We have a tiny kitchen. All the walls have kitchen cabinets. There are no spaces for radiators. Plinth heaters lose heat quickly. Electric underfloor heating is expensive. The solution?
Wet underfloor heating!
Having already modified my central heating a few years earlier by adding drain points and additional radiators, there was little room for messing around with pipes due to carpet and laminate being laid down in the finished rooms.
The kitchen was the last space in our house to get a worthy upgrade. Because of the problems listed at the top, I sought the challenge of retrofitting wet underfloor heating in the kitchen. At the same time I was upgrading our dumb thermostat to a smart TRV Tado system so I could control all the central heating over the internet.
Challenges to consider for wet underfloor heating
- The subfloor
- The additional height
- Pump placement
- Having multiple central heating zones
- Space to route new piping discretely
Splitting the central heating for underfloor heating
In this post I am going to focus on the biggest bottleneck. You could technically install wet underfloor heating on a traditional single zone combi system. The only problem would be that whenever the thermostat would call for heat for any of your normal radiator rooms, it would also trigger the underfloor heated room up as well. Likewise, if you had a thermostat for the underfloor heating room, then it would start heating the radiators up in the house.
By splitting the central heating zones, it means you have more control over how the house is heated. This means you need at least two thermostats for each of the zones. If you have a smart system such as Tado then you can have multiple thermostats for one zone. Each of these thermostats can independently call for heat on their respective zone. The wiring allows the thermostat to only trigger that specific zone valve and to leave the other alone. This allows whole sections of pipework to remain cold and save you money on your gas bill.
The basic idea is that the Hot flow pipe shown above, splits off into two hot flow pipes. The original one feeds the radiators across the whole house and the new one feeds just the underfloor heating. Theoretically you can have multiple zones for examples such as a zone per floor plus a zone for the underfloor heating.
Planning for underfloor heating
Before cutting any pipes it was essential to make sure I had space to split the central heating zones. In addition, I would also need to make sure the path to route the new pipes was clear towards the underfloor heating pump. This was also the same for the return journey for the water. Once the water has passed through the underfloor heating circuit, it goes back to the pump to recirculate if it’s at the right temperature. For water that is too cool, it then needs to rejoin the blue cold return pipe to get reheated by the boiler.
Fortunately my boiler is in an alcove cupboard in the bedroom. This meant downstairs there’s also another alcove behind a door that I can make plumbing modifications to. If your boiler is in an awkward location and you can’t run piping through to the new retrofit area, you may struggle to make modifications without damaging the walls / floors.
I had a rudimentary method of identifying the hot and cold pipes. Turn the central heating on and feel for which one heated up first, that’s the hot flow. Eventually the cold return will heat up once the water had travelled through the radiators. I labelled them straight after to not get them mixed up for when I installed the underfloor heating pump later on.
Shopping list for zoned central heating
- 22mm Zone valves x 2
- Automatic bypass valve x 1
- 22mm Yorkshire Copper elbow x 2
- 22mm Compression T x 2
- Boiler wiring centre x 1
The amount of plumbing fittings will vary depending on the space you have to deal with. I had limited flexibility between the hot and flow pipes so had to do some awkward angles to fit the bulky fittings.
Drain the central heating
It’s only necessary to drain to the level at which you’re making the modifications. Hypothetically if you’re installing a new system on the first floor and your boiler is on the first floor. You could just drain enough water so that the top floors have no water left in them. I was working at the lowest point in the house so had no choice in the matter.
If you haven’t got a drain point, this is how I added one.
Dry run of the new fittings
It was essential for me to figure out the angles and lengths the new fittings would take. I was soldering new connections with copper so there was absolutely no give and I needed mm precision. Doing a basic mockup as shown below gave me an idea of where to cut my hot flow and cold return pipes.
The connections on those two pipes were compression fittings so I could have a bit of flexibility and room for error. All the soldering bits of my pipework was saved for the new bits. This meant I could do it somewhere else and bring it as one piece to the alcove with the boiler.
Fit the new section
I understand I’ve made quite a leap here from the previous step. Once upon a time I was a complete novice with soldering copper pipes but by following these steps I’ve never had a leak!
Automatic bypass valve
You have two zones that are controlled independently from one another, should it all go to plan.
Zone valves can fail. In the worst case scenario you could have one stuck closed, calling for heat. The other zone valve could be closed, not calling for heat. Where does this water go to? It has nowhere to go putting significant stress on the boiler.
The automatic bypass valve is a safety design that when a certain pressure is reached within the pipework, it will allow water to go straight from the hot flow to the cold return. Whilst it’s not compulsory to spec this into your modifications, it would be foolish to not make the effort for peace of mind.
A side benefit for me was that all my radiators have TRVs on them. You’re generally meant to leave one radiator without a TRV. Traditionally this is the towel heater in the bathroom. This is because of the same principle. If a thermostat calls for heat but all the TRVs on the radiators are closed, where does water go? Now my automatic bypass can support the two zone valves and the unlikely scenario of all my TRVs being shut.
Prepare second zone connections for underfloor heating
I won’t go into the underfloor heating pump or wiring specifics (S plan vs Y plan etc) in this post. This post is merely to highlight the process I went to configure the plumbing components of splitting a combi central heating. The second zone hot flow pipe I had hanging down on the right until I was ready to continue the project. Once I had the pump installed correctly I made a cut out of the original cold return flow to “seal” the system.
If you’re curious as to why I’ve used plastic pipes between the new and old zones. Laziness. I was just too tempted to bend some plastic pipe rather than measuring out more copper and using an array of elbow fittings. It doesn’t looks as nice but all this piping will be behind MDF as it forms the backing of our coat cupboard.
I wont win any plumbing awards with this but it definitely did the job!
