Insulated hot and cold water pipes?

[Alan Ambrose]

I've been looking at these Geberit FlowFit Pre-Insulated pipes to use as supply pipes to the bathrooms running in a 50mm service cavity. Probably 20mm ID to bathroom and split locally to taps / bath / showers? (Kind of assuming there's nobody else in there when someone is taking a bath/shower). I've highlighted the ones that have max insulation but just fit in my service cavity. No need to insulate cold? Anyone have any thoughts?

 

https://www.pipestock.com/geberit-flowfit-pre-insulated-system-pipe-blue-coils

 

 

[JohnMo]

2 hours ago, Alan Ambrose said:

Probably 20mm ID

That's ok for cold, but you will waiting for ever for hot to arrive. I used a 15mm pipe from cylinder to a central manifold then 15mm to each wet room then split it to each outlet in the room. I would keep it simple just use Hep2O and add insulation if you have a secondary return system in place. Maybe some thin insulation if you feel you need it otherwise.

[MortarThePoint]

The concern with uninsulated cold pipes is condensation forming on them.

 

I think a 15mm pipe is something like a litre per 10m of run. A shower at 6L/min would clear 10m of pipe in 10s. I've even wondered about the benefits of a circulatory hot system Vs just filling the toilet cistern from the hot supply. The cistern fill would clear the pipe of the cold water before the basin tap get turned on.

 

Insulating hot pipes for between uses is pointless in my mind. Others may have experience to the contrary though. Also, regs require it.

[Big Jimbo]

9 minutes ago, MortarThePoint said:

 just filling the toilet cistern from the hot supply. The cistern fill would clear the pipe of the cold water before the basin tap get turned on.

 

I like that idea.

[sgt_woulds]

Also the toilet cistern becomes a heat source.  But Legionnaires' might be an issue

[Iceverge]

If you're set on crimp fittings and mlcp I'd run 12mm to everything hot except showers and baths and maybe the kitchen tap in a radial fashion. 

 

Colds can be Teed off within a room and 16mm. 

 

You shouldn't need 20mm pipes unless you have a gravity system.

 

A hot fill for the cistern could easily dump 150l per day of DHW down the bowl for a family. It's be less wasteful to just run the tap while you used the toilet until its hot. 

 

However....just use Hep2O. 

 

Don't insulate anything apart from the hot at the cylinder, it's a waste of time. I have a thread about my woes somewhere.

 

 

 

 

 

 

 

 

 

 

 

[MortarThePoint]

On 16/01/2026 at 19:33, Iceverge said:

A hot fill for the cistern could easily dump 150l per day of DHW down the bowl for a family.

 

Probably still less wasteful than whizzing hot water round a circulation system all day long

[BadgerBodger]

Has anyone used one of the Grundfos recirc pumps with the autoadapt feature??? So it learns the house schedule and records at the tiles you’re most likely to call for hat water. 

 

Seems to be a happy compromise for generally having hot water when you want it and not circulating unnecessarily… pipes would need lagging still though.