Undergound hot pipes

T

TellMeWhy

Hi all,

I have a house with an ATAG 32 kilowatt combi. It heats only a warm air unit with a copper coil unit nothing else. It works very well. I have a single floor, flat roof out-building at the bottom of the garden that is being converted to living accommodation, which is about 20 foot from the main house. Getting a gas pipe to it is not on as the meter is not big enough to supply another combi. I am going to install a small thermal store as this is the best solution to the needs.

The main house needs about 12 kilowatt of heat from the boiler. The boiler is man enough to heat the house and the out-building. This means I have to run underground two hot pipes, the flow and return, from the main house to the out-building. I intend to run two continuous plastic pipes in 110mm plastic drain pipes at 1 metre deep. The run maybe about 35 foot in length. If the pump on the ATAG is not man enough I can also put in a smart pump as a booster for the out-building. The flow and return pipes having insulation around them. Once in the ground the 110mm pipe will have a layer of insulation foam over the pipe to give extra insulation. A separate conduit, or direct burial cable, will be run for the control wiring.

Doe anyone see any problems with the underground hot pipes?

Is there special pipe for hot water with integrated insulation available?

Views are most welcome.
 
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Is there special pipe for hot water with integrated insulation available?
I recall seeing a display in my local plumbers merchant for a pipe designed for just what you are doing. Two plastic pipes, embedded in insulation, and the whole lot encased in a flexible(ish) outer of about 4" diameter. For good measure, the insulation was colour coded red and blue for easy identification. Idea being that you just order the length you want, drop it in the trench, job done - with no need for threading stuff together (I think assembling pipes with insulation, then into a 100mm pipe, and then into more inuslation would be "fairly difficult").

No idea what it's called or who makes it though.
Oh, scratch that, it was Uponor, and while looking for that, I found that Polypipe also do it.
 
T

TellMeWhy

Is there special pipe for hot water with integrated insulation available?
I recall seeing a display in my local plumbers merchant for a pipe designed for just what you are doing. Two plastic pipes, embedded in insulation, and the whole lot encased in a flexible(ish) outer of about 4" diameter. For good measure, the insulation was colour coded red and blue for easy identification. Idea being that you just order the length you want, drop it in the trench, job done - with no need for threading stuff together (I think assembling pipes with insulation, then into a 100mm pipe, and then into more inuslation would be "fairly difficult").

No idea what it's called or who makes it though.
Oh, scratch that, it was Uponor, and while looking for that, I found that Polypipe also do it.

Thanks just what the doctor ordered. The polypipe stuff looks like yellow MPDE gas pipe. They say it is not suitable for fresh water but OK for heating water. I can't see why not as plastic pie simplistic pipe, only the temperature is relevant. That is not an issue here as only CH water will be running through it.

I assume 35-40 foot of this stuff is expensive :(
 
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T

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Are wide sweep bends available fro 110mm pipe to get 2mm plastic through?
 
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Squirting expanding foam in the 110mm pipe would give good heat insulation.
 
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You also need to consider what insulation is suitable. If it can absorb water (and it'll be difficult to find stuff that really won't when underground) then it'll lose effectiveness over time.

And I had a feeling that the pre-made stuff might be eye wateringly expensive. But on a commercial project you'd also need to consider the manpower (cost) saving (and improvement in reliability) by using a pre-made system.
 
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You can do it, but a more elegant way is to separate the outdoor from the indoor water circuit with a heat exchanger. This stops any leaks etc in the outdoor water circuit affecting the indoor one. You can also use antifreeze etc on the outdoor water without it coming into contact with the boiler.

If you put a pump at the outhouse end on the outdoor + outhouse water circuit, with a local programmer/thermostat, then you can use a flow switch on the house end of the water circuit to provide the call to heat for the boiler, and not have to run a boiler control cable between the two buildings.
 
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Owain: I did consider a flow a switch as you mentioned but it would not work as the boiler water and thermal store water are separate and the plate heat exchanger is at the thermal store end. A trench has to be dug, it is simpler and more reliable to run a cable in the trench. I will run two, or more, cables in case one fails at anytime in the future and extra communication is needed between the two buildings for any unforeseen reason in the future.

I do intend to have the thermal store water not run underground by using a plate heat exchanger and pump at the thermal store. This way sludge is prevented from forming. Apart from a few pumps, there is no ferrous in the system.

It is all about doing the costings and some research. If spray in foam is waterproof and not that expensive then it may be in the frame. The 110mm pipes will be waterproof of course and no ground water will enter. Two 22mm pipes with normal pipe insulation can be rammed in the 110mm pipe. Rigid foam insulation then placed on top of the 110mm pipe when in the ground to prevent frost and the cold earth above getting at the 110mm pipe.

I see that the pre-made insulated piping may be cost effective in certain circumstances as it cuts down on labour cost and time. In this situation I doubt it would be viable. But costings have to be made first. 110mm pipe can be had for around £10 per 3m length plus bends. I can't see materials coming to more than approx. £140. Far less than £350 to £500.
 
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I would buy a 50m roll of lay flat 22mm poly then thread insulation onto it, tape the joints then feed inside rain water down pipe, silicone & tape the joints. That's about the cheapest way to do it in my opinion.
 
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When it comes to polyurethane foam, for the sort of thing you are talking about you can buy it in larger "two part" packs - probably a lot cheaper than in cans. Mix, pour, stand back ! I'd try and get the 110mm pipe sat on top of a layer of it so you have a complete encasement - IIRC the PU foam is itself waterproof so should nicely seal everything.
To do it I think you'd need to do two pours. A small one, and lift the pipes a bit so it can get underneath them, and then a second to cover them over. I suppose you could support the pipe on something like fishing line and sticks across the top of the trench and do one pour.

Might be worth running a piece of tape round each joint so the PU foam doesn't interfere with the rubber seal - but that's just belt-n-braces !
 

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