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.
 

 

[Iceverge]

2 hours ago, MortarThePoint said:

 

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

 

Could do the sums on it.......

 

150l of DHW water heaters to say 50 deg is about 7kWh. 

 

30m of 22mm looped pipe and a room temp of 20deg and a water temp of 50deg is a heat loss of 250w. 

 

Say you run it 24hrs a day and you get 6kWh. 

 

Put it on a timer or a PIR sensor and you'd probably half it. 

 

Of course you might have less flushes in the first place. 

 

Your mileage will vary. 

 

 

 

I reckon it's still nothing compared to running a 10mm pipe run radially from the UVC. 

 

Will do sums later ...

 

 

 

 

 

Edited January 17 by Iceverge

[Andeh]

We have a very large recirculation, probably 100m? We're a bungalow with high ceilings, and fairly spread out layout. 

 

I thinks it's a 22mm circulation loop outbound, 15mm inbound, with very thick Armaflex pipe insulation wrapped all around it. 30 to 40mm thick maybe? I wanted high flow as much as possible. 

 

The circulation is on for about 14 hours of the day, but is timed to come on in the cheap night tariff when DHW comes on, so the whole loop warms up during cheap period.

 

It drops a 49 degree 300L cylinder 3 degrees from DHW being turned off around 5.45 am, and me walking past it now 90 mins later. Some natural cylinder loss as well. 

 

I'd say that's totally worth it for what is otherwise a near 1 minute wait at the kitchen tap for hot water. 

 

Two core bathrooms on the circulation loop is within 10 inches if the tap.. Instant... The kitchen/other bathrooms is annoying as it's in the ceiling above it, so takes around 5 seconds (!) to get to the tap, I shoulda got them to loop it into the cupboard. Can't win them all!! 

Edited January 18 by Andehh

[BadgerBodger]

Yes, my thoughts were that we are a pretty habitual family and our DHW usage is pretty much confined to two distinct blocks of time 0500-0800 (showers and washing up) and 1700-2000 (baths, showers and washing up) with random intermittent use inbetween. 
 

I’m putting the tank right above the main cloakroom which will get the majority of the use during the day. But the longest run is likely to be 55m if I did it radially (one of the kids bedrooms) and when it’s bathtime it’s bathtime if you know what I mean. 
 

Reckon I’m pretty convinced that recirc is the most water efficient (we’re on a pumped treatment tank) and could well be the most energy efficient if well lagged AND suitable automation/control is installed. 

[Alan Ambrose]

Can I ask - does the recirc pump somehow not interfere with the mains pressure supply from the UVC?

[BadgerBodger]

Someone smarter than me will have to answer that precisely. I haven’t researched the install that far yet. 
 

I’ll have a stab though and someone can correct me  My logic says no. The cylinder is topped up with mains water via mains inlet (top if I recall last time a was meddling with one) and then the flow which becomes the pump feed exits bottom and the return would be another top entry. So as long as the cold water supply keeps up with the usage (there would be no interference with flow) only the usual dilution of heat caused by the cold water top-up 

 

this is roughly based on my current immersion set-up so it may differ with ASHP but again my logic says no. 

[Nickfromwales]

3 hours ago, Alan Ambrose said:

Can I ask - does the recirc pump somehow not interfere with the mains pressure supply from the UVC?

No. The pipe running from the UVC / hot manifold is direct to the outlet. 
 

The HRC pump is on the return, at the UVC, so has no effect on the delivered lpm. 
 

Sizing the pipework is critical for differing distances from the plant / UVC, and working out which outlets are needed most frequently etc, so needs a bit of thought, but a good design will reap excellent results. 
 

As stated above, even taking the return off at a point not immediately behind the outlet has significant penalties, and my thoughts are why the feck go to all this trouble, cost etc and still suffer any kind of compromises?

[Iceverge]

6 hours ago, BadgerBodger said:

But the longest run is likely to be 55m if I did it radially

 

55m!? 

