Assessing underfloor heating systems

The last article in The Fix (Quarter 4, 2023) looked at how to assess the feasibility of existing radiators for low temperature heating. However, challenges arise when assessing existing underfloor heating (UFH) systems for suitability at lower flow temperatures and higher flow rates.

Calculating heat output

While radiator outputs are relatively similar based upon their surface area, UFH systems have quite a wide band of heat outputs based upon many factors.

BS EN 1264 part 2 is the standard that is used for calculating both theoretical and real heat outputs of a UFH system. The key variables that determine the heat output are the heat conduction material, construction make-up including floor covering, the pipe diameter and pipe spacing, the flow temperature, system delta T and room temperature.

Every UFH system has its own unique KH value based upon each layer of the floor construction, which is determined from BS EN 1264 part 2. The formula is as follows:

KH * (MWT - Air) = floor heat output (W/m²)

The CIPHE low temperature guide covers three main system types: screed system (Figure 1), pipe in aluminium plates (chipboard intermediate layer), and pipe in aluminium plates (screed board intermediate layer – Figure 2.)

Once you know both the pipe centres and system MWT it gives you a typical heat output per m². From this you can make an initial assumption of the existing system.

In the absence of design documentation, the user can use a thermal imaging camera to at least determine the pipe centres and the active heated floor area. Once you have determined the type of UFH system and selected the floor covering, then you can use one of the three nomographs (Figure 1 and 2 – nomographs with a floor covering of 0.1) to make an assumption of the output. Once you know this then you can see if the system will deliver enough heat to deal with the peak heat loss of the room.

Another important factor is the pipe diameter. If a heat pump is coupled up to an existing UFH system, the existing pipework may not be able to deliver the required flow rate at the new delta T.

This does highlight the need for standardisation across the UFH industry to ensure that all installed UFH systems are coupled with documented designs to ensure the heating works effectively and at its maximum efficiency.