Wet Underfloor Heating systems: Domestic Heating How To series Part 2

  • 29 July 2020
  • 8 replies
Wet Underfloor Heating systems: Domestic Heating How To series Part 2
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Part 2: Wet Underfloor Heating systems


Whatever is the final floor surface, water needs supplying to the heating pipes at around 45°C. That should allow the surface temperature to be maintained at about 22-27°C.

Wooden floors can be damaged at higher temperatures whilst tiled/stone floors cease being comfortable in bare feet.

The optimal method to supply the required 45°C to the distribution manifold is to use a Thermal Store.


A Thermal Store looks similar to a traditional hot-water tank. However, instead of the main body of water being that which comes out when you turn on a hot tap, the tank is filled with the water that supplies the UFH pipes and is heated by the boiler.


The tank must be tall enough to provide a thermal gradient from bottom to top. Taking out water at a midway point creates the required 45°C for UFH.


In the above diagram I’ve added one top-coil to generate domestic hot water (DHW) on demand, and one lower coil to provide a second heat-source besides the boiler. In practice you might have yet another heating coil and one or two electric immersion heaters too. The whole point is that the thermal store can collect and retain heat from a number of different sources.


So what do you do if you want UFH but don’t have the space or finances to install a thermal store?


The answer lies with a mixing manifold:

This 6-port mixing-manifold at £350 is one from Underfloor Heating Express although there are numerous suppliers online with a variety of options available.


The mixing manifold connects straight to your main boiler feed. To lower the water temperature to the required 45°C it takes a proportion of the colder water from the Return side and mixes it back into the 65°C + water from your boiler.


Return to Domestic Heating Part-1: Strategies


UFH Manifolds


The Manifold is the distribution point from which all UFH loops are run. There is usually a separate one for each floor, and it may have up to a dozen pairs of ports to which pipes are connected.


Once connections are made, access to the manifold is rarely required so they are usually sited in a cupboard somewhere near the centre of the house.


The photo above shows a ground-floor manifold with six possible zone connections. Four currently have pipework connected, three of which have the temperature-controlled actuator fitted to the return side.


This manifold is made of brass, but stainless steel is also now common. It is made by Tiemme in Italy and has ¾” BSP standard male ports for each zone, and a 1” BSP female thread for the flow & return main connections.


By way of comparison, here’s a 7-zone Tiemme manifold with the more common butterfly valves on the main flow & return feeds. The flow connection has a thermometer, whilst the return connection has an empty thermometer pocket.


The main butterfly valves on this manifold are partially closed in order to restrict the maximum water-flow and provide a level of “balance” with another manifold which is further away from the heat-source.


Pipes and connections


UFH pipe must prevent oxygen migrating through the walls, leading to corrosion within metal parts of the heating system. The type most often used in the UK is PEX (cross-linked polyethylene). This is lightweight and flexible.

I most cases I use 3-layer PEX with a central layer of aluminium. This allows it be bent around a curve and then remain in that shape.


PEX pipe is prone to degradation from UV light. Although it can also be used above floor level to connect radiators, this should be done with care so as to eliminate locations with sunlight.Larger diameter pipework between the heat-source and the manifold can be Polybutylene, HDPE or copper. All three types are widely used within the plumbing trade. The UFH pipe within each room is usually laid in a complete run without any connections. For that reason it is supplied in lengths of 100-200m.


Try to choose a generic pipe, available from several suppliers. You don’t really want a pipe of a special style or size which would be difficult to find in future. Sizes commonly used are those with external diameters of 10mm, 15mm and 16mm. The PEX pipe in the photo is 16mm; that’s 2mm wall thickness, and thus a 12mm central bore. The smaller the bore, the higher pump pressure you’ll need to circulate the water.


I’ve standardised on using manifolds with ¾-inch BSP threads and 16mm under-floor pipes. The connectors in the above photo are made by several manufacturers, including Reliance. They can be bought from suppliers such as Underfloor Heating Supply and Underfloor Parts. Easy-fit push-connectors are available from a number of manufacturers. In some cases that means you must also purchase the manifold from that same company. Invariably this comes at increased cost.


Forward to Part3: Installing UFH in solid and suspended floors.

8 replies

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Part one here - Heating Strategies


Part 3 here - Installing wet underfloor heating

hi there,

after reading your post I’ve found this review *edited by mod*. Could you recommend any of this?


best regards

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Hi @SarahSimm and welcome to the [color=#3b6889]F[/color][color=#298070]o[/color][color=#138b55]r[/color][color=#339931]u[/color][color=#54a03a]m[/color].

