Radiators Slow to Heat Up - Replace 8mm Microbore?

12 Apr 2015
Reaction score
United Kingdom
Installed new radiators when doing up house two years ago but they take a while to heat up.

All are installed with 15mm tails that then connect to 8mm microbore under the floor, which all connect to 28mm (I think, could be 22mm). Plumber who done the work then said I should get it swapped out when £ permitted.

Should it be a straightforward job of swapping out the 8mm for 15mm or am I best sticking a diagram together (perhaps sharing on here) and checking ive got the optimal connection / flow etc?

Got plenty of room under the house to work, so planning on swapping it out myself. Would plan to get the new pipework in position, then drain the system and swap the connections out in a day.
Sponsored Links
My old house had a Myson fan assisted fed with micro bore which really is a no no, but it worked well, micro bore will actually speed up the heating as it takes less to heat the pipes, the ideal method to rapid heat a home is fan assisted radiators, normally using 15 mm pipes but idea is the less water circulating the faster the air is heated, however the problem with fan assisted radiators is they have so little water in them they also cool down fast, so with non modulating boilers you feel the room warming up and cooling down each time boiler stops and starts.

The standard radiators has a problem if it warms up too fast, especially if the TRV is on the return, TRV heads are slow to react, even the electronic type can take 3 minutes or more to open/close, so you don't want radiator to heat up too fast, so we also fit a lock shield valve, this allows us to set each radiator so on start up when all TRV heads are wide open each radiator has a share of the water, if radiators are left wide open, then the house will heat up one room at a time, what happens is as the radiators close to boiler heats up the room that TRV closes then forcing water to next radiator, so it seems the furthest room takes a long time to heat up, well it does take a long time to heat up.

Heating guys don't have enough time to set the lock shield spot on, they may fit a pair of thermometers to supply and return and set 15°C between them, but the fine tuning really has to be done by the home occupant. So I would close the lock shield valves, then slowly open them 1/4 turn at a time until I felt warm water at the supply side, and at that point stop, I would set the TRV to 19°C and put a thermometer in the room, and slowly open the lock shield valve more until the room temperature is what it says on the TRV head. If the lock shield is open too wide, the radiator heats up quicker than the TRV can react so room temperature over shoots, and if not open enough then clearly room does not get to temperature.

Once I set the lock shield valves, with slight tweak over a week or two, then I found rooms actually got to temperature set, so as the program on the TRV head asked for more heat so the room warmed up, and clearly reverse as the TRV program called less heat so the room cooled.

However the electronic TRV heads have a built anti hysteresis software, so if I want a room to heat up fast, I have to cheat, so 7 am it moves to 22°C and 8 am down to 20°C then 11 pm down to 17°C otherwise it is 11 am before it will hit 20°C. Where I had the problem was mechanical TRV heads which did not have temperature marked in °C so never quite sure if the TRV or the lock shield needs adjusting. The way I did it was fit a electronic head (borrowed from another radiator) then set lock shield valve then refit wax head, so now I know lock shield set and over or under temperature was due to TRV setting, why TRV heads have *123456 rather than °C I don't know, but with electronic heads at £10 or with bluetooth £15 I have now got electronic heads on most radiators, OK the wifi version that links to my wall thermostat are £40 each, but only use them in main rooms.

What is more of a problem I found was not warm up speed, but cool down speed, if not set correct it can take hours for a room to cool down, so room too hot, 1/4 turn down on lock shield but it's next day before you know if that was enough.

But my house mixture of 8 and 10 mm micro bore and no problem with heat up speed, within 5 minutes of boiler firing the fan on the Myson will start and that's the furthest radiator from boiler, that one is on 10 mm micro bore, but in Mothers house with 15 mm pipes, it could take 10 minutes for furthest radiator to get warm, as there is so much water in the system, both in pipes and in radiators, however on plus side when boiler turns off, they retain some heat, so room temperature more constant.
it will be fine just swapping the 8mm for either 10 or 15mm. Then google or search on here for "balancing a heating system" and follow a guide for that.

Obviously the type and size of boiler will have an impact on how long it takes for the radiators to heat up so with the info we have so far we couldnt say how much difference, but 8mm was never really suitable for most heating systems so definitely a reasonable idea to swap it out with one or two sizes up
Thanks both, very helpful. Wasnt aware of how to balance a system, could be very useful.

Might follow-up with a couple of clarifications. Thanks
Sponsored Links
Thanks both, very helpful. Wasnt aware of how to balance a system, could be very useful.

Might follow-up with a couple of clarifications. Thanks

I think balancing should be your 1st step. If that don't work the change pipework.
But do think 8mm is not the norm and could well get blocked if you have a very dirty system that has not been maintained..

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local