 

Is that a typo? 

 

 

[BadgerBodger]

No. But thinking about more deeply a good bit less. Tank is at east end nearest heat pump.  Length of build from on end to the other 32m + 6m elevation between GF and FF. ad in a bit for not being able to make the most direct route. Call it 45m on the safe side. 
 

Nonetheless. Having to draw down on 45m pipework to have a shower, run the water to wash your hands is gonna waste a good lot of water and I imagine, in the case of the WHB, you’ve probably washed your hands before you get the hot water in the most extreme case.

 

The kitchen and master bed are still 15 and 25m respectively! 

 

[JohnMo]

18 hours ago, BadgerBodger said:

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. 

No not used one, but I have a pump with a thermostat and a simple timer. Works fine.

 

We have a manifold in the centre of the house, from UVC, then to each wet room. Secondary circulation is only to the ensuite which is furthest away from the cylinder. But with the secondary return off the shower is flowing hot by the time I have undressed a d ready to get in - 20m pipe run in 15mm.

 

[Iceverge]

26 minutes ago, BadgerBodger said:

No. But thinking about more deeply a good bit less. Tank is at east end nearest heat pump.  Length of build from on end to the other 32m + 6m elevation between GF and FF. ad in a bit for not being able to make the most direct route. Call it 45m on the safe side. 
 

Nonetheless. Having to draw down on 45m pipework to have a shower, run the water to wash your hands is gonna waste a good lot of water and I imagine, in the case of the WHB, you’ve probably washed your hands before you get the hot water in the most extreme case.

 

The kitchen and master bed are still 15 and 25m respectively! 

 

 

Jebus ....

 

Recirculating loop on a PIR I imagine is the way to go. 

 

 

 

 

[Stu789]

I'm thinking over a similar issue on a much smaller bungalow - is running 15mm pipework for bath/shower hot water & a 2nd 10mm hot water pipe for the sinks almost as good as HWC?

 

2 bathrooms and kitchen sink, longest run from DHW cylinder is ~10m to Bathroom sink / bath, mains water pressure is pretty decent at over 3 bar.

At 4l/min flow out of the tap & 15mm pipe it could take 20secs to get hot water to the furthest tap (6-8sec if using 10mm OD pipe)

Alternatively run 15mm pipe to shower and bath and a 10mm pipe to ensuite sink and then on to bathroom sink

It does mean there are 2 hot water pipe runs to the bathrooms but they are not particularly long & a HWC approach would have a 2nd hot water pipe for the return anyway.

 

 

[Nickfromwales]

2 hours ago, Stu789 said:

I'm thinking over a similar issue on a much smaller bungalow - is running 15mm pipework for bath/shower hot water & a 2nd 10mm hot water pipe for the sinks almost as good as HWC?

 

2 bathrooms and kitchen sink, longest run from DHW cylinder is ~10m to Bathroom sink / bath, mains water pressure is pretty decent at over 3 bar.

At 4l/min flow out of the tap & 15mm pipe it could take 20secs to get hot water to the furthest tap (6-8sec if using 10mm OD pipe)

Alternatively run 15mm pipe to shower and bath and a 10mm pipe to ensuite sink and then on to bathroom sink

It does mean there are 2 hot water pipe runs to the bathrooms but they are not particularly long & a HWC approach would have a 2nd hot water pipe for the return anyway.

 

 

Is this built yet?

[Iceverge]

I would do 10mm to every hot tap except the shower and bath. 

 

6l/min is what we get at the kitchen tap and I think it's plenty. 

 

If you want 10l/min I just blend in a bit of cold with the tap mixer. It also limits less economical types than yours truly pouring endless quantities of hot water down the drain too if you're feeling a bit scrooge like. 

 

 

 

 

[Stu789]

1 hour ago, Nickfromwales said:

Is this built yet?

Not yet - Planning app is Pending Consideration.