3m run of plastic CH pipes through an insulated (UFH) slab - risks? What pipe to use?

[Fallingditch]

Victorian stone house renovation. Existing CH system with distribution in the ground floor ceiling void: 15mm copper pipes dropped down feeding 4 radiators on the ground floor.

 

We have dug up the existing concrete and laid 150mm PIR insulation. We are about to pin the Pert al Pert UFH pipes to the insulation and then cast a 100mm concrete slab in a couple of weeks.

 

So we will be having two independently controlled circuits: new UFH on the ground floor, but retaining existing radiators on the first floor.

 

However, we will be retaining/installing a matched pair of small T3 convector radiators on the ground floor at the far end of the house. Most of the time they will be manually switched off, but on cold days when the UFH turns out to be insufficient (the house is poorly insulated), we will manually turn on the radiators to top up the temperature in the room. (Small matched pair for aesthetic reasons).

 

So here's the question. Again for aesthetic reasons, I want to bring the 15 mm plastic flow and return pipes down from the ceiling behind the plasterboard, and then run them 3m through the slab (and away from the UFH pipes) to the new radiator, so no pipes or boxing will be seen.

 

I will be letting the slab dry out for 4-6 months before turning on the system.

 

I can't find any advice as to the risk of running a short length of  central heating pipes through a concrete slab?

 

Do people do this?  Is it risky?  What sort of pipe is best to lay?  Should I lay them in conduit?  

 

It's such a short run, and the radiator will be used only sporadically, that I don't think there’s any point insulating the pipes.  
 

[JohnMo]

If you are concerned just use ufh pipe (pert-al-pert) and use a 16mm to 15mm adapter at the radiator, you can do it any length you like with that pipe joint free.

[PeterW]

3 hours ago, Fallingditch said:

It's such a short run, and the radiator will be used only sporadically, that I don't think there’s any point insulating the pipes.  

Ironically I would !! Use 15mm pipe and make any joins above ground and then use 15x9 insulation on them, duct tape the joints in the pipe insulation and then bury in the bottom of the screed.

[MikeGrahamT21]

Or alternatively do what I did, and stick an appropriate euro cone in a standard TRV and Lockshield, and have the 16mm MLCP straight into the radiator, no connectors needed. Let me know if you want any photos, and I can get some.

 

As for laying MLCP in concrete, i can't see an issue, pretty much what they were designed for.

Edited March 17, 2023 by MikeGrahamT21

[Fallingditch]

21 hours ago, MikeGrahamT21 said:

 Let me know if you want any photos, and I can get some.

 

 That would be a good idea.  They were discussed in the Plastic vs Copper thread, but I didnt see any images of what their final appearnce is ... how do they compare to (eg) sheathed 15mm plastic?

 

(The room has an original victorian panelled bay window, so I am planning on installing two 600 x 400 x 160 T3 ultraheat convector radiators either side of the bay, so they are as unobtrusive as possible.  Getting neat F&R pipes rising to them is also pretty important).

[MikeGrahamT21]

3 hours ago, Fallingditch said:

 

 That would be a good idea.  They were discussed in the Plastic vs Copper thread, but I didnt see any images of what their final appearnce is ... how do they compare to (eg) sheathed 15mm plastic?

 

(The room has an original victorian panelled bay window, so I am planning on installing two 600 x 400 x 160 T3 ultraheat convector radiators either side of the bay, so they are as unobtrusive as possible.  Getting neat F&R pipes rising to them is also pretty important).

they end up looking like this…

 

 

this is a standard 15mm TRV with a Tado smart radiator stat and 16mm MLCP pipe going into a 16mm eurocone. The only discernible difference is the compression nut is slight larger as you can see vs the standard ones for 15mm

[Nickfromwales]

Defo insulate, as they're T3's! They'll need the heat and flow preserving so 2 runs each fed off 22mm supplies ideally.

[Lofty718]

Why not run those radiators from the UFH manifold at the same low flow temp. Will help with efficiency

[markocosic]

I would:

 

 

(unless they're HUGE output)

 

 

Run 15 mm Hep2o (completely idiot proof fittings) to each radiator

 

Use 15 mm conduit so that it can expand / contract

https://www.screwfix.com/p/jg-speedfit-15blk-con-25c-conduit-pipe-15mm-x-25m-black/72840

 

Chase the wall

 

Use a Hep2o elbow converting to 15 mm copper where it pokes out the wall

 

And have the pipework come horizontally out of the wall so that you don't get muck collecting around the rad pipes poking out the floor

 

 

If the "drop" is some distance from where the rads go then just run the 15 mm in the conduit under the slab. 

 

No need to insulate per se. It won't lose much heat provided that it isn't' in direct contact with the concrete. Do protect with the conduit though.

 

 

1 kW on 15 mm Hep2o ought to be ok. That's about 3 litres/minute at a 5C deltaT (for heat pump use - worst case scenario), at 0.5 metres/sec, and will drop 0.4 kPa per metre run. You probably want to keep the head loss in the pipework below 2 metres head (20 kPa); so that gives you 25 metres of flow and 25 metres of return (equivalent length including fittings) if coming directly off a manifold. Plenty. 

 

22 mm only if they're chuff off massive output (in which case you'll need DN20 TRVs and lockshields anyway...) or the runs are long. What do you say @Nickfromwales?

 

Misc spreadsheet of nonsense attached that may be useful - it was for sizing a heat pump setup but includes some pipe sizing calcs and a link to http://www.pressure-drop.online that's handy.

 

 

Knoll Pipe Calcs.xlsx

[markocosic]

Better yet use euro style rads that have bottom entry and the TRV in built. Spendier but easier to make look neat?

 

e.g. 

https://www.stelrad.com/wp-content/uploads/2014/11/Radical-ME-Consultants.pdf

https://www.stelrad.com/radiators/range/radical-series-radiators/

[Fallingditch]

Thanks for all the excellent advice. Apologies for not replying sooner - have been away.

 

I should have been clearer about a couple of things:

- there is no heat pump supplying the system, just a 28kw Vaillant combi (propane) boiler which feeds the radiator circuit (at say 75 C)  and will feed the new ufh circuit (thermostatic valve controlled down to say 25 degrees.

 

- the pair of T3 convector radiators (which I have already bought) were chosen to be as unobtrusive as possible whilst able to emit significant heat (1.1 kW AT50). They will be manually controlled with standard TRVs

 

- the existing (old) circuit is copper, partly 22 mm and partly 15 mm. No manifold. If I T'd off the 15mm there would be an 8m run; off the 22mm there would be a 14m run.

 

So seems to me the key points are:

 

If the point of these T3 convector radiators is to occasionally get a belt of heat quickly into the room, it's less risk to supply each radiator individually with 15mm F&R?

 

and chasing the pipes into the wall does have advantages (for the same reasons as wall hung toilets) - so maybe I should drop the idea of casting them into the slab after all?

 

and they really must be insulated where they run down the channel in the wall?

 

and I might as well run them back to the 22mm (for an extra 20 m of hep2o)?

 

Many thanks to all for your comments 😊

 

 

[HughF]

On 19/03/2023 at 13:59, markocosic said:

Better yet use euro style rads that have bottom entry and the TRV in built. Spendier but easier to make look neat?

 

e.g. 

https://www.stelrad.com/wp-content/uploads/2014/11/Radical-ME-Consultants.pdf

https://www.stelrad.com/radiators/range/radical-series-radiators/

Very nice indeed….