First fix of point-to-point plumbing PEX

[CurvedHalo]

Hi all, sorry if I'm repeating other questions, I can't seem to find anyone asking this same thing? 

I'm doing a self-build and I'm currently at the point of fitting my domestic pipes. I'm using point-to-point plumbing, so plastic pex.

Q1: My plumber says that they usually fit the pipes directly to the subfloor/slab, and then cut insulation around it, and screed on top etc. While I guess this works for cold water, would this have a negative impact on the DHW, as it's essentially outside the thermal envelope? I know we could use pre-insulated pipe, but I just wanted to know about this. 

Q2: I have always had problems with leaks in copper pipes buried under the ground, which is why I've chosen this point-to-point approach, so no joints are buried. However, my first question is 

- Is there a risk of this pipe rubbing as it expands/contracts, and therefore leaking anyway? Are there any mitigating factors I can do to reduce this?

- Is there any benefit in burying this pipe within a conduit so I can replace down the line if I ever need to? 

 

Q3: as a last point.. in the plumbers design, he is proposing to take one pipe from the manifold to the bathroom's toilet, and one to the shower, and tee off this to supply the bath and sink as well, saying that the connections will all be above-ground, and the likelihood of any of these 3 consuming water at the same time is next to zero. Is this common practice? I was under the assumption that every draw off point would receive it's own supply?

 

Sorry if this is really basic, I just want to get my head around some of these decisions and most people round my are just use trunk and branch copper piping. 

[Nickfromwales]

Bonjour, as we say in Wales  

 

9 hours ago, CurvedHalo said:

Q1: My plumber says that they usually fit the pipes directly to the subfloor/slab, and then cut insulation around it, and screed on top etc. While I guess this works for cold water, would this have a negative impact on the DHW, as it's essentially outside the thermal envelope? I know we could use pre-insulated pipe, but I just wanted to know about this. 

It's not the biggest crime in the world, I did this christ knows how many times (before I stepped out of the dark and into the light) lol. I would always use the brown horsehair / hessian type of 'wrap' to give some kind of thermal separation between the pipe and the sub floor or masonry of a wall, plus I have never buried a copper pipe that wasn't first mummified in duct / gaffa tape.

 

Plumbers should know nowadays that anything that gets hot should NOT be in direct contact with something cool or cold, it's just not cricket. If, however they can fit suitably sized pipe insulation around these and THEN fit the floor insulation around that, then they can crack on. Less of a problem on pipes where they heat sporadically (hot water drawn), vs heating pipework that is very hot for a very long time.

 

The biggest issue is you cannot bury pushfit fittings in the slab, so unless the depth of finished floor is deep enough for the pipe to bend upwards and outboard of the slab and finishes, and THEN have a fitting put on, it;s "no dice".

 

9 hours ago, CurvedHalo said:

Q2: I have always had problems with leaks in copper pipes buried under the ground, which is why I've chosen this point-to-point approach, so no joints are buried. However, my first question is 

- Is there a risk of this pipe rubbing as it expands/contracts, and therefore leaking anyway? Are there any mitigating factors I can do to reduce this?

- Is there any benefit in burying this pipe within a conduit so I can replace down the line if I ever need to? 

I've had the same phone number for over 25 years, and nobody's rung me to say their pipes leaked (they damn well would too!). Soldering joints and wrapping the pipe in duct tape to prevent corrosion etc, plus working to a high standard prevents these issues afaic. Leaks come from poorly made joints, rather than pipes corroding between joints. I've seen original (bare) copper pipe that's been buried directly into concrete or mortar etc for 30+ years, and still doesn't leak (doesn't look like it would be far off by then, but what would you expect there?!). Leaks are caused by shit plumbing, in a nutshell.

 

Pipes will  be free to expand / move in the thinnest of insulation, so 9mm for cold water, 13 or 19mm for hot water, and 19 or 25mm insulation wall thickness for heating (if buried).

