Long Run for Hot Water - Pumping it round

[Phil Addison]

You can achieve a similar result with a pumped flow and return hot water

> system..and it probably costs no more in the long run if well insulated.

I've been thinking of doing exactly that. My system has a hot water tank in the airing cupboard and the kitchen sink at the far end of an extension. It takes 4.5 litres of run-off before the hot comes through from cold. The first 6 or 7 meters of pipe is 22mm because that also feeds the bath, the remainder being 15mm.

The scheme I have in mind is to tee off a return under the kitchen sink as close to the tap as possible, and route this back to the DHW cylinder in plastic pipe, insulating this and the feed at the same time.

I then have to think of the best place to put the pump and how to switch it on or off. As the tap is only used in burst of activity I discounted continuous (or more likely time switched) running as probably being at least or more wasteful as running off the 4.5 litres a few times a day.

I then thought about switching it with a temperature sensor in the tee. This maybe a bit better than continuous run but not much, as the although the pump would cycle on/off, the water would still be maintained at a hot temperature.

Then I wondered about a simple switch in the kitchen to turn it on whenever someone wants the tap. Hmmm... would we actually do that in advance of needing hot water. Probably not in many instances, so we would still be cussing while the 4.5 litres ran off.

Then I had the idea is to have a PIR sensor to detect activity in the kitchen preparation/sink area and have this activate the pump. This fits in well with my plumbing and power arrangement as the hot feed comes in along the top of wall units, so the pump could go on top of those, and there is also power up there feeding the hob extractor fan.

Next is how best to use the PIR detection signal. I'm thinking of powering the pump, once the trigger is received, for just long enough to get the 4.5 litres through, so we don't have continuous hot in the return. The flow will then start to cool if the tap is not actually used, so the pump will need re-triggering after maybe 5 or 10 minutes, so long as the PIR is still detecting a person present.

I'm not sure whether to put the pump in the flow or return. Because of the long pipe run the flow rate is not that good, so a pump in the flow side could also boost that. It would however need some way of ensuring it is switched on whenever the tap is turned on (otherwise it would impede the flow instead of boosting it), so it needs a flow sensor to over-ride the PIR system, and this sensor needs to be between the return tee and the tap, otherwise it will sense the return flow and keep the pump running for ever. The alternative of putting the pump in the return allows a smaller, cheaper, quieter pump to be selected, but doesn't have the boost benefit.

The pump obviously has to be potable water rated, so shower pumps are out.

Now to the return pipework. At first I though that because only 4.5 litres need be drained back through it, it can be 10 or 12mm, but then the flow rate would be fairly slow, and we may finish up still having to wait for hot to arrive. I need to do some calculations on rates, pump pressure and bores. However, putting the pump in the flow may give enough circulating pressure to permit a 10-12mm return. That would certainly simplify the installation, and minimise heat loss.

Finally the connection of the return to the DHW tank. Feed it into the

22mm cold feed at the bottom? No, that would stir the cold layer at the bottom of the tank up to the top! It needs to feed in near the hot top, but perhaps not the very top, given it will dump up to 9 litres of cold into the tank when first switched on (9 litres is the max assuming the return is the same bore as the flow). So do I need to make a new tapping into the tank, say 1/4 or 1/3 the way down (how?), or can I get away with a surrey/essex flange at the top. This will have to operate in reverse flow to the normal use as a shower take off. And what if I later decide to install a pumped power-shower? The shower at present is fed from the bath tap, and could do with a boost. This needs a bit more thought.

So to summarise, I would put a flow switch close to the tap, a tee just upstream of it, a plastic return pipe (10, 12, or 15mm?) from the tee back up to the DHW tank, connection method to be decided. The boost pump goes in the flow on top my wall cupboard (or possibly fixed to the floor if noise is going to be a problem). A PIR is fixed high up, pointing at the work area, and a bit of delay/logic circuitry goes in a Maplin box nearby, and the mains power gets taken off the extractor-hood supply. This allows the unit to be isolated by the fan isolation switch for servicing.

Any comments, and especially suggestions for the make of pump and connection to the DHW tank.

Thanks

Phil

Phil The uk.d-i-y FAQ is at

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[meow2222]

The ideal control system would be PIR plus fast response thermostat. That way you get idealised pumping, under all conditions and at all times. A stat can be implemented with electronics for fastest response.

2 connections to the copper pipe, spaced apart, would respond quickly to temp changes. The pipe itself is the detect element, changing R with temp. An opamp could measure the delta R. Very low Rs being measured though, the opamp circuit would need to be picked and made fully to instrumentation levels, it would not be a basic opamp cct. The water is so many orders of magnitude higher R thats its R can be ignored. Stat circuit should be (transformer) isolated so no anodic/cathodic currents can flow. Connections to the copper would need soldering to ensure R stability. You'd ideally want the opamp to reset itself after an hour's non pumping so that changing connection Rs would not upset the set point. Not quite as simple as I was initially thinking then.

A small low power pump in the return would be the most energy efficient.

Since youre returning cool to the hw tank, it can to go in low down, better than high up. Use slow flow and it wont stir it all up, and will be quiet too.

