I made my decision in retrofitting water underfloor heating! I needed to research the various options available to me in this mental market.
You don’t need a new build to install underfloor heating
Electric underfloor heating is significantly easier to install in any home project. I got 1 square metre of electric underfloor heating for my bathroom. My thoughts? Not great. It’s under ceramic tile and I feel the heat isn’t retained well, it seems to dissipate as quick as it comes on. This doesn’t even touch on the point that over a larger surface area it costs a fair bit more to run.
Having decided to pursue retrofitting water underfloor heating, I was glad to find that the market had options for all sorts of variables.
Concrete screed system – Castellated Panels
If time and money are on your hands then this is the best way to retrofit water underfloor heating.
You start with a suitable subfloor such as a concrete base or plywood screwed onto timber floorboards. Following that, building regs would want you to lay insulation boards. It’s important to do so to prevent losing all the heat generated down into the ground. The plastic pipes are then laid out in the pre planned pattern on a castellated plastic panel or “egg crates”. These trays allow bends and turns to allow your pipes snake around the room before returning to the manifold. Depending on the size of space that needs heatings you may have 150 – 200 mm gaps between pipes. The benefit of the trays are that you can use the pipe spacing that suits you.
Concrete screed over these panels and pipes is the best because of the ease of installation. The concrete will set around the pipes entire surface area. This means when the heat goes through the pipes, they will be disperse evenly throughout the new layer of concrete. This will generate a radiant heat and avoid hot and cold spots. The concrete will also retain the heat for a significant amount of time after the heating has been turned off.
Likewise, this concrete screed will also take some time to heat up. In newer houses and systems, heat pump heating may be on for hours before the room gets to the right temperature.
As I mentioned, you need time on your hands. Depending on the climate it can take several weeks before the concrete has fully dried out. It can take even longer before you can turn the underfloor heating on!
Pipe clips
Using pipe clips follows the same procedure as the castellated panels. The difference is the pipe clips are “stapled” in place directly onto the insulation boards which sit on top of the original concrete floor. This is certainly a cheaper option but requires more work to ensure the correct spacing is followed and that the angles are wide enough for turns.
Once laid, this will then take a concrete screed or self levelling compound ready for the correct floor to be laid. At this stage there is no difference between the castellated panels or the pipe clips.
Overlay panels
This is the system I used. I didn’t have several weeks on hand to wait for a perfect finish. I needed to have some form of floor ready for my kitchen delivery. There was literally nowhere else I could store the kitchen other than the room itself. This process uses tile adhesive to glue the overlay panels onto the original concrete floor. Once you have room covered in these overlay panels, you feed the pipes into pre routed slots. The panels have a foil membrane on the top which disperses the heat produced. The thickness is made of a polystyrene foam which adds some insulation to the ground below. It’s not ideal as this is less than 20mm whereas insulation boards of 50mm can be used for the above methods.
Because of the pre routed paths, you have a lot less flexibility in how the pipes will flow around the room. This wasn’t a problem for me as I only had a basic shape of around 9 square metres.
The boards were light and easy to adhere to the subfloor. For retrofitting water underfloor heating on short time and a low budget, they were perfect for my job.
What’s going on top of the underfloor heating?
The major benefit to this type of system was that you had the choice of doing a concrete screed over the final panel floor, or tile directly onto this. If time is on your hand then it is definitely worth doing a concrete screed on these panels to get the benefits mentioned above. Also tiling onto a smooth flat floor is significantly easier than a floor with loads of pipes bulging around!
If you are laying laminate or a wooden floor, then you will have to use a concrete screed as it would be impossible to lay a flat surface without it. The benefit of tiling was being able to use the tile adhesive to manipulate the final level.
You also need to cut open all the unused grooves and ensure that they’re full of tile adhesive. Doing this prevents hollow gaps which may collapse under the weight of a step on a tiled floor. This would compromise the integrity and the tiles could end up becoming loose.
Another downside I experienced was a pipe kink. When you have a kink in the pipe you are advised to use that kinked area in a straight run to prevent flow problems. My kink happened to occur on a return bend where I had no alternatives. In the end I had to cut my own groove at an earlier point and continue on the path. Now I have a noticeable cold spot where this pipe should have been!
Other considerations for retrofitting water underfloor heating
Floor height
Retrofitting water underfloor heating can add a substantial amount of height to the floor. This needs to be considered with how it works with the adjoining rooms. Will there be a noticeable step? How would the transition look? Is there an external door? If so would it actually open and close? I actually had a new external door fitted and because I didn’t tell the fitter I was adding wet underfloor heating, he installed it too low! It was my fault but he was happy to reinstall it at the increased height, otherwise the tiles would have prevented it from doing so.
Heat output
You can get standard and high heat outputs. There are calculations you can do depending on the size of your room and how many watts it would take to get to the right temperature. These will be wider diameter pipes and smaller distances between the pipe spacing. At this point it’s a cost to benefit ratio of whether you need the high output system, for example conservatories.
Pump location
The pump has to be in an accessible area and cannot be hidden behind a plasterboard wall. I had the benefit of using my victorian alcoves with the boiler and new pump placement. If you have multiple locations to heat then your system will have a manifold system. The manifold will divert water to the different zones that call for heating. The pump and manifold spacing will have to be sorted before investing in retrofitting wet underfloor heating.
Thermostat location
The thermostat will most likely be a wired connection. You will want this to be discrete so it would have to be planned which wall it would go on. I had mine on the side of a kitchen wall panel so the cable was fed behind the kitchen cabinets. Make sure that it’s out of view of the sun, otherwise it will think the room is hotter than it actually is.
To conclude retrofitting water underfloor heating
This is part of a series of posts related to my thought process in retrofitting wet underfloor heating. The wiring itself was a steep learning curve for me which I do intend on writing about. You may find interest in the first bit of plumbing I did in splitting the central heating into two zones in preparation for this project.