Recirculating Hot Water Systems

I'm contemplating setting up a recirculating system for the hot water in my bungalow.

The HW cylinder, (OSO Pressurised Stainless Steel, heated by nearby Oil Boiler on a separate zone) which I would want to retain, is in the attached garage, and the Kitchen is about 40m away. That's a lot of pipe to empty down the drain every time you want a hot sink, it also takes a long time.

But finding information on how to do it is a big problem, can anyone point me to a good information source?

I'm guessing a well insulated pipe run with a bronze pump in the return switched by a pipe thermostat at the highest/furthest point is the idea, but connected how to the cylinder? Feed from the top and return to the bottom with cold feed tee-ed in somehow. Check valve in the circuit and another on the cold feed?

Does anyone have any experience of actually doing one? Advice?

If I could get the waste down then a meter makes sense (high rateable value, 2 people).

Thanks, R.

Reply to
TheOldFellow
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Not done one. Treloar on Plumbing has one-paragraph on it, basically saying non-corrosive pump on pipe returning from furthest point, with connection back to top-third of cylinder to prevent cooler water in bottom of cylinder mixing.

As far as controlling the pump, a flow switch on the cold inlet to the cylinder would seem the way to do it. Available in the Farnell catalogue.

Reply to
dom

That won't work, you need circulation before anyone turns the tap on. Run it on a 24hr timer with a pipe thermostat.

NT

Reply to
NT

I've also not done it, although am planning to.

Grundfos do a range of pumps specifically for the purpose:

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're low power designed to run constantly, although a timer so they only run during the likely usage periods cuts cost.

A
Reply to
auctions

Thanks for that. I've just discovered my OSO cylinder has a special 'Secondary Return' tapping, for exactly this purpose. It's about half way up the cylinder, so that lines up with what you say. And it's very encouraging that the cylinder manufacturer planned for it, and even has a diagram in the installation instructions (page 16):

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don't understand the flow-switch idea, the plan is to maintain hot water at the furthest point, not to assist the flow of the 40 meters of cold when a tap is opened. It has to be a thermostat and timer I think, with possibly a push switch to override the timer.

R.

Reply to
TheOldFellow

I've just done a back-of-an-envelope calculation, and you're quite right.

Assuming all 80m (of the 40m there and 40m back pipe run the OP mentions) needs to be cleared, and it's 22mm throughout - thats 121 litres to clear. If pump performance is similar to a CH circulator (around 0.3 l/s) - that comes out as 6-7 minutes to clear!

I was thinking the heat loss from the pipe would make it essential to be demand-only.

Reply to
dom

Possibly counter intuative but have you considered smaller bore pipework? You've got a pressurised cylinder so 10mm shouldn't be a problem and maybe even 8mm would work satisfactorily. That would cut down the dead space a lot.

I hasten to add I have no experience of doing this but on the continent is seems to be the norm to use much smaller bore pipework that we use here and the flow has always struck me as being perfectly adequate for sinks.

Tim

Reply to
Tim

Temp on the pump inlet is, AFAIK, more usual. There's also a yard or two of uninsulated pipe here, so that the thermostat cools down more quickly than the water in the pipes to the taps. The pump is run to keep the entire system at reduced pressure, i.e. immediately before the cylinder return.

The return pipe can be pretty small (I've seen 10mm on a big install) as it doesn't need any appreciable flowrate - if you're in a hurry, the taps were open anyway. This also helps the pump return cool faster than the tap feed.

Treloar also notes that the return to the cylinder should enter 2/3rd up, so that you're not trying to circulate the entire cylinder volume, including any stratified cold water at the bottom of it.

In principle it might be nice to have a way to stop the pump if a tap's open (so that the pump isn't stealing flowrate when you need it), but that would need an extra pressure-drop sensor that was also insensitive to the pump's demands, otherwise the whole system would start oscillating. I can't imagine you'd add a recirculation system to any system that already had such a demand sensor, otherwise you'd be firing up a combi continuously etc.

Reply to
Andy Dingley

If the tap is used frequently then adding lots of insulation will help. You will want lots if its re-circulating so you may as well try it first.

