JohnMo Posted July 28 Posted July 28 45 minutes ago, mikeysoft said: And we like our reception rooms carpeted and at 21 to 22 degrees in the cold months. I would be just doing radiators, decent carpets kill UFH no matter what the spacing. Design a system just like you would for a heat pump. So oversized radiators, high gain cylinder - A system similar @marshian has, it would be cheaper to run, and install than anything you could do with your UFH, not what you want to hear, but... Run whole system as a single zone on either weather compensation or via an opentherm (if your boiler does that). @SimonD is the man to advise that. A quick review off downward heat losses U value about 0.2 Area 100m² UFH floor temp average 30 running WC. Radiator floor temp 20 Ground temp 6 degs Heat loss is U value x dT x area 0.2 x 14 x 100
mikeysoft Posted July 28 Author Posted July 28 14 minutes ago, JohnMo said: I would be just doing radiators, decent carpets kill UFH no matter what the spacing. Thanks, but we don't want radiators. Are you are saying all UFH retrofits are invalid, if you can't put down 100 to 150mm of PIR? Anyone on here with UFH and carpets? If so, what floor temp are you running? What floor temp are you running @JohnMo? 14 minutes ago, JohnMo said: Design a system just like you would for a heat pump. So oversized radiators, high gain cylinder - A system similar @marshian has, it would be cheaper to run, and install than anything you could do with your UFH, not what you want to hear, but... Running cost is not the only factor. 14 minutes ago, JohnMo said: Run whole system as a single zone on either weather compensation or via an opentherm (if your boiler does that). @SimonD is the man to advise that. We don't want a single zone - we want different temps in different areas of the house. Why do we have to have the whole house the same temp?
SteamyTea Posted July 28 Posted July 28 17 minutes ago, mikeysoft said: We don't want a single zone - we want different temps in different areas of the house. Why do we have to have the whole house the same temp Bit late to the party, but when looking at different temperatures in different rooms, you need to know the thermal losses for each individual area. Once you know that, you can play around with pipe spacing to increase or decrease output. A low temperature heating system is not run like a traditional heating system i.e. blast of heat, then switch off. They can be set up to match the varying building losses as they change during the day, seasons and extremes. Regarding the floor insulation and carpets. If there is low thermal resistance between ground and floor, and a high thermal resistance between floor and room, then you are heating the ground more than necessary. As this is a 20 year old house, do you have any idea what the air leakage is like? Have you thought of ventillation strategies? On 21/07/2025 at 21:39, Nickfromwales said: me old china mug It is china plate
marshian Posted July 28 Posted July 28 (edited) 32 minutes ago, mikeysoft said: We don't want a single zone - we want different temps in different areas of the house. Why do we have to have the whole house the same temp? You don't - You can have different temps in different areas of the house by controlling the flow rates (you can do this with flow rate supplying each loop) what you don't want to do is control temps with thermostats/actuators @JohnMo is stressing the importance of having a nice open system with no thermostat/actuator controlled zoning which is bad for efficiency (boiler cycling) I have 13 rads - only two have set backs using "Smart" TRV's - rest all are controlled by matching flow rate to heat loss to achieve the desired room temp means system volume is ~100 Litres so plenty of water to keep the boiler happy Edited July 28 by marshian Clarify
Nickfromwales Posted July 28 Posted July 28 26 minutes ago, mikeysoft said: We don't want a single zone - we want different temps in different areas of the house. Why do we have to have the whole house the same temp? If you have a well insulated, airtight house, with MVHR, then the rooms will all try to get to the same temp, eg you'll be constantly fighting a losing battle. If the house in question is not the pinnacle of energy efficiency then you'll be fine having some rooms loitering around 19oC and others pushed up to 20.5oC for eg, but cool attracts heat, laws of physics, so a room that's cooler will be elevated by it's surrounding ambient. Not an exact science, and @JohnMo and I have very different views on thermostatic mixing valves, buffers, and the what-not, but there's nothing stopping us both being reasonably right. Take it all with a pinch of salt, is my 2 cents, there's a great bunch on here and every person who takes time to comment in their own time is simply trying to help and offer their opinion, no need to get rubbed up the wrong way as there are plenty of other forums you could join if you want to be subject to that type of nonsense, . The purpose of BH is for like-minded folk to share information, knowledge, and hands-on experience (from actually having already done it, so the facts are the facts). Sit back, relax, and enjoy the ride
marshian Posted July 28 Posted July 28 36 minutes ago, mikeysoft said: Are you are saying all UFH retrofits are invalid, if you can't put down 100 to 150mm of PIR? Running cost is not the only factor. Two ways to do do UFH - With a lot of insulation under the floor or with none Skip to 11:30 to the bit that covers UFH in an un-insulated slab @JohnMo is pointing out you are in "no mans land" where a little insulation can actually be worse than none
