Hot water recirculation pump control

[DanB]

Sorry for lack of build updates from Antigua, but construction site has been hectic and not had much time...Will upload some new pics in coming weeks. I do have a potential snag in designs this week with the plumbing – specifically hot water.

 

The issue is that the house design is several buildings, spread out over the site: one main building (which will contain the hot water cylinder – an air to water heat pump HW cylinder) and 3 remote bedrooms. Now water is more precious than electric for us (since the HW cylinder will be solar powered, but water is 100% harvested from rain water and we can have months of drought) and so there is a problem in that the furthest bedroom with have around 200ft of pipe from the hot water cylinder. Using ½” pipe then 200ft gives aprox 2 gallons / 8 litres of waste water in order to get hot water through…

 

Solution is obviously a recirculation pump, with a dedicated return line (or in fact one big HW loop encircling all the buildings, with a short spur off the loop to each building). But I’m looking for suggestions on how to manage the pump. Since the HW cylinder is a heat pump, then the manufacturer recommends against a continuous recirculation pump because it will cause a lot of compressor short cycling... So I was considering perhaps fitting PIR motion sensors in each bathroom with adjustable timers to activate the pump. Or alternatively a fixed interval timer that runs the pump for a couple of minutes at a time (and switched off at night). Any suggestions on the fixed interval time? (Assuming ambient temp circa 30 deg C and water temp 55 deg C, with plastic pipe uninsulated and buried in ground – probably want it to recirculate when it drops to 40 deg C) And also does anyone have any recirculation pump recommendations? (Final thought, I also have around 70ft altitude difference between highest and lowest building…obviously the water will be sufficiently pressurized but wondering if this “head” difference might present a problem to a recirculation pump…)

[Simplysimon]

2nd hot water cylinder for the furthest room?

[JohnMo]

I'm using a Lowara Ecocirc Pro pump, which has an integrated timer.  Works great, real easy to install, just power needed.  Several sizes are available.

 

 

[Temp]

One option is a pipe stat on the loop so the pump runs until hot water returns to the cylinder. Then switches off until it cools down. Ideally you want to be able to control the hysteresis (temperature at which it switches on/off).

[DanB]

2 hours ago, Temp said:

One option is a pipe stat on the loop so the pump runs until hot water returns to the cylinder. Then switches off until it cools down. Ideally you want to be able to control the hysteresis (temperature at which it switches on/off).

That's interesting, so I suppose I would put the pipe stat on the loop just before it returns to the tank?

 

I had done a few calcs to work out how often I might need to recirculate....

With 60 deg c water and 30 c ambient, using Q = c m dT  I've calculated that a 1ft length of 1/2" pipe full of water holds around 1.2 Wh of energy and heat loss using Q/t = σeA(T₂⁴- T₁⁴) is around 2.5W ...so in other words the uninsulated pipe would lose most of it's heat in about half an hour. Also my total volume of loop pipe is around 40 litres so if I had a 40 LPM pump then I could program it to run for 1 minute every half hour. If I insulated the pipe I could probably get that to 1 minute every hour, and also have it turned off overnight.

 

What about pump head? Most are quoting max 5m, but height difference from top building to bottom is around 20m in my case (with the pump sited roughly in the middle).. If the system is closed and pressurized this doesn't matter from what I've understood, is that correct?

[DanB]

2 hours ago, Temp said:

One option is a pipe stat on the loop so the pump runs until hot water returns to the cylinder. Then switches off until it cools down. Ideally you want to be able to control the hysteresis (temperature at which it switches on/off).

Thinking about it more, pipe stat is a good shout. If I put a stat on the loop at the furthest distance away, and also use a digital timer to turn off the pump completely at night, and finally use pipe insulation then I think that will do it with the least amount of cycling and be the simplest solution...

[TonyT]

What about a pressure switch, operating when the tap is opened?

[JohnMo]

5m head in this case isn't the height difference as you have a loop, as the water would find its own level, imagine a 'U' tube.  The water coming down will be pushing the other water in the circuit up.  So the 5m is the head loss from pipe friction and bends.

 

I just time mine on for 30mins for the morning, as get up the same most days and the same in the evening.

 

But I would be tempted to work out how often your cylinder would initiate start the heat pump to start running circulation continuously.  I think you will be surprised, as I don't think it will be that often (every few hours).  But insulation on pipes would be a good idea.

[DanB]

2 minutes ago, JohnMo said:

5m head in this case isn't the height difference as you have a loop, as the water would find its own level, imagine a 'U' tube.  The water coming down will be pushing the other water in the circuit up.  So the 5m is the head loss from pipe friction and bends.

 

I just time mine on for 30mins for the morning, as get up the same most days and the same in the evening.

 

But I would be tempted to work out how often your cylinder would initiate start the heat pump to start running circulation continuously.  I think you will be surprised, as I don't think it will be that often (every few hours).  But insulation on pipes would be a good idea.

