hot water 'dead-leg' solution please


Postby gazpaz » Mon Sep 10, 2007 2:18 pm

I believe the term is deadleg but in case I'm wrong ... I have a storage hot-water system and like it or not, a bathroom that is at the end of a considerable run of pipe, with the result that it takes a while for hot-water to come through first thing in the morning. I'd like a solution that unobtrusively bridges that gap.
I've been told that I need a return loop on the hot water pipe back to the hot-water cylinder together with a circulating pump that circulates hot water around this loop. OK, that means a new pipe-run which will be difficult to thread through the building -> expense and mess.
I'd like something that is more straightforward. From first principles, what I think I'd like is something along the lines of a small local hot water cylinder connected in-line with the hot-water pipe. The cylinder would have a modest wattage electric heater (it won't have to produce instantaneous hot-water) and will only have a slightly larger capacity than the deadleg storage (say ca 5 - 10litre). The total assembly could be small enough to tuck away under the bath. When a bathroom hot tap is opened, cold water from the deadleg displaces the hot from the local cylinder to the bathroom tap, until hot water arrives from the main storage, when by thermostatic valving it bypasses the local cylinder and flows direct to the tap leaving the local cylinder with cold/cool water that can be heated ready for the next time. An appropriate control system to switch the local cylinder heating on/off appropriate to the pattern of hot-water use would complete the picture.
Does anyone know of some such (or another) solution?
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Postby The Heating Doctor » Mon Sep 10, 2007 9:18 pm

Trust me what you are trying to do is far more difficult and expensive than the correctly suggested secondary return with a bronze pump to circulate the water. Electrical work in a bathroom or Kitchen must be carried out by a part P registerd electrician no ifs or buts get caught trying to do this as a DIY job can result in very large penalties. Now to the pipe work you would require an expansion vessel to take up the expansion of the hated water and a multipoint heater. If children or elderly people are to use the taps from which this hot water is to be drawn you would want to fit thermostatic blending valves. Of course you could always live with the long draw off. Good luck either way.
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Postby gazpaz » Tue Sep 11, 2007 9:50 am

Many thanks Heating Doctor for your response.
I was hoping there might be a ready-made solution along the lines I'd described rather than lashing my own together.
But out of interest, why would there be any need for an expansion vessel for the heated water - with the device in line with the hot-water supply pipe why couldn't the expansion simply be accommodated via that pipe back to the hot-water cylinder?
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Postby The Heating Doctor » Wed Sep 12, 2007 11:35 pm

Hi gazpaz,

You could not fit the unit in line with the cylinder it would have to be an independent circuit, ie a vessel of 10 litres fed by mains cold water and heated by an immersion heater, these are made by companies such as Heatrae, Santon & Zip. To try and couple a extra storage vessel on to your hot water draw off from your cylinder would be a pointless excercise, one reason is depending upon pressure and type taps can deliver anything up to 9 l/min, there for your 10 liters could be gone in just over one minute. Then you are back to waiting for the dead leg to catch up again. Also you now have a vessel which acts like a large pipe and the pressure drop across it would be huge and can reduce the flow from your taps to a trickle. Blow down a straw, place your hand at the other end you will feel air movement, now blow down a 6" dia pipe, when you place your hand at the other end you will feel nothing. The same volume of air is moving but down a straw the pressure drop is small. Same principle applies.
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Postby DONFRAMAC » Thu Sep 13, 2007 4:26 am

Many years ago I fitted an under-sink water heater to supply hot/blended water to a small wash-basin. In its more normal overhead location, the expansion dribbles into the sink ( the output pipe swivels ).
In the under-sink application, the unit is positioned upside-down, and the element righted. The cold feed comes via a 3-pipe special blender swan-neck tap, which has a cold feed in, cold out to the heater, and the hot line from the heater to the blender tap. the cluster of 3 pipes below the tap are in 1/4 inch dia.,chrome.
The water heater is 13KW, 10 litres, with variable thermostatic control.
20 years ago the water heater cost £86, and the special tap ~£60, but it is essential as it is the vent from the 10 litre tank.
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