Hws circulation


Postby Woodside » Wed Sep 08, 2010 10:05 am

I want to add a pumped circulation system to my existing domestic hw system, as the distant draw-off points result in excessive waste of water (and hot water at that!) whilst waiting for hot water to emerge at the tap. My existing distribution is in copper, but I think it would be easier to install any circulation pipework in plastic (?). I need a small pump, suitable for dhw service, but assume that it only needs to be large enough to handle heat losses in circulation pipework, so very small flow rate and head. I would much appreciate any advice on pump and pipe size selection, and how (and where) to connect circulation pipework to existing system.
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Postby plumbbob » Fri Sep 10, 2010 5:10 pm

I can completely understand why you might want to consider this but I can see so many implications galloping towards you, I really don't know where to start.

You mention it currently causes a waste of hot water, but removing hot water from an insulated cylinder to circulate it around a pipe will increase the waste even more and that is not counting the waste of energy required to run the pump continuously 24/7.

Even with over 30 years experience, I cannot recall ever seeing such an installation and believe me I have seen some odd things!

I think another big point will be the hot water cylinder. Circulating water may prevent this from working efficiently and cause all sorts of problems.

Sorry, but I just can't imagine how you could make this work.
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Postby htg engineer » Fri Sep 10, 2010 6:49 pm

What type of hot water system do you have ?
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Postby Woodside » Sat Sep 11, 2010 9:17 am

Type of hws system:
Mains pressurised, indirectly heated from dual coils: one from gas-fired heating system (in upper section of cylinder), one from solar system (lower part of cylinder). One problem is the location of the connection of any circulation return pipe to the cylinder. As far as I can make out, the only connection is at the bottom of the cylinder, so any recirc circulation would heat up the entire cylinder, so reducing the effectiveness of the solar input in winter, when the solar coil is primarily used to preheat the water being presented to the upper (htg circuit) coil. I live in a bungalow, so the hot water points are widely dispersed, resulting in excessive cold water draw-off before hot water arrives at the outlet.
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Postby Woodside » Mon Sep 13, 2010 5:33 pm

Re: plumbbob Posted: Fri Sep 10, 2010 5:10 pm Post subject:
________________________________________
Sorry, but I just can't imagine how you could make this work.

Installing a pumped dhws circulation is normal practice in most medium size commercial and industrial buildings as it gives dhw at point of use, so I am surprised that it is unusual for domestic application if the draw-off locations are not compactly located. My primary objective is for convenience of instant hot water at point of use. However, I have attempted to measure my wasted water waiting for hw to arrive. This totals about 15litres/day, which would cost me about £25/year in water cost only, not including significant energy lost down the drain. I can run a 30W motor 15hr/day (on timer) for less than that using utility electricity (but I generate from pv cells, so elec cost would be negligible for me). If I run the recirc pipe (small bore) inside hw lagging, I can substantially reduce the energy losses. I wonder how these losses compare with the wasted hot water energy? Incidentally, I also have a solar dhws, so wasted heat cost is of seasonal significance only.
Why would this jeopardise the working of the cylinder? (I could return it to the cylinder via the cw inlet, and it would only be a minimal flow rate).
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