 

Insulation will act like a protective conduit, but if you think you can pull these in/out retrospectively then you're mistaken, sorry, unless they're arrow straight, short runs, with access both ends where they emerge from the slab. Plumb properly, fit & forget  

 

9 hours ago, CurvedHalo said:

Q3: as a last point.. in the plumbers design, he is proposing to take one pipe from the manifold to the bathroom's toilet, and one to the shower, and tee off this to supply the bath and sink as well, saying that the connections will all be above-ground, and the likelihood of any of these 3 consuming water at the same time is next to zero. Is this common practice? I was under the assumption that every draw off point would receive it's own supply?

In a true manifold arrangement, you are right, there should be a single pipe to each outlet / appliance. Question is, do you need that? It is perfectly acceptable to plumb a single 15mm cold feed to a single bathroom, as your chap suggests, and how many time are there more then 1 person in that room using water at any one time? If this is not a multi-bathroom, 6 bedroom mansion, then I'd suggest your plumbers logic is sound. This is both practical and far more cost-effective imho.

 

Last job I did: 13 outlets, 13 valves, 13 pipes.

 

 

 

 

[JohnMo]

10 hours ago, CurvedHalo said:

Q3: as a last point.. in the plumbers design, he is proposing to take one pipe from the manifold to the bathroom's toilet, and one to the shower, and tee off this to supply the bath and sink as well, saying that the connections will all be above-ground, and the likelihood of any of these 3 consuming water at the same time is next to zero

That's how I did mine, one valve per wet room and branched out in the room. Even with shower and sink running hot no issues, no issues flushing toilet while in shower etc.

[G and J]

Hmmmm.  I’m thinking dedicated 10mm hep2o pipe to each hot basin tap from cylinder manifold to deliver quick hot water.  After that just one 15mm hot and cold to each bathroom. 
 

Given the above this sounds like overkill. 

[JohnMo]

8 hours ago, G and J said:

deliver quick hot water.

We have secondary circulation also to our ensuite is about 15m from the cylinder. It's only switched on in the morning for a couple of hours. So when you want to use the sink hot water is nearly instant. Pump has built in timer and thermostat, so only runs when needed. Not really needed for the shower as by the time you are undressed the water is already started coming out warm, so only a short wait.

[G and J]

39 minutes ago, JohnMo said:

We have secondary circulation also to our ensuite is about 15m from the cylinder. It's only switched on in the morning for a couple of hours. So when you want to use the sink hot water is nearly instant. Pump has built in timer and thermostat, so only runs when needed. Not really needed for the shower as by the time you are undressed the water is already started coming out warm, so only a short wait.

Fortunately our house is small enough so the runs are short enough so I don’t need (I hope) a hot return thingy.  Fingers crossed.  

[JohnMo]

30 minutes ago, G and J said:

Fortunately our house is small enough so the runs are short enough so I don’t need (I hope) a hot return thingy.  Fingers crossed.  

I would just keep the plumbing simple.

 

Manifold good for hot and cold, locate in central location, with easy access, feed in pipes 15mm.

 

Take cold water to cold manifold from combined valve on UVC.

 

Cold feed to DHW UVC combined valve in 22mm.

[CurvedHalo]

On 27/01/2025 at 22:48, Nickfromwales said:

Bonjour, as we say in Wales  

 

It's not the biggest crime in the world, I did this christ knows how many times (before I stepped out of the dark and into the light) lol. I would always use the brown horsehair / hessian type of 'wrap' to give some kind of thermal separation between the pipe and the sub floor or masonry of a wall, plus I have never buried a copper pipe that wasn't first mummified in duct / gaffa tape.

 

Plumbers should know nowadays that anything that gets hot should NOT be in direct contact with something cool or cold, it's just not cricket. If, however they can fit suitably sized pipe insulation around these and THEN fit the floor insulation around that, then they can crack on. Less of a problem on pipes where they heat sporadically (hot water drawn), vs heating pipework that is very hot for a very long time.

 

The biggest issue is you cannot bury pushfit fittings in the slab, so unless the depth of finished floor is deep enough for the pipe to bend upwards and outboard of the slab and finishes, and THEN have a fitting put on, it;s "no dice".

 

I've had the same phone number for over 25 years, and nobody's rung me to say their pipes leaked (they damn well would too!). Soldering joints and wrapping the pipe in duct tape to prevent corrosion etc, plus working to a high standard prevents these issues afaic. Leaks come from poorly made joints, rather than pipes corroding between joints. I've seen original (bare) copper pipe that's been buried directly into concrete or mortar etc for 30+ years, and still doesn't leak (doesn't look like it would be far off by then, but what would you expect there?!). Leaks are caused by shit plumbing, in a nutshell.

