs to trigger a smart plug. I'll look into that, but it might be that I need some extra kit which will negate any cash savings. But hopefully at least I might be able to eek out some environmental savings.

If I do end up trying the pipestat, is it a high limit or low limit variety that I want @stem?

 

[stem]

That's a definition I am always careful with, as some will make different use of the term depending on its application.

For example, someone may wish to run the heating somewhere at a lower temperature overnight than during the day. The nighttime thermostat could be set to 12 degrees, and the day time thermostat to 18 degrees. Both thermostats are physically exactly the same, but because of their different settings, the night time one may be referred to as the low limit and the daytime one the high limit.

However, generally, a thermostat that interrupts the electrical supply when a certain temperature level (in your case 40 degrees) is reached, is usually referred to as a high limit stat, and a low limit thermostat refers to the type used with solid fuel heating systems that works in reverse, in that it doesn't provide an electrical supply until a certain temperature is reached.

Whatever you want to call it; you would require a pipe thermostat that can be set to interrupt the electrical supply when 40 degrees is reached. If you have a particular one in mind, post the details on here and someone will be along who can confirm its suitability, or otherwise.

 

[Harry Bloomfield]

I agree with the mechanical pipe stat being a poor solution, because those I have tested had a fairly wide hysteresis (difference between switching points). There is also the problem that the pump would run continuously, if tank temperature fell or the heating failed, was turned off. A little better rather than a mechanical pipe stat, would be a system which took an actual measure of temperature and used a variable speed pump.

Another way would be to have a more local store of hot water to where it would be needed, some people fit more localised electric heated cycinders, filled from a main gas heated cylinder..

I really like the PIR idea, as the best/cheapest solution.

 

[cross thread]

we normally install a timer so the pump is on at the mainly used times

 

[stem]

It already is on a timer.

At the moment it's just on a mechanical timer, meaning that it's pumping way more than it needs to and more heat than necessary is being lost through the pipes.

 

[StephenStephen]

Personally, what I've done is to massively insulate the pipe run - it means I can still have reasonably hot water quite a long time after the pump has had it's 4 minutes worth of timed pumping. I've more insulation to do, particularly on the return of the loop.
In terms of energy efficiency, and in terms of hot water availability, in a pumped loop, I would suggest that a completely anal and over-the-top approach to insulating the pipes will make a lot more difference than any details of how and when you pump hot water round what is effectively a long and wiggly radiator running round your house.

Constantly pumping hot water round a poorly insulated pipe running round your house is, by definition, inefficient. I can't see how a thermostat is going to significantly alter that, but having a 3 minute boost switch easily accessible in the location you're likely to be 3 minutes before you want the hot water could cut down on waste.

 

[bernardgreen]

Out of the box idea that might work for a hand wash basin in a toilet that is some considerable distance from the hot water cylinder.

Supply the toilet cistern from the hot supply. Then after the toilet is flushed the cold water in the pipe fills the cistern allowing hot water to come along the pipe ready for when the tap is turned on.

As an enhancement a thermally operated valve could be fitted on the inlet to the cistern to switch to cold water when hot water reached the cistern.

 

[dilalio]

Supply the toilet cistern from the hot supply. Then after the toilet is flushed the cold water in the pipe fills the cistern allowing hot water to come along the pipe ready for when the tap is turned on.

It's really only cloakroom and wc basins that need instant hot water for handwashing after going for a number 1 or 2!
I've pondered before about plumbing in the associated toilet to the hot feed (and maybe with a blending valve) so that the flush before you use the basin draws off enough cold through the combi to get the water at the basin up to temp. The blending valve set so as to not fill the toilet cistern with very hot water. Could work perhaps but plenty of fine tuning required

 

[bernardgreen]

Great ( inventive ) minds think alike.

That said I would not consider the idea for use with a combi hot water supply, firing up the combi every time the loo was flushed would be a waste of gas.