Thanks for confirming that on the pump head, that's exactly what I thought but just couldn't get my "head" around it!? (sorry, that is truly terrible humour). I do like pipe stat (in combination with a timer). I think I've decided to try and keep it flexible for now (i.e. put in conduit and a few spare control / signal wires to a few different locations during construction) then as you suggest I can see how the cylinder operates once I actually have it.

[DanB]

7 minutes ago, TonyT said:

What about a pressure switch, operating when the tap is opened?

So I had thought about that, but had ruled it out because I had reasoned that once you open the tap then you are wasting water and you still have to wait for the hot water to come around? So would it make any difference even having a recirculating pump in that case? What I really want is for the water to be hot locally before opening the tap.

[Nickfromwales]

1 hour ago, JohnMo said:

But I would be tempted to work out how often your cylinder would initiate start the heat pump to start running circulation continuously.  I think you will be surprised, as I don't think it will be that often (every few hours).  But insulation on pipes would be a good idea.

A man after my own heart.

 

Pulsing the hot return in this scenario would be an epic fail, and using a pipe stat to decide when to pulse would still not yield an acceptable result afaic.

 

Just insulate this run(s) very well and switch on and off via a dusk to dawn sensor for ‘daytime’ use, and then fall back to occupancy switching for nighttime use ( where someone moving at night would trigger a recirc event ).

 

Make sure the hot return pipe is the same size as the delivery pipe, or evacuation times for the delivery ( aka flow ) leg will be hindered by the return pipe size.

[DanB]

17 minutes ago, Nickfromwales said:

A man after my own heart.

 

Pulsing the hot return in this scenario would be an epic fail, and using a pipe stat to decide when to pulse would still not yield an acceptable result afaic.

 

Just insulate this run(s) very well and switch on and off via a dusk to dawn sensor for ‘daytime’ use, and then fall back to occupancy switching for nighttime use ( where someone moving at night would trigger a recirc event ).

 

Make sure the hot return pipe is the same size as the delivery pipe, or evacuation times for the delivery ( aka flow ) leg will be hindered by the return pipe size.

Thank you Nick, from Wales, much appreciated. Will do on the insulation and will do on the pipe sizes. But I thought that the pipe stat on the return pipe was a good idea? Set to something like 40 deg C kick in, it would recirculate for a couple of minutes and then switch off circulation pump when circuit back up to temp, what am I missing (I'm missing lots these days I've realised, but I mean specific to this!)

 

I personally don't mind running the pump continuously as we are 100% off grid/solar, it was only the heat pump HW cylinder manufacturer instructions that spooked me on that. This is what it says in the manual "Stiebel Eltron advises against the installation of this appliance in any recirculation loop. Always install a recirculation loop with a thermostat cut-off or a timer. Installation without these devices will cause the appliance to run continuously, resulting in high electricity use and extended compressor run times which will reduce the life of the appliance and void the warranty. The Accelera energy efficiency will drop when installed with a recirculation loop due to the reduction in thermal stratification of the tank." Thermal stratification I think is the key point I suppose, as continually mixing up the water will keep triggering on the compressor.

[Bitpipe]

8 hours ago, DanB said:

Thank you Nick, from Wales, much appreciated. Will do on the insulation and will do on the pipe sizes. But I thought that the pipe stat on the return pipe was a good idea? Set to something like 40 deg C kick in, it would recirculate for a couple of minutes and then switch off circulation pump when circuit back up to temp, what am I missing (I'm missing lots these days I've realised, but I mean specific to this!)

 

I personally don't mind running the pump continuously as we are 100% off grid/solar, it was only the heat pump HW cylinder manufacturer instructions that spooked me on that. This is what it says in the manual "Stiebel Eltron advises against the installation of this appliance in any recirculation loop. Always install a recirculation loop with a thermostat cut-off or a timer. Installation without these devices will cause the appliance to run continuously, resulting in high electricity use and extended compressor run times which will reduce the life of the appliance and void the warranty. The Accelera energy efficiency will drop when installed with a recirculation loop due to the reduction in thermal stratification of the tank." Thermal stratification I think is the key point I suppose, as continually mixing up the water will keep triggering on the compressor.

 

We use PIR sensors in the bathrooms to activate both MVHR boost and the DHW recirculation pump. 

[Nickfromwales]

21 hours ago, DanB said:

Thank you Nick, from Wales, much appreciated. Will do on the insulation and will do on the pipe sizes. But I thought that the pipe stat on the return pipe was a good idea? Set to something like 40 deg C kick in, it would recirculate for a couple of minutes and then switch off circulation pump when circuit back up to temp, what am I missing (I'm missing lots these days I've realised, but I mean specific to this!)