SimonD Posted July 28 Posted July 28 1 hour ago, mikeysoft said: Are you are saying all UFH retrofits are invalid, UFH is perfectly valid in retrofits but it's only efficient and effective with minimised down losses, which means lots of insulation and flooring covering with good heat transfer. E.g. not long ago I had a call from a new customer who had just bought a very large property nearby. He and his wife wanted me to come and have a look at the underfloor heating that was on but not producing any heat. I went and had a look and quite correctly there was no heat, especially in the kitchen but the UFH was on. I asked to go and have a look in the basement and found the unility down there very nice a toasty. I touched the ceiling to find out it was lovely and warm. The UFH was working wonderfully as a radiant ceiling due to poor floor insulation. 1 hour ago, mikeysoft said: Anyone on here with UFH and carpets? You've just got to be very careful with tog values of any carpets laid on top of ufh see here from one of my renewables catalogues: 1 hour ago, mikeysoft said: We don't want a single zone - we want different temps in different areas of the house. Why do we have to have the whole house the same temp? There are many strategies for room based control, the most common of which is to use on/off switching with room by room stats, through to designing the system with proper room by room heat loss, varying pipe spacing like @SteamyTea says and then varying the flow rates as @marshian suggested. @JohnMo is at the more extreme end using self regulation and thermal mass with low flow temps. What you'll find though is that regulating individual room temps successfully is quite difficult and in well insulated houses you might as well just treat the whole thing as one room and forget about it, just as @Nickfromwales says. I've been there and tried, tying myself in circles with it all and now find myself having arguments with desk based heat pump system designers who don't seem to understand the basics of thermodynamics and why you'd want one room at 22C next to another at 18C and making sure it theoretically stays that way with the doors always shut... Now different temperatures can be achieved across different floors, but that's another story. 1 1
Nickfromwales Posted July 28 Posted July 28 1 hour ago, SimonD said: UFH is perfectly valid in retrofits but it's only efficient and effective with minimised down losses, which means lots of insulation and flooring covering with good heat transfer. E.g. not long ago I had a call from a new customer who had just bought a very large property nearby. He and his wife wanted me to come and have a look at the underfloor heating that was on but not producing any heat. I went and had a look and quite correctly there was no heat, especially in the kitchen but the UFH was on. I asked to go and have a look in the basement and found the unility down there very nice a toasty. I touched the ceiling to find out it was lovely and warm. The UFH was working wonderfully as a radiant ceiling due to poor floor insulation. You've just got to be very careful with tog values of any carpets laid on top of ufh see here from one of my renewables catalogues: There are many strategies for room based control, the most common of which is to use on/off switching with room by room stats, through to designing the system with proper room by room heat loss, varying pipe spacing like @SteamyTea says and then varying the flow rates as @marshian suggested. @JohnMo is at the more extreme end using self regulation and thermal mass with low flow temps. What you'll find though is that regulating individual room temps successfully is quite difficult and in well insulated houses you might as well just treat the whole thing as one room and forget about it, just as @Nickfromwales says. I've been there and tried, tying myself in circles with it all and now find myself having arguments with desk based heat pump system designers who don't seem to understand the basics of thermodynamics and why you'd want one room at 22C next to another at 18C and making sure it theoretically stays that way with the doors always shut... Now different temperatures can be achieved across different floors, but that's another story. Amen! 1
marshian Posted July 28 Posted July 28 Lovely post @SimonD @mikeysoft "Experience is what you get when you don't want it" My house built in the early 80's had T11 rads throughout - they were originally fed by a high temp non condensing boiler and I'd changed that to a condensing boiler and got a little energy saving Over the years single glazing got swapped out for double glazing - 25mm of loft insulation got upgraded to 75mm - empty cavity wall got insulation blown in - all giving small but incremental improvements in energy consumption A few years ago I "upsized" some of the rads in my house (mostly bedrooms, living room and hallway) from original single panel, single convector rads to double panel double convector rads - my assumption was I could run lower flow temps improve boiler efficiency and reduce my gas usage It was to be frank a bloody disaster Rooms with upsized rads heated up far faster than ones with the original rads, the TRV's closed to limit the room temp This meant that the heating circuit was half the total volume once the TRV's shut down - the boiler then cycled like crazy wasting loads of gas and my energy costs went up not down Lower flow temps meant that the rooms with original rads didn't heat up as quickly but now with a circuit volume that had shrunk and a boiler that cycled like crazy they never came up to temp unless I increased the flow temp Increasing the flow temp meant that any condensing eff gains was lost and I learnt a valuable lesson I upgraded the remaining rads and was then able to run lower flow temps for the whole house - downside was my boiler wasn't capable of running the lower flow temps to not overheat the house 45 deg C was as low as it could go with sensible burn times I did a room by room heat loss calcs - swapped some rads about and replaced a couple of rads where the move from T11 to T22 wasn't balanced to the heat loss and ended up with a house that could be heated with 30 deg flow temp even at zero outside Except my gas boiler would short cycle when set to anything below 45 So I changed the boiler for one that could run low flow temps and now I can heat 24/7 low and slow and get the benefits of boiler efficiency It doesn't matter if you use Rads or UFH you really need to have a very good idea of heat loss and align flow temps with the size of the rads or spacing of the UFH pipes to meet the needs of the room Oh and micro managing room temps to save energy doesn't work either - there is a great heat geek video on that "Stop turning off radiators in unused rooms" it covers the reasons why it can actually cost you more So in summary people on here are trying to stop you making the same mistakes we've already made
SteamyTea Posted July 29 Posted July 29 7 hours ago, SimonD said: thermal mass Try using Thermal Inertia or Thermal Effusivity depending on context (usually surface area with buildings).