Your link is to a comparison review of different approaches to “dry” electric Underfloor Heating. This series of Topics is about “wet” UFH systems with water flowing through pipes.

It is definitely my intention to cover Dry UFH, and I have practical experience of installing this beneath two types of flooring:

  • thin laminate “wood” floor (rapid, but no heat retention)
  • 50mm thick concrete & tile floor (heat retention with longer heating time)

Until I get around to writing that Topic, can I just point out that you need to first consider the relative costs of the fuels which provide the heat. Electricity for dry UFH costs about 4x as much as Gas to heat water for wet UFH.

There are a number of ways to reduce that margin:

  • use off-peak (Economy-7) electricity, but that will require a thick solid floor to retain the heat during the day, and it will be coldest in the evenings
  • choose a company offering a [color=#703000]Time Of Use tariff[/color] with lower costs outside the evening peak time (and use a heat retaining floor)
  • install an overly-large array of [color=#102090]PV Solar Panels[/color] and/or a wind turbine and store the surplus in a [color=#A08010]Home Battery[/color] once the floor is up to temperature
  • use super-insulation in the walls/roof to significantly reduce heat-losses

These will still be more expensive than using water in a wet UFH system, but you might have savings with the initial installation, depending on materials and labour costs.

Electric UFH is most often specified in two domestic situations:

  1. a tiled bathroom floor; this provides comfort but is usually in addition to the heated towel rail/radiator
  2. a new conservatory which will probably be used mainly in summer. The heating is therefore just to take the chill out of the air, and avoids extending the existing radiator pipework into the extension

Does that start to put the issue into perspective?

Oh, I got it. Thanks a lot for a comprehensive reply! :blush:

The general idea of how it works and what would be cheaper for me, personally, is clear. But I’d really love to hear a professional opinion about these models in the review.

Or is there a thread where I can ask about it? Sorry, if here isn’t appropriate

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Thanks for re-posting the Review Link @SarahSimm and I’m unsure why it was removed by a moderator. I’ve sent a Private Message to find out!

The first and most obvious thing about the six reviews is that all these electric UFH systems operate at 120v AC. This is unsurprising because the website houseweather.org is registered in Arizona USA! That’s why all the dimensions are given in inches and square feet. Aagh!

Since the voltage is half that in the UK, the current (Amps) is doubled for the same amount of heat output. However, all but one of the authors of this article are from a journalism/web-publishing background.

That’s why they also make a mess of the electrical units:

Technical features include an in-built air sensor, 120/240 voltage frequency and 10 feet of cable.

That doesn’t bode well if I want to put my trust in the accuracy of their reviews!

Secondly the Review assumes that the electric heating mats can’t be fitted around objects

These mats are great to have, although they generally cannot be cut away to make way for obstacles.


If your room or the space under consideration is irregularly shaped, you would be better off using an electric heating kit like the Warming Systems Cable Set. It is more flexible and lets you get the optimum layout for the space.

That’s simply untrue. The mat-based electric UFH systems I’ve seen and installed in the UK come with instructions as to how the mat is cut to fit the required space.

The features by which an electric UFH system should be purchased would be:

  • Does it work under wood/laminate or within a cement screed?
  • What is the power per unit area (Watts /m²)?
  • Does it have both an air thermal sensor and a floor sensor?
  • What safety cut outs does it have?
  • What type of controller does it come with, or require?

I suggest you re-start your investigation by visiting the websites for UK outlets selling products approved to British Standards.

The UnderfloorHeating Store, Underfloor Heating Direct and Floor Heating Warehouse are all sites which I’ve used previously to evaluate alternatives.

Having said that, on the (few) electric UFH systems which I’ve installed, I have bought individual components separately rather than a bundle in a box.

Remember that in the UK, you must use a professional qualified electrician to connect these electrical heating systems. He/She may agree that you can lay out the UFH cable or mat, but you will need to leave it exposed whilst the safety checks are carried out.

You certainly don’t want to have laid a floor over the top only to be told that the insulation-test has been failed!

You will also need the test certificate for house insurance and any future sale.


{quotations in this post have been taken from houseweather.org/best-electric-radiant-floor-heating}

Thanks a lot, @Transparent !!!

I’m happy someone can give me a good explanation. 

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Wow - Just reviewing this great guide (as well as this one) as part of my optimising journey - I’m definitely not best placed to fact-check this one! - Any thing that might need updating, @Transparent ? 

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Well I never actually got around to writing the section on electric UFH.

I do have the photos… but time is in short supply  :disappointed_relieved: