Help needed with pipework for oil-fired central heating!

21 Jan 2022
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United Kingdom
Hi there, we're hoping to install oil-fired central heating in our house but just needing a bit of help/advice with the pipework. We want to open the floors and get the worst of the runs in ourselves as our budget is limited with a baby on the way!! We have the boiler, oil tank and radiators already so almost good to go. Luckily my partner is pretty handy but we just have a few questions before we start and would appreciate any input from you lovely people:

1. What diameter of copper pipe should the runs be? 22mm for main piping and 15mm for runs to each room?

2. We've got uninsulated hung joist floor so where would you recommend putting pipework? Notch it in, put it through or under? We were thinking of hanging the pipes under the joists rather than notching but just wanted to check this was the right way to go. Also, any advice as to how wide we should cut the floors open to do the runs? Thinking about 12"?

3. What's the best pipe insulation to use?

4. We do have a pipe bender and just wondered if pipe bends are more beneficial than pre-bent soldered joints?

Got a picture of the layout of our house if that helps:


Once installed, we're going to get an engineer/plumber to check over the work. Any advice on the install would be greatly appreciated.

Many thanks in advance,
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Well, with copper pipe, you tend to notch the top of joists. With PVC pipe, you can drill joists and push the plastic pipe through. I know Speedfit fittings cost more, but much quicker to install. You can use Speedfit on copper pipe, but they are bulkier. And yes, 22mm main pipes, reduced to 15mm in the rooms. Many people place the rads under windows because you've often lost that wall space to a window, why lose more wall space elsewhere with a rad.

But as always, heating systems should ideally be designed.

As for insulation, I've seen pipes left as they are, some in grey foam, others wrapped in loft insulation.
1. 22mm for main runs, drop to 15mm to feed the last couple of Rads, (balance system when done to achieve optimum performance.)

2. Notch joists. Concrete floors, copper pipe will need sleeving to protect it, or run it in ducts so it can be got at in future if need arises.

3. Pipe Insulation, the thicker the better.

4. Formed bends are always preferable where possible, each joint is one more chance of a leak.

If you're doing a lot of soldering, a decent blowlamp is a must.
If the void under the joists is deep enough (200mm plus) I'd put the pipes under- saves problems later on with rocking floorboards and people putting screws through them. Specially with the 22mm main run, if the joists are fairly old they'll likely not be very deep so the notch depth you'd need would be outside current regulations.
While you're messing about with floorboards, consider getting some insulation under there- strawberry netting and loft roll will do nicely.
Pipe insulation- in the ground floor voids as thick as you can afford.
Joints- soldered under the floor is my preference. Speedfit etc are good but they have moving parts so should ideally be accessible. The speedfit etc. couplers are also a pain to insulate....
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Assuming enough void we would always put pipework under joists easier to insulate use galvanised band to hold pipework up.
No problem using 15mm pipe till heat load adds up to 15.000 btu then up to 22mm.
If you want to use push fit fittings try tectite at end of day that's what's in your Grant boiler! Yes bend as much as possible.
I take it that's a external boiler? If it is seriously think about fitting a plume kit they do tend to run much cleaner with one when installed in that type of position.
Also try to get a Grant registered engineer to commission will extend warranty.

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