JamesPa Posted July 29 Posted July 29 @mikeysoft I can only echo the good advice above. We have been brainwashed by the manufacturers of central heating controls into thinking that (a) we can control rooms individually to our hearts content and (b) we save money by doing so. In a very lossy house where the insulation between rooms is better, or at least as good, as the insulation between the house and the outside world, both of these are true. However in a reasonably well insulated house, and certainly any modern house where insulation between rooms is far poorer than the insulation between the house and the outside world, neither is true. This is simple thermodynamics, heat will travel from hot to cold and thus will tend to equalise temperatures, and there is nothing you or the heating control manufactures can do about it. Nevertheless they have to maintain the fiction in order to remain in business. In most cases micro management neither works nor is it possible, if you try hard enough you can maintain modest temperature differences between rooms where doors are kept shut and which are physically separate, but in most cases nothing more. You can also maintain some difference (usually the wrong way round unless you are careful or sleep downstairs) between two floors. Neither I nor anybody above is saying you should ignore this, but if you start with the assumption that the whole house is a single zone and depart only when its (a) actually going to work and (b) absolutely necessary, you are very likely to get a better outcome. Of course this is magnified if you have a heat pump, but still applicable if you have a boiler. Similarly with carpet over UFH. The heat for which you have paid will escape by the easiest possible route. If that's downwards, out of the house, the physics doesn't care one iota, and no amount of hoping its otherwise is going to change things. Given how much people like carpet, I'm surprised that there isn't one with embedded conductive (eg carbon) fibre - maybe there is!
marshian Posted July 29 Posted July 29 29 minutes ago, JamesPa said: @mikeysoft I can only echo the good advice above. We have been brainwashed by the manufacturers of central heating controls into thinking that (a) we can control rooms individually to our hearts content and (b) we save money by doing so. In a very lossy house where the insulation between rooms is better, or at least as good, as the insulation between the house and the outside world, both of these are true. However in a reasonably well insulated house, and certainly any modern house where insulation between rooms is far poorer than the insulation between the house and the outside world, neither is true. This is simple thermodynamics, heat will travel from hot to cold and thus will tend to equalise temperatures, and there is nothing you or the heating control manufactures can do about it. Nevertheless they have to maintain the fiction in order to remain in business. <snip> ^ spot on @SimonD I have a good example of exactly this to throw into the mix My house is T shaped, one edge of the T is north facing and has front hall and bed 4 (PC room/study) above both have 3 external walls - heat loss from these areas is quite high and they are infrequently used so it's they are the only rooms/areas where I run heating to a scheduled heating and at a much lower target temp (Typically 16 deg C). Next to bed 4 is the bathroom - it's radiator should be capable of getting the room to 23 deg C but it struggles to get above 20 and I was getting to the point in Nov last year where I was going to up size it - right until the xmas break when I spent a bit of time in the study doing an annual paper shredding exercise and had reverted the schedule to 24/7 because running low flow temps the warm up time can be a day or two to react to a change Bathroom temp went up by 1.0 to 1.5 deg in this period................ A really good example of the radiator in a warm room losing the battle because it's having it's input to the room being leached out to heat the colder room next door. I've now increased my loft insulation from 75 mm to 325 mm to reduce my ceiling losses in the whole house and my winter plan is to run a far less aggressive set back temp in bed 4 - possibly even go 24/7 with a smaller differential. There was another good video from Urban Plumbers recently where he was doing an install and the existing rads in the bedrooms were on the edge (output wise) at the design flow temps - all the bedrooms were off a shared landing and the action to resolve was not replace/upsize all the bedroom rads but add a rad to the landing to reduce the differential between the areas. The more I think about what UP's did the more I realise that my bathroom shares 1 wall with Bed 4, 1 wall with the landing (only heated from 1 rad downstairs) and 1 wall with the stairwell so the hot has plenty of cooler areas to go
