MDPE pipe for pre heating water for unvented cylinder - safe?

[Andyinver]

I have a wood burning stove in the livingroom of my bungalow. During a look around my house with an infrared camera a couple of winters ago I
discovered that the 6ft by 4 ft masonary chimney stack which sticks up about 6 ft in the loft had a surface temperature of
around 40oc on the same side as the fire above the loft insulation when the fire had been on for a few hours.

As an experiment I lifted the insulation around the hot side of the stack and put 120litres of water in 12 10litre stackable
containers against the chimney and put cardboard cover and insulation over it, upshot was 28oc water at furthest point from
stack under the insulation over a 4 week period in winter.

Incoming water temp in winter =<5oc, I figure if i can scavenge heat from stack i should save approx 30% of water heating in winter.

Im having a new gas boiler and central heating fitted in september, the boiler is going to heat my unvented immersion/solar water cylinder by utilising its solar coil.

My plan is to put a in a loop of 150m mdpe 32mm pipe fed from the incoming mains water, coiled on the top of my stack of water containers,
(this should hold about 70litres of water, average shower usage 50ltrs), and then feed this into the cold supply of my cylinder before any of the valves etc

Can anyone foresee any problems with this, or have any advice. Any comments greatly appreciated.

My main concern is bacteria but if the water is flushed out at least twice a day during occupancy would this be sufficeint, or as the water is being heated in the cylinder before use anyway is it not a problem?

 

[Madrab]

I guess the question is, is the saving worth it after the expense of the installation? The other question is, can the mains pressure/flow handle that extra length of pipe.

 

[flameport]

and then feed this into the cold supply of my cylinder before any of the valves etc

That will result in your bacteria laden warm water appearing at some of the cold outlets, as an unvented cylinder should have a cold takeoff for use with mixer showers and similar, so that the pressure of the hot is identical to the cold.

 

[Andyinver]

Would the cold takeoff be below the floor as I can't see any pipework going to cold taps after stopcock on cold feed to cylinder.

Or would connecting pre heated water after the valves in picture prevent preheated water appearing at cold taps?

 

[James1991]

Sorry to cast a shadow on your heat recovery contraption, but if the chimney breast in the attic is getting to 40°C, does this not give you any cause for concern?

 

[Andyinver]

Yes, in that I'm paying money for wood and would like to recover some of the heat.

If you mean something else, then no. Its been in operation for 8years as is. I guess my chimney breast acts like an inefficient eastern European style masonary stove.

 

[flameport]

Or would connecting pre heated water after the valves in picture prevent preheated water appearing at cold taps?

Picture is inconclusive as it only shows a very small section of the pipe.

You CANNOT connect anything after the valves. They are safety devices, anything connected after them would remove the safety features.

 

[James1991]

The chimney shouldn’t get that hot. If anything, the further away from the heat source, the cooler the masonry should get. Do you get the chimney swept annually, does it have a metal liner? I suppose your sweep would have highlighted a problem if there was one.

On the mdpe pipe conundrum, I’d have thought using a conductive material like copper would make more sense, or even better, fit a stove with a water jacket that can carry hot water directly to one of the coils on the cylinder.

 

[Andyinver]

I don't think there's any problem with the the stove, its swept annually. Its inset, heat rises, masonry gets heated.

The stove is in the livingroom of bungalow, which is 2 rectangular blocks connected by a flat roofed hallway. hot water cylinder is in hallway cupboard on far side of hallway.

I wanted to utilise the pressurised water from mains to move the heat from chimney breast into hot water cylinder (a showers worth at a time). No pumps etc required.

Flameport, what if I connected pre heated water before valves with a closed stopcock below it, so that pre heated water can only feed cylinder, and the original cold feed feeds taps, shower etc.

 

[SpecialK]

Pipes in the loft? How will you stop them freezing when the stove is not on?

 

[Andyinver]

They're under the loft insulation, the roof of room below is pine tongue and groove, they will be as warm as the warmest part of the room below, ie never freezing.