Hello. I am new to the blog and am working my way through all the posts. There is a lot to take in and a lot to learn. I am after some advice on where to look for underfloor heating design.
We have an 1800s farmhouse that has been well insulated and is running a 10 kW Vaillant aroTHERM Plus. The house is undergoing an extensive renovation, so to future-proof it I have decided to install a new insulated floor with underfloor heating. I am happy with the structure and foundations, which can take the depth needed for the insulation, and I am aware of the amount of work and potential cost involved.
Iâm looking for advice on how to design the underfloor heating so that it works most efficiently with the heat pump. Can anyone recommend any resources or companies that specialise in this area? I am based in Anglesey, North Wales. I do most of the work myself but would be happy to collaborate with the right company.
Youâre not wrong there, and donât forget the âDocsâ section, which is where you can find technical details of the OEM products as well as, in the âLearnâ sub-section, some theory if you need it.
Itâs good to hear youâre willing to invest in improving the underfloor insulation in an old building and to use the opportunity to install underfloor heating at the same time.
I donât believe we currently have a Docs page specific to underfloor heating. The Docs page on heat loss calculations is directly relevant, but I can think of a few UFH-specific considerations itâs probably worth capturing somewhere. Perhaps we could use this thread to collect those, with a view to turning them into a UFH-specific Docs page in due course?
Here are some notes based on my experience from installing UFH in my own new-build and from other forum posts by people retrofitting a heat pump to previously-boiler-fed UFH. They wonât all be relevant to your specific circumstances (since you already have a heat pump - and youâll have a well-insulated floor slab) but might be helpful to others reviewing this thread later:
The key to achieving the best performance from a heat pump is to transfer the heat the building needs at a low flow temperature while maintaining a high flow rate
The heat pump will choose its flow temperature to account for variations in heating requirements (typically based on a Weather Compensation algorithm) - with water kept flowing through all the UFH pipes all the time, but at a slightly higher or lower temperature as required
The UFH mixing valves which âdiluteâ the flow from the heat source with some of the return water from the UFH should never be used with a heat pump
These are generally required with a boiler, to prevent 80C water going into the UFH loops, but would significantly reduce the efficiency of a heat pump
Complex multi-zone UFH controls which work by âswitching offâ the flow to UFH zones when those are up to temperature are generally not heat pump friendly - because they reduce the flow rate once rooms start to get warm enough (and only âcall for heatâ again when those rooms have cooled)
The key point here is that the heat pump should be âin controlâ - rather than trying to make the heat pump respond to a âcall for heatâ from another controller, like a boiler would
The number of UFH zones per room and the UFH pipe spacing will need to be designed to reflect the heat loss in individual rooms (and also any differences in target temperature - e.g. slightly higher in bathrooms)
The objective here is to have the heating system ânaturallyâ deliver the right proportion of heat to the right rooms, without per-zone controls
A very tight pipe spacing should allow a very low flow temperature - but if the UFH is only on the ground floor and there are radiators on upper floors those might need a higher flow temperature to work well (which means the UFH will receive that higher temperature too)
Rooms which can receive significant passive solar gain (or internal heat gains from other activities like cooking) probably need the ability to dial-back their heating, compared with other rooms, using valve actuator heads fitted to the manifold (under thermostatic control)
The choice of floor covering can have a significant impact on the effectiveness of low-temperature UFH:
For maximum heat transfer, the best option is no floor covering at all. A polished concrete finish on the floor slab containing the UFH pipes can be very effective - but is not to everyoneâs taste and can be expensive
Ceramic tiles are generally good
Wood flooring can be OK, but it should be âengineeredâ wood flooring with a plywood backing, for stability
Carpet - especially with thick underlay - can be problematic
Take the opportunity to embed some empty tubes in the concrete slab (20mm electrical conduit or similar) which can accept temperature probes, to be able to monitor the temperature of the floor slab - rather than relying on monitoring the air temperature in the room
These are difficult / expensive to retro-fit and can be used with thermostatic controls that limit the slab temperature, for rooms that can be prone to overheating from solar gain
On a very simple installation with one UFH manifold and no radiator circuits, the heat pumpâs own circulation pump will probably suffice. With multiple UFH manifolds and/or a mix of other types of emitter, secondary circulation pumps will probably be required - likely also introducing a requirement for âhydraulic separationâ via a Buffer Tank or Low Loss Header.
Will you just be adding UFH to the ground floor and keeping radiators on the upper floor(s)? Do you already have a Buffer Tank or similar, with secondary circulation pump(s)?
Thank you so much David for such a comprehensive answer. Iâm still digesting a lot of what you have said.
There already loads of questions.
As far as I can tell I donât have a buffer tank.
I am still in the design phase the area Iâm preparing is a kitchen and a large living room. I will need to consider having a lower temp in the kitchen and design this into the system. One slight complications is that the ground floor is on 3 seperate levels each seperated by 2 steps. Are there any consideration i need to take? I was planning to have each level as a seperate UFH loop.
The upper floor will still have radiators.
Your response was very detailed thanks so much. Have you got any links or recommendations on books oâr resources where I can learn more about UFH?
Youâre very welcome Ifan. This is a topic Iâm sure will come up again, so I responded as much with a view to helping other people with the same question as addressing your specific situation.
One book Iâd recommend without hesitation is Heat Pumps for the Home by John Cantor. While that covers a wide range of heat pump topics and is very much not specific to Underfloor Heating, it does have a few pages on UFH - and addresses driving UFH with a heat pump (rather than a boiler).
Iâve not read any specific âunderfloor heatingâ books myself so other people are probably better placed to advise, but publications from well-respected professional organisations should be trustworthy - for example Underfloor Heating Design & Installation Guide Bookhowever watch out for publications which assume the UFH is being fed from a boiler, and add complex controls and mixing-down arrangements as a consequence.
I very much expect you will end up with quite a number of loops / zones. Ideally youâd want to bring those together at a single UFH manifold, so youâll have to consider if that might work or if youâd need multiple UFH manifolds.
dMb . I used the CIBSE guide (ref 1) and found it valuable when discussing UFH zones, pitch sizes and flow temperatures with my contractor/plumber. It is a good guide in my opinion.
I would like to expand on point 4 above. âThe number of UFH zones per room and the UFH pipe spacing will need to be designed to reflect the heat loss in individual roomsâ. This is based on the room heat loss calculation to obtain the W/m2. However, you may find the contractor has a âtypical pipe pitchâ for a mean water temperature. You may consider future proofing the UFH with a smaller pitch to allow for a potential lower mean water temperature. Once the pipe is in the screed, then it is literally set in cement. If you intend to always have radiators in your house, then this is probably not an important point for your renovation.
This was an important point in my own house renovation. I went from a pitch of 250mm at MWT of 35C to 100mm with a MWT of 30C. John Cantor makes a similar observation from his blog. Most importantly, my house is completely UFH with no radiators (no mixing valve). The extra length of pipe loop is not that costly in the grand scheme of things. I now get the benefit of a lower flow temperature (better COP) than initially proposed.
The loop length has a maximum limitation, depending on the stockist ~100m.