I am considering my options for a renovation project which will require substantial changes to my central heating and domestic hot water system. I was considering going a thermal store with a biomass boiler (wood pellet) and solar thermal, to supply:

  • domestic hot water
  • high temperature water for radiators
  • low temperature water for underfloor heating

While researching these options I've discovered that underfloor heating needs carefully controlled water temperature. Installation guides for floor coverings make explicit requirements about how the materials are commissioned with respect to the underfloor heating.

So I assume that the underfloor heating system, when used in conjunction with a thermal store has some method of modifying the available water, mixing hot with cooler if hot is too hot? If this assumption is correct what part of the system manages this and where does the mixing occur?


Yes, this is supposedly possible with a thermal store. Quoting from a manufacturers' website:

The Thermal Store can be used to capture energy from a variety of heat sources, such as solar, heat pumps, biomass and CHP, and is capable of providing hot water at varying temperatures to suit different heat emitters within a building. For example the Thermal Store can supply an underfloor heating system that may require 40°C, radiators requiring 70°C and hot water at 55°C, all from the same unit.

Yes, qualified engineers would be involved in the commissioning of such a system. However, there's an amount DIY in designing the system in the first place and I may take on some of the installation work myself (e.g. laying the pipework for the underfloor heating).

I'm seeking understanding in how such a system balances the temperature requirements.

  • You're not thinking of radiators and radiant flooring, are you? Oct 15, 2012 at 19:56
  • @TheEvilGreebo: I'm not sure if you're questioning my use of terminology or my intentions. My research has suggested that a thermal store may be used for this purpose, I don't quite understand how, hence the question. Yes, I'm considering using two different thermal emitter technologies.
    – MattH
    Oct 15, 2012 at 20:05
  • I'm questioning your intentions. So you want some rooms with radiators and some rooms with radiant flooring? Oct 15, 2012 at 20:17

4 Answers 4


This isn't a DIY project. You need to have someone who has experience engineering these systems give you input. Steam radiator systems are not something that a DIYer should be designing; there are a lot of design aspects that are not intuitive and need an expert's hand.

On top of that, the question that you're asking here is one that you need to take up with the manufacturer of the floor heating system that you're considering and the manufacturer of the thermal storage and furnace system.

Again, this is not a DIY project -- it's not something where you can head on off to your local hardware store and they'll have all of these products in stock. You need to at least consult with all of the manufacturers of the components that you're considering, or you need to have an experienced firm design the entire system.

  • Is my question beyond the scope of this stackexchange site?
    – MattH
    Oct 15, 2012 at 20:46

Thank you all for your input.

After a third night of searching, I've found an answer on this website that satisfies my curiosity and it turns out that the answer is quite simple: The hot draw from the thermal store is mixed with the cold return from the underfloor heating as necessary.

The temperature of the water entering the UFH zone is regulated by a thermostatic mixing valve. Water returning from the UFH zone is directed back into either the thermal store or the heat pump depending on the function of the heat pump and the position of the return diverter valve.

The schematic isn't an exact match for my envisioned scheme (I don't have the space for a ground-source heat pump and I'm not convinced by the efficacy of air-source), but it explains the details I was missing.


The system you are considering needs to be designed by a professional with experience in such systems. It is a common issue where the temperature of the transfer medium is inappropriate for the device moving the energy in or out of the building envelope. There are a number of solutions depending on your unique situation. Cold water is commonly used for tempering since it is usually readily available, is compatible with most piping systems, and has decent heat capacity. Air or refrigerant may be viable media in some situations.

Where and how the tempering is done will depend on your specific system. It may be direct mixed or use some sort of intermediate transfer device, depending on the compositions of the main transfer and tempering media. In your situation where a range of temperatures are required for various methods of heating the dwelling, it is common to first supply high temperature devices first, then use the cooler return media to supply lower temperature devices. Obviously, further adjustment of temperature may be required between the two types of devices, but since the temperature differences involved are much smaller, the energy required is much smaller, making a very efficient system.

The details of such systems are beyond the comprehension or interest of many ordinary people, so information targeting lay people will be frustratingly short of specific useful details. If you want to learn more, focus on materials that target professionals, in particular those specializing in alternative energy systems. Even if you don't fully understand everything, you'll still get a better understanding of what's involved, and will become a well informed consumer.

  • It's not so much a lack of comprehension or interest that keeps details hidden, but liability and market protection.
    – Adam Davis
    Oct 16, 2012 at 23:09

Radiant heat systems use a boiler system which heats the water to the desired temperature for the system in question.

It keeps that water in a continuous loop, optionally with a water reservoir (although that to me seems wasteful - reservoirs (aka tanks) are for "on demand" heat as in for the shower/tub/sink), to minimize the amount of water used overall (it also is cheaper to reheat warm water than to heat cold water).

I've never heard of a system which injects cold water to get the right temp - that's self defeating - the systems simply just heat it up enough.

If your intention is to supply both radiators and radiant flooring, and they have different core temperature requirements, then you're going to need multiple systems, one for each different temperature needed.

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