 

I personally don't mind running the pump continuously as we are 100% off grid/solar, it was only the heat pump HW cylinder manufacturer instructions that spooked me on that. This is what it says in the manual "Stiebel Eltron advises against the installation of this appliance in any recirculation loop. Always install a recirculation loop with a thermostat cut-off or a timer. Installation without these devices will cause the appliance to run continuously, resulting in high electricity use and extended compressor run times which will reduce the life of the appliance and void the warranty. The Accelera energy efficiency will drop when installed with a recirculation loop due to the reduction in thermal stratification of the tank." 

Stiebel Eltron are the Bentley of the HP world, and are Uber-fussy with the installation criteria, ask me how I know!! Not a bad thing, but they are very focussed on correct and immaculate execution wrt to the implementation of their products.
 

A simple and acceptable ‘adaptation’ would meet their criteria; eg the employment of a much larger UVC for one, plus some manipulation of the flow temps of the HRC vs the stored temps of DHW ( and flow temps to it ). The HRC can then be made “invisible” to the HP so as to not upset SE.

All in a days work lol.

22 hours ago, DanB said:

Thermal stratification I think is the key point I suppose, as continually mixing up the water will keep triggering on the compressor.

Not exactly   If the HRC has no demand and managed losses then there would be no heat energy lost, ergo nothing to then keep triggering as demand aka call for heat, at least no more than acceptable latent losses attributed to the system and its components.

A HP is there to provide heat energy, end of. It must do its job, simples. How the install is designed and executed will dictate the efficiencies of the system as a whole. I think it is very narrow-minded of SE to state the above, but that’s if it is designed for a domestic installation. If it is not, and you apply it to an adverse scenario, then expect the fleas to come with the dog.

[DanB]

22 hours ago, Nickfromwales said:

Stiebel Eltron are the Bentley of the HP world, and are Uber-fussy with the installation criteria, ask me how I know!! Not a bad thing, but they are very focussed on correct and immaculate execution wrt to the implementation of their products.
 

A simple and acceptable ‘adaptation’ would meet their criteria; eg the employment of a much larger UVC for one, plus some manipulation of the flow temps of the HRC vs the stored temps of DHW ( and flow temps to it ). The HRC can then be made “invisible” to the HP so as to not upset SE.

All in a days work lol.

Not exactly   If the HRC has no demand and managed losses then there would be no heat energy lost, ergo nothing to then keep triggering as demand aka call for heat, at least no more than acceptable latent losses attributed to the system and its components.

A HP is there to provide heat energy, end of. It must do its job, simples. How the install is designed and executed will dictate the efficiencies of the system as a whole. I think it is very narrow-minded of SE to state the above, but that’s if it is designed for a domestic installation. If it is not, and you apply it to an adverse scenario, then expect the fleas to come with the dog.

Many thanks. Final answer (as they say on Millionaire) - I've given it some thought and I'm going to go with PIR sensors in the en-suite bathrooms, and a demand push button at the kitchen sink (don't want to use PIR in kitchen as it is an open plan kitchen/dinning/lounge so would trigger all the time).

 

Just so that I learn something, I understood a lot of what you said...but what is "HRC"? - from context I'm guessing that is the recirculation loop?? And "UVC" I couldn't figure out at all? Sorry for my ignorance!

[Nickfromwales]

16 hours ago, DanB said:

Many thanks. Final answer (as they say on Millionaire) - I've given it some thought and I'm going to go with PIR sensors in the en-suite bathrooms, and a demand push button at the kitchen sink (don't want to use PIR in kitchen as it is an open plan kitchen/dinning/lounge so would trigger all the time).

 

Just so that I learn something, I understood a lot of what you said...but what is "HRC"? - from context I'm guessing that is the recirculation loop?? And "UVC" I couldn't figure out at all? Sorry for my ignorance!

HRC = Hot Return Circuit 

UVC = Unvented Cylinder.

 

You could put a flow switch on the hot pipe feeds to the kitchen / utility sinks which would trigger the HRC pump when hot water is drawn. 

Edited April 18, 2022 by Nickfromwales

[Jenni]

On 16/04/2022 at 00:25, Nickfromwales said:

Make sure the hot return pipe is the same size as the delivery pipe, or evacuation times for the delivery ( aka flow ) leg will be hindered by the return pipe size

 

Hi Nick. 

I've just been mentally prepping myself to do my HRC, some other threads in the past have said to use 15mm on the supply and 10mm on the return leg. But here you're saying all the same.

Are there different circumstances for when each method is preferable? 

For context, my UVC is near the bath+shower so the loop will be running the kitchen + utility.

Thanks. 

[Jenni]

Also, I realise I've not given enough thought to how it might efficiently operate. My sparky is coming to do first fix next week. 

Is it possible to have the pump triggered in 3 different ways -

- A timer say 7am-10am and 5pm-10pm. (potentially different at weekends) 

- From front hall occupancy sensor. 