SimonD Posted July 29 Posted July 29 4 hours ago, SteamyTea said: Try using Thermal Inertia or Thermal Effusivity depending on context (usually surface area with buildings). I put that one in there thinking of you 😜 I know how it grates, but I have succumbed to the laziness of using the term because everyone in the industry does - I've sat through numerous lectures, and even hydronics course lately where it's bandied around like it means something, and if we were really serious about it, we'd be building houses with water and wood in preference of conrete and stone because they actually have better thermal mass, at least in one measure, IYKWIM 😊 1
marshian Posted July 29 Posted July 29 12 minutes ago, JohnMo said: Our ensuite sits at about 20 no more, As does ours and that is fine for daily showers in the winter - it shares a wall with bed 1 which is heated to 20 Deg (Mrs Alien is not someone who copes well with a bedroom at only 18 deg C - compromises in life we all have to make them 😉 ) Bathroom and En-suite are identical from the perspective of 1. dimension (L x W x H) 2. Insulation above the ceiling 3. Insulation in the brick cavity 4. Radiators are also identical as is the flow rates & temp thro them However the bathroom needs to be a lot warmer than 20 Deg C when she wants a bath and this was a source of some discussion (hence considering the rad upsize until I realised that it can be achieved by heating the adjacent room with a less aggressive setback
marshian Posted July 29 Posted July 29 Anyway individual bathroom temp discussions are taking this slightly off topic. I'm hoping @mikeysoft has a little more insight into the subject and how looking at the whole house heating needs requires both a room by room analysis paying careful attention to the flow temp and heat input (either rad sizing or UFH spacing and pipe size)
SteamyTea Posted July 29 Posted July 29 7 minutes ago, SimonD said: we'd be building houses with water and wood Damp chalk and timber. The thing is, any contribution to thermal stability is overridden by the form factor and window to surface area ratio.
mikeysoft Posted July 29 Author Posted July 29 3 hours ago, marshian said: Anyway individual bathroom temp discussions are taking this slightly off topic. I'm hoping @mikeysoft has a little more insight into the subject and how looking at the whole house heating needs requires both a room by room analysis paying careful attention to the flow temp and heat input (either rad sizing or UFH spacing and pipe size) Yes, thanks for everyone's input and time 🤩 In all honesty, doing a room-by-room analysis seems pointless and contrary to what some of the comments have been saying: that a retrofit low-build-up system is doomed to failure in a house like ours. It really doesn't feel like there's much variation possible. I came into this knowing that a retro-fit system is a compromise. We are not a PH. We are not ASHP running 24/7. We are not after nor can achieve ultimate efficiency - we are after comfort. We can't remove our house and slab and rebuild with 300mm insulation under the slab and a 150mm insulated cavity. So I have to make the best of what we have, as do plenty of other retrofit homeowners. My basic philosophy was to put a reasonable amount of pipe in the floor (16mm on 100mm centres), as that is permanent, in order to have plenty of opportunity to put heat into the house (distribution, controls and zones aside), given that we have a low insulation house. I'm aware that we would have bigger downward losses into the slab (reminder: our 100mm raft has 100mm EPS underneath from 20 years ago), and that will increase the running cost. How we configure and run the system can be varied over time with trial and error, perhaps needing some 'equipment' changes and or reconfiguration. I'm also aware that some carpets and underlay can insulate too much, but as you all know, there are low tog options for the rooms where we want that, even though these are not a magic bullet, and we may need design changes or other mitigations in the carpeted rooms. I'm looking forward to piping up soon, and having the Cemfloor Therm go down, so we can then start the UFH on the first floor and second floor! 😉
SteamyTea Posted July 29 Posted July 29 Have you thought about additional plinth heaters (radiators with fans in them) to supplement the UFH in the colder areas? You really should do room by room heat loss calculations, they are not hard, just a bit laborious, but spreadsheets make repetition easy.