 

Pipes will  be free to expand / move in the thinnest of insulation, so 9mm for cold water, 13 or 19mm for hot water, and 19 or 25mm insulation wall thickness for heating (if buried).

 

Insulation will act like a protective conduit, but if you think you can pull these in/out retrospectively then you're mistaken, sorry, unless they're arrow straight, short runs, with access both ends where they emerge from the slab. Plumb properly, fit & forget  

 

In a true manifold arrangement, you are right, there should be a single pipe to each outlet / appliance. Question is, do you need that? It is perfectly acceptable to plumb a single 15mm cold feed to a single bathroom, as your chap suggests, and how many time are there more then 1 person in that room using water at any one time? If this is not a multi-bathroom, 6 bedroom mansion, then I'd suggest your plumbers logic is sound. This is both practical and far more cost-effective imho.

 

Last job I did: 13 outlets, 13 valves, 13 pipes.

 

 

 

 

Would you suggest that this system of 13 outlets/13 valves/13 pipes is the correct correct approach? For high pressure, in an unvented cylinder system?

[Nickfromwales]

Just now, CurvedHalo said:

Would you suggest that this system of 13 outlets/13 valves/13 pipes is the correct correct approach? For high pressure, in an unvented cylinder system?

Depends on the remit from the client, as no two instances are ever the same. Further depends on whether there's a very good cold mains water supply, with plenty of l/p/m dynamic flow rate. As I've said elsewhere, I design and install to the requirement.

 

I've always done a pipe / valve to each single feed to each outlet / appliance, as that's just how I prefer to do these things, as this is defo the better solution for equal flow / pressure to every outlet (comes mostly into play when multiples of outlets are being used simultaneously).

 

One client wanted to have 4 people showering in different en suites, all at the same time, and also said there's a likelihood that will be happening whilst the kitchen sink or cloak WC was also possibly going to be used by other occupants, so I went 'nuts' on that one.

 

Most UK homes have a scabby pair of 15mm hot and cold pipes run right through the house with all the outlets T'd off willy-nilly. They seem to manage  so best to take with a pinch of salt sometimes, but also know it is fine to dial this back if it's just unnecessarily adding to cost with no real benefit.

[G and J]

25 minutes ago, Nickfromwales said:

Depends on the remit from the client, as no two instances are ever the same. Further depends on whether there's a very good cold mains water supply, with plenty of l/p/m dynamic flow rate. As I've said elsewhere, I design and install to the requirement.

 

I've always done a pipe / valve to each single feed to each outlet / appliance, as that's just how I prefer to do these things, as this is defo the better solution for equal flow / pressure to every outlet (comes mostly into play when multiples of outlets are being used simultaneously).

 

One client wanted to have 4 people showering in different en suites, all at the same time, and also said there's a likelihood that will be happening whilst the kitchen sink or cloak WC was also possibly going to be used by other occupants, so I went 'nuts' on that one.

 

Most UK homes have a scabby pair of 15mm hot and cold pipes run right through the house with all the outlets T'd off willy-nilly. They seem to manage  so best to take with a pinch of salt sometimes, but also know it is fine to dial this back if it's just unnecessarily adding to cost with no real benefit.

In our current house I carefully provisioned for high flow rates.   Quite a lot of 22mm copper.  Sadly as a result some hot taps take ages to get hot water.  Sigh.  One tries one’s best.  
 

Next time a radial system. Defo. 

[Gus Potter]

Good points from all.

 

But the houses you all seem to be talking about have more than one bathroom.

 

For all self builders that just have a small house the above probably does not apply to you.

[Nickfromwales]

Just now, Gus Potter said:

For all self builders that just have a small house the above probably does not apply to you.

 

37 minutes ago, Nickfromwales said:

Depends on the remit from the client, as no two instances are ever the same.

 

38 minutes ago, Nickfromwales said:

but also know it is fine to dial this back if it's just unnecessarily adding to cost with no real benefit.

Amen. 🙏