(Hopefully) Redoing CH with PEX, need advice

[b166er]

Hi all,

I looked in the Sticky's 'n' Wiki's, but couldn't find an answer, so I'm hoping I can pick someones brain

I'm redoing the HW and CH system as part of renovating my bungalow.
I understand from the GasSafe regulations that I can't plumb any gas pipe or commission a boiler, but I can do 'wet work'. Is that correct?

The CH is currently plumbed with 22mm to manifolds and then 8mm to the rads (Scotland, so the water isn't very hard)
At the moment there are 4 rads and 2 four-way 8mm manifolds (Flow/Return).
I want to add 2 more rads and copper pipes/manifolds are expensive.
I'd like to replace the lot with PEX if you think that's wise. The pipes would run under the floor, but would be clipped to the joists. Would rats still be a problem (approximately minimum 2 foot clearance)?

Am I right in thinking that it's 22m to the manifolds and then 8mm to provide the right pressure to each rad?

The boiler I was thinking of using is a Glow-Worm Flexicom cx. From what I can understand, it comes with copper tails, so can PEX be connected to those tails?

If you think I must stick with copper, is there a minimum length for the 22mm run? I would need to tee the existing 22mm and take them over to near the new rads, then use 2 two-way manifolds? This could be less than 4m from the boiler to the manifold, is that a problem? Does it also matter that there are 2 runs of 22mm with different lengths?

Basically the existing 22mm runs are 8m long with 4 rads and I would be adding 4m runs with 2 rads from the floor under the boiler if I can't do it with PEX.

Hope I've explained things clearly and that someone can help me

b166er

 

[twgas]

I would NEVER recommend either plastic pipe or microbore, most manufacturers stipulate a minimum of 1metre of copper pipe from boiler.
22mm main circuit and 15 to each rad. Having flow and return slightly diferent lengths won't be a problem

 

[bazdaman]

As above

 

[b166er]

Thanks for the replies gentlemen.

Microbore is VERY common in this part of the world and I'm unaware of anyone having major CH problems because of it, so why are you both against it?

Cheers,
b166er

 

[SandyLyon]

Nearly all new build ( Redrow, Persimmon, Taylor Wimpey, Barratts etc) will use plastic and 22mm /10mm systems.
As long as the system is dosed with the correct strength of inhibitor and maintained at the correct level you shouldn't have a problem.
Problems seem to occur when non-Barrier pipe has been used instead of Barrier and the system has been used with no or low inhibitor levels.

Copper connections from the boiler is generally accepted as 1mtr, but always check in the boiler I & M manual, this should always take preference.

If you do use plastic, you will find PB much easier to handle and work with than PEX.

 

[doitall]

Thanks for the replies gentlemen.

Microbore is VERY common in this part of the world and I'm unaware of anyone having major CH problems because of it, so why are you both against it?

Cheers,
b166er

System noise is the biggest complaint with microbore

 

[Agile]

When the cost of DIY work in copper and plastic are about the same I can never understand why anyone would choose plastic for most installations.

Nor would I recommend anyone using 8mm.

10 mm is the minimum that I would ever suggest should be used.

When its clean there is no problem but with a little dirt the 10 mm gets blocked very quickly and is difficult to clear.

Its a pity that 12 mm has not been used in the UK as its pretty much normal in France and has little disadvantage.

Tony

 

[b166er]

Thanks again for everyones input.

Agile, I can buy 50m of 15mm PEX for £32. The same in copper is £135!
How is it the same?

If I leave the existing 8m runs as 22mm/8mm copper, can I do the additional 4m runs as 22mm/10mm PEX? I take it any inhibitor will work or is there a particular type if it's mixed copper/plastic?

I read that PEX is better at higher temperature and pressure than PB?

Cheers again

 

[SandyLyon]

Thanks again for everyones input.

I read that PEX is better at higher temperature and pressure than PB?

No it isn't, both PEX and PB with withstand a short malfunction at 114 deg C at 3 bar.
The safe operating pressure decreases as the temperature increases with both materials, there's nothing to choose between the two, other than PB is much more flexible and easier to work with.

I only ever install PB.

 

[Agile]

Copper tube is far more expensive than plastic.

But plastic fittings are far more expensive than endfeed for copper.

So overall there is little difference for the average installation.

Copper will withstand about 10 bar pressure and about 250 C and both together!

 

[hanns21]

I use qual pex in all the new build a do. Tested to 10 bar before. Plastic is fine, just don't use any unnecessary fittings and always use inserts. If your using it on plumbing always fit a pressure reducing valve.

 

[SandyLyon]

Copper tube is far more expensive than plastic.

But plastic fittings are far more expensive than endfeed for copper.

So overall there is little difference for the average installation.

Agreed on the price, but the installed cost should be cheaper using plastic, if you use coils of pipe, you should use less fittings (couplings ane elbows) than copper, as the pipe can be "cabled" . Also the joints are much quicker to make than with copper so the install overall should work out cheaper.

 

[Agile]

Yes, Sandy for a paid for installation where labour is to be paid for.

But this thread is from a DIYer.

In that case using copper costs little more when there are no labour costs and in my opinion gives a better and more professional job in my opinion.

I see plastic pipe as a cheap job often used on new builds to save money.

Have to agree that properly fitted it rarely gives any problems. But nor does a Ford car although most people would prefer to be driving a BMW!

 

[SandyLyon]

Yes, Sandy for a paid for installation where labour is to be paid for.

But this thread is from a DIYer.

I see plastic pipe as a cheap job often used on new builds to save money.

Point taken with the DIY part, although how many novices are ok with end feed and blow lamps (keep the fire extinguisher and burns cream handy)

As for the new build, I think it's as much about construction methods as cost, I beam joists that have to use knock outs or drill holes rather than notching restricts what lengths of copper can be installed.

Having said that, there are some brands of plastic that I wouldn't want in my house.

 

[b166er]

Thanks again for the debate guys. Helps me make a decision.

I've decided I can accomplish what I need with 4x3m 22mm and 10m by 8mm in keeping with the existing pipework. I only need 2 compression tees and the manifolds.
There are some endfeed 2-way manifolds available, so I'll use those. (I'll have the fire extinguisher handy )
I decided copper in the end because the John Guest website warned its product would need to be protected from vermin and the CH will be underfloor.

I'll do the hot and cold water via the walls in plastic. Does the plastic need protected from cold between a concrete outer skin and sheet insulation?

Interesting the car analogy showed up
I'd rather have the Ford and pocket the change


Thanks again for all contributions