marshian Posted July 29 Posted July 29 29 minutes ago, mikeysoft said: Yes, thanks for everyone's input and time 🤩 In all honesty, doing a room-by-room analysis seems pointless and contrary to what some of the comments have been saying: that a retrofit low-build-up system is doomed to failure in a house like ours. It really doesn't feel like there's much variation possible. The room by room heat loss calc will help you to understand the heat loss of each room and that means you can get the pipe spacing right for the needs of each room Just using 100mm spacing everywhere doesn't make sense to me - if a room has a low heat loss then you are going to have to restrict the flow thro that section to achieve the target room temp - now that may well be fine for the odd room but your boiler is going to need a level of flow to be happy - why put excess pipe under a floor for a room that doesn't need it and then shrink the circuit flow to compensate? 29 minutes ago, mikeysoft said: I came into this knowing that a retro-fit system is a compromise. We are not a PH. We are not ASHP running 24/7. We are not after nor can achieve ultimate efficiency - we are after comfort. We can't remove our house and slab and rebuild with 300mm insulation under the slab and a 150mm insulated cavity. So I have to make the best of what we have, as do plenty of other retrofit homeowners. My basic philosophy was to put a reasonable amount of pipe in the floor (16mm on 100mm centres), as that is permanent, in order to have plenty of opportunity to put heat into the house (distribution, controls and zones aside), given that we have a low insulation house. I've got an 80's house - yes it now has blown CWI, SUDG windows and doors, 325 mm of Loft insulation and 75mm of PIR under the suspended ground floor as improvements over the original build spec but the heat requirements of each room can be very different (due to number of external walls in some rooms, patio doors in living areas) as a result the rad sizes are very different and where they are similar the flow rates aren't I'm using a gas boiler not ASHP but I do heat 24/7 from Sept to May I've sized my rads to meet the heat loss needs with flow temps linked to weather compensation (34 Deg C at -2.5 DegC OAT) with the flow rates controlled so I can heat 24/7 maintain a stable comfortable house temperature with a boiler that is happy because the circuit never shrinks) This is the most comfortable the house has ever been - certainly massive improvement compared to higher temperature scheduled heating 29 minutes ago, mikeysoft said: How we configure and run the system can be varied over time with trial and error, perhaps needing some 'equipment' changes and or reconfiguration. Trial an error whilst it works, takes time and is no substitute for a good system design right from the start
JohnMo Posted July 29 Posted July 29 He said he used Loopcad to design the UFH, if he did and built the house correctly the room by room calculation is done for you. But 1 hour ago, mikeysoft said: doing a room-by-room analysis seems pointless So only he knows what he has or hasn't done. I am bowing out now, he doing what he's doing, no matter if it's good bad or indifferent. Most likely end up with a thermostat in every room and boiler short cycling, like many thousands of installs. But gas/lpg is cheap...
mikeysoft Posted July 29 Author Posted July 29 1 hour ago, marshian said: why put excess pipe under a floor for a room that doesn't need it and then shrink the circuit flow to compensate? Because variables might change over the years? Use of room; floor covering; change of heat source (boiler to heat pump or some other technology); external insulation; change of owner (and their needs); etc. @Nickfromwales you do many installs - what spacings do you use and how much do you vary that room-to-room?
mikeysoft Posted July 29 Author Posted July 29 1 hour ago, JohnMo said: He said he used Loopcad to design the UFH, if he did and built the house correctly the room by room calculation is done for you. But So only he knows what he has or hasn't done. I am bowing out now, he doing what he's doing, no matter if it's good bad or indifferent. Most likely end up with a thermostat in every room and boiler short cycling, like many thousands of installs. But gas/lpg is cheap... Your advice was not to do UFH. That doesn't really help me.
marshian Posted July 29 Posted July 29 10 minutes ago, mikeysoft said: Your advice was not to do UFH. That doesn't really help me. Not actually true - @JohnMo’s advice if doing UFH was to either fully insulate or do no insulation at all. If you continued with you current floor insulation levels you’d be better of using rads I occasionally disagree with @JohnMo but on this issue I completely agree with his viewpoint I kinda agree with him with regard to you have already made up your mind and are going to do it anyway so it’s kinda pointless to comment any more. Heat loss calcs - design UFH to meet the heat loss at whatever min winter OAT is typical of where you live - set the boiler up with WC and DHWP and don’t complicate the system with buffers, mixers and thermostats because that will increase both the capital costs and the running costs
mikeysoft Posted July 29 Author Posted July 29 Just now, marshian said: Not actually true - @JohnMo’s advice if doing UFH was to either fully insulate or do no insulation at all. @marshian How do I fully insulate with 8 to 9cm build-up to play with?
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