It might be ideal in one sense to return it to whichever part of the tank matches the return temp at the time, but implementing that would be work, complication, time and money. I reckon this method might get used in future with solar hw systems, a row of takeoff points enabling both ins and outs to go to which ever temp zone matches them best. But its not an off the shelf low cost option for now.

NT

[David Hansen]

On Wed, 25 Oct 2006 16:18:58 GMT someone who may be Phil Addison wrote this:-

The problem is bad design, with too long dead-legs. One solution is to install a local heater in the kitchen. These can be gas or electric and will only heat water as needed. There are now some fairly powerful electric ones, though they still don't match stored water.

If you want the standing losses of long runs of pipe, which will be reduced but not eliminated by insulation, then rather then anything complicated simply running the pump from a time clock is as good as anything.

[manatbandq]

been thinking of doing exactly that. My system has a hot water tank

But what are you going to gain?

At the moment you "waste" 4.5l waiting for the hot water but this could be collected for the garden. Then 4.5l is left to go cold in the pipes.

With your scheme pumping only when activity is detected you are going to leave 9l of water in the pipes to go cold between periods of activity. OK these pipes are now insulated so it stays warmer for longer. It still seems like alot of fuss and bother.

I would just insulate the existing hot pipes.

MBQ

[fred]

In article , Phil Addison writes

Bit of lateral thinking Phil, could you run a separate feed in parallel with the

22mm, just to feed the kitchen? 15mm plastic would be a nice alternative, a little less volume than 15mm copper & the makers claim equivalent flow due to being smoother inside.

Is it gravity feed that means you need the 22mm to the bath? I recently replaced a kitchen run of 3/4" with 15mm plastic and it has made the run- off time just about bearable. The pipe run was a bugger so I ran the plastic inside the old copper, passing through formed bends and one joint nicely once the joint was opened and the (internal) olive removed.

I think you're right about returning the water to the bottom of the cylinder, if your system is working correctly the return water should be cold or lukewarm. Don't expect the water to remain hot in the dead leg for long, I have 25mm wall climaflex over my long 15mm run to the bathroom and it's too cool to be useful after 20mins or so, suggesting quite a loss and keeping that hot will certainly use a more than keeping a telly on standby.

[The Natural Philosopher]

been thinking of doing exactly that. My system has a hot water tank

Indeed, With our Klargester feeding ALL 'waste water' into ditch down the side of the garden - where it promptly vanishes- all our waste water invisibly waters the trees that side.

They then make good firewood :-)

Yes. That helps a fair bit.

[Phil Addison]

Not quite sure what you are trying to achieve here.

Maybe, but I said I want to boost the supply pressure so its going on the flow. I have 15 litres/min from the hot tap and would like to double that. After all, its not even twice a combi flow ;-)

Its only cool initially, unless you take steps to shut it off when it heats up.

However, I think now I will have a two speed pump that goes fast when a person is first detected (unless a thermocouple indicates the tap already has hot water), and then reverts to a very slow re-circulating speed to maintain hot in the dead-leg. To control the pressure boost there will be a flow switch to over-ride this and set the pump to max whenever the tap is turned on.

Once hot has reached the tap I will shut off the re-circulation return with a solenoid or motorised valve. Thus the return to the tank will be (1) a fast slug of up to 9 litres of cold, or (2) a maintenance trickle of hot re-circulation water. With this addition I can take the return to the bottom of the tank as the hot trickle won't cause any stirring problem. In fact I can tee it into the cold feed from the storage tank so avoiding any tank modification.

Any suggestions for a suitable pump?

Phil The uk.d-i-y FAQ is at

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[Phil Addison]

Perhaps I didn't make it clear that my aim is eliminate the delay in getting hot water first time you use the tap, and then the annoyance when you use it again 10 minutes later and find it has cooled.

I did say insulation will be included in the scheme. Then there is the pressure boost that I want.

Phil The uk.d-i-y FAQ is at

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[Phil Addison]

So basically you suggest I don't bother with a return to re-circulate the water, but just discharge it outside. I feel tempted to quote Dr D but will just say I'm not going to do that.

Phil The uk.d-i-y FAQ is at

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[Phil Addison]

Good idea. I realised an extra benefit too, the hot tap in the bathroom will also be primed with hot, though an extra PIR in their would be needed to make that always the case.

Not sure about that.

Yes

Neat trick.

See other post for latest scheme.

Phil The uk.d-i-y FAQ is at

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[meow2222]

Thats precisely where I think you're going to hit a problem. Theres a huge difference in flow and thus pump speed between pressure doubling and a slow return flow, and I dont see any impellor pump doing that range of speed without being very likely to stall.

NT

[Phil Addison]

OK, but there are ways round that.

Put a bypass round the pump with a solenoid to open it for the trickle phase, or

Pulse modulate it. The inertia of the water in the system should more or less smooth it out.

Pulse modulate the shut off solenoid already proposed in the return

Put a restrictor in the return pipe and bypass it with a solenoid for the high flow phase. Yes, I like this one, I can use the solenoid proposed for shutting off the return, and tolerate a trickle return all the time instead of shutting the return off.

So, any suggestions for a suitable pump? I am a complete newbie on those.

Phil The uk.d-i-y FAQ is at

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