- From a boost button. 

 

Thanks again. 

[Nickfromwales]

3 hours ago, Jenni said:

 

Hi Nick. 

I've just been mentally prepping myself to do my HRC, some other threads in the past have said to use 15mm on the supply and 10mm on the return leg. But here you're saying all the same.

Are there different circumstances for when each method is preferable? 

Yes, different instances get different responses here, so be mindful to look at the OP comments to see if the “shoe fits”   

The above info for same pipe size is because the system I’m advising on there is a “district” system servicing satellite dwellings from a central plant source, so if the HRC return pipe is smaller than the delivery ( supply ) side, the rate off delivery to the point of consumption will be extended somewhat, but, as per my advice in other threads, in a domestic ( small ) installation this is fine. 

3 hours ago, Jenni said:

For context, my UVC is near the bath+shower so the loop will be running the kitchen + utility.

Thanks. 

How far away are they? And are you plumbing with a radial setup, or regular series plumbing? If radial you may get away with just having the HRC running a loop for the large bore delivery side, eg from UVC > DHW manifolds > UVC and not then bother / need to run the HRC pipework throughout, all depends on frequency of use and how you control the pump.

2 hours ago, Jenni said:

Also, I realise I've not given enough thought to how it might efficiently operate. My sparky is coming to do first fix next week. 

Is it possible to have the pump triggered in 3 different ways -

- A timer say 7am-10am and 5pm-10pm. (potentially different at weekends) 

- From front hall occupancy sensor. 

- From a boost button. 

 

Thanks again. 

I’ve implemented many different control types tbh, but my favourite is still to trigger with the burglar alarm output relay, so if the alarm is fully armed the HRC goes off, when you return it comes back on when you disarm the alarm. Cheap as chips home automation lol. 

A second relay can be installed for when the alarm is only part-armed, eg sleeping plus guests etc, where the kitchen and utility / downstairs cloakroom basin etc ( that you know cannot be used at night ) are shut off by a zone valve on the HRC manifold. All depends, as I say, if you go radial or series plumbed though, as with series plumbed you’re stuck with all or nothing HRC in most instances as the whole houses large bore pipework will get warmed through even if just one basin is used for a few seconds for hand wash or a quick squirt under the kitchen sink tap is needed to rinse a dish off, with those ‘calls’ for HRC flow seeing a significant amount of time delay and losses ( as with series plumbing there is usually a lot more large bore primary distribution pipe which the DHW has to replace before premium temp water arrives at these high-frequency / low volume outlets ).

 

An occupancy switch ( motion / PIR ) allows a single burst if it detects movement at night, but daytime I just say to leave it running as the water going warm in the pipe is a dead leg, and if you need full temp DHW to wash a dish, you’ll still leave the tap run to get the warm water to run hot again, so still a waste of time, water, money and effort AFAIC. A couple of solar PV panels will easily offset moats of the years energy bill for running this, if detailed and insulated well, so point the purse-strings at more PV instead of complex controls which give mediocre results is my 2-cents.

 

All this advice differs from instance to instance, so if yours is one where you just have a kitchen and utility loop, I’d just allow their HRC to run on the larger bore primary stuff, to only keep the ( very well insulated ) point of origin / distribution pipework hot, and just wait for premium temp DHW to make its way to you, that’s if the pipe runs are sub 25m? That option only needs a time clock and negates any sensors etc. FYI, I always run. A 10mm feed to ants wash basin hot tap that is NOT on HRC.

 

Buy some 10mm pipe ( Hep2O is what I use because of the quality of the fittings and how small the inserts are ) and mimic the run to the kitchen and utility. Buy the kitchen tarp now, and hook it up and test it. If you are happy with the flow rate ( pipe size vs length of run ) then work out the volume of water in the pipe and how long it will then take to discharge the cold water and decide if you actually NEED an HRC first .

 

I treat each of my M&E clients uniquely, so cater for what they want / need / how they live / number of occupants / PV or not / radial or series plumbed etc etc and so forth, so picking off info here can be a bit ‘random’ tbh, but if you read up here there’s a shed full of discussions from various members installations and what they’ve done successfully ( and unsuccessfully ).

[Ajn]

On 15/04/2022 at 17:27, DanB said:

the furthest bedroom with have around 200ft of pipe from the hot water cylinder. Using ½” pipe

Thst seems a small pipe if you have more than one outlet or shower.

 

Ours is 40m with two kitchens and two showers on that run. The feed is 22mm and return is 15mm (all well insulated) the pump is on the return with a timer set for morning, lunch time and evening with weekends a bit longer. There is a stat on the oulet of the pump. The pump is set to it's lowest flow and runs for less than five minutes of the initial time period before the stat switches it off.. We have just short of two Bar on the cold water feed to the HW tank. It all works and flows very well without control overload.