Reply to
dennis

Good idea, especially from the bathroom ( 1/3rd way) to the kitchen. Certainly for the return as others have said.

R.

Reply to
TheOldFellow

As others have pointed out, it would be far too slow to do it that way.

Reply to
dom

From experience, 10 & 8mm cause a disproportionate restriction on flow, even at mains pressures, for 40m I'd say 15mm is borderline but worth a try.

Reply to
fred

To reply to my own. SWMBO has suggested another idea, a simple push button demand system. Button in the kitchen, calls a relay which starts the pump, thermostat cancels the relay when hot water reaches the kitchen. Saves 100l of water every time, but doesn't save any time.

By the way Dom's calculation of 6-7 minutes is wrong, it's only the

40M that needs to clear, and at the moment it's more like 3-4 minutes, much of the pipe is 15mm. But he's right about the heat loss meaning it should be demand only, no matter how thick ( say 25mm ) I lag the pipe - actually I was thinking of putting the bulk of the pipe in a box of foil celotex 120mm. All the runs are in the loft, so there it's all loss, no heating gain, but on the other hand room for thick insulation.

R.

Reply to
TheOldFellow

The thermostat would maximise the tap flow if its mounted at the tap end on the main pipe. Once the water delivery point is hot, the pump stops, and any water use keeps the pipe hot. ... I also haven't done this though.

NT

Reply to
NT

I was wondering about a tirck to get round this.

Ideally, I could just run a 10mm to the basin (fast to heat up, but low pressure) and 22mm to the shower (slower to get hot, but high pressure) , but that would mean taking up the bathroom floor.

So, how about using a thermostatic mixer and running both a a 22mm and a 10mm hot supply from boiler to bathroom. The two would split at the boiler and join again at the bathroom with the 10mm going in to the 'hot' inlet of the thermostatic mixer and the 22mm going in to the 'cold' inlet (even though it will carry hot water).

When you first turn on the tap your get hot through the 10mm only (thermostat senses outlet too cold so shuts off the 22mm inlet) so it arrives fast but at reduced pressure. Then once the hot water arrives in the 10mm the thermostatic mixer mixes cold from the 22mm. Eventually the 22mm runs hot as well and you have high pressure hot.

So, you'd get hot water quickly (but at lower pressure) at first and then hot water at high pressure.

Has anyone ever done this?

Robert

Reply to
RobertL

That's a nice idea, as it limits the pumping to the minimum needed, but in practice I would suspect that it depends on how near the pipe runs are. You have to run this long extra pipe (the tap is by definition far, or else we wouldn't be bothering), but are probably unlikely to run an extra cable.

It's worth doing this though - the point isn't to save pump energy (small) or noise at night (use a time switch), but to reduce heat losses from the pipework.

Reply to
Andy Dingley

Nice try but the disproportionately higher frictional losses on the smaller pipe would mean that the 22mm pipe would go hot before the 10mm.

The pipe volume varies with the square of the radius but the frictional losses go up disproportionately, at something like another power or 2 higher than the inverse ratio of the area so the 10mm pipe would not 'empty' first.

I tried 10mm as an experiment on a 10m run with a 3bar head and it was useless, barely a trickle of flow.

Reply to
fred

At first, I thought that's what Dom's suggested flow switch on the cold inlet was for - but he was actually suggesting something different.

*However*, if you had a flow switch on the cold feed which switched the pump *off* when flow was detected, that should have the desired effect. With no taps open, there's be circulation only - but as soon as you opened a tap, there would flow in the cold feed, and the pump would stop - preventing it from detracting from the flow to the tap.
Reply to
Roger Mills

Just what I was going to suggest. It seems wasteful (& pointless) to keep water circulating once HW has reached your kitchen sink. The tricky bit it getting the pump to know in advance when you're going to want hot water. ;-)

Perhaps you could trigger it with a wireless doorbell type switch? Then you could stick it in your pocket and trigger it on you way to the kitchen. Possibly a silly idea but you could have a few buttons so that you could have his & hers buttons as well as a few fixed ones.

Reply to
Tim

PIR occupancy sensor over the sink

Reply to
Andy Dingley

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