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New Radiators Questions and Getting Larger Rads than Required

Postby GForce2010 » Fri Oct 19, 2018 6:43 pm


I am currently planning on replacing all my old radiators and I have a couple of questions.

I'm OK with the plumbing side of things and I have used an online tool to work out what BTU's I need from the rads.

I will have a TRV on every rad and I am wondering about getting larger rated rads than I need for an average day. If I understand it the TRV will limit the temp in the room so the larger rad would still use the same amount of energy to heat the room with the added benefit that I could have the room much hotter if I wanted.

I'm just thinking that by using TRV's and turning down the boiler temp it will be easier to make a more powerful rad cooler than it would be to try and make it hotter if the rating doesn't quite work out to be enough.

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Simply Build It

Postby ericmark » Tue Nov 06, 2018 1:11 pm

Yes too large with TRV not really a problem maintaining the temperature, however it will take longer getting to the temperature and at some point you will also need extra expansion tank.

So taking it in the other direction, if you fitted all fan assisted radiators they hold far less water so heat up far quicker, so if you turn on heating with a 6 to 28 kW boiler for example you can likely heat a three bedroom house in half an hour, so you could use a thermostat with geofencing and it would auto reheat house before you get home.

With over size radiators the boiler has to heat the radiator first, then the heat is only put into the room by natural convection so possibly two hours to heat rooms so geofencing will not work because your journey time home is not long enough.

I have found with this house radiators are far too big, they are rarely more than warm, and upstairs is always too hot as heat raises from room below, and over heating is a problem.

For an example the living room, huge bay windows, at 6 am heating switches on, so radiator starts to get hot, by 9 am sun shining through windows if clear day so want radiator cold to stop over heating, but still warm an hour latter, not helped as wax heads on TRV's are slow to react, so room set to 20 deg C but by 10 am at 28 deg C. Remembering to close curtains helps, but what I needed was a faster more accurate TRV head, using an electronic head still hits 22 deg C but much improved mainly as set to 18 deg C in the day and only 20 deg C in the evening then 16 deg C over night.

Next to consider is how the boiler works, today the boiler does not switch on/off it modulates so typically between 6 kW and 28 kW output, so basic idea hot water comes out, to each TRV, as each one gradually closes more water is pushed through the remaining until the by-pass valve opens, this in turn means the return water gets hotter and hotter, as the boiler detects this it modulates the flame height until it is at minimum output.

At this point the boiler starts to cycle, (switch off/on) anti cycle software alter how long before boiler switches on again, but boiler will continue to cycle all through the summer unless you turn it off. This is why we fit wall thermostats, to turn off boiler on hot day, not to control room temperature the TRV does that.

I really don't know how a over size radiator will affect the boiler.

There are other methods, with Bosch we have the Wave thermostat which tells the boiler to modulate with out having to wait for return water, however it only works in one room, better quality Boilers allow third party control gear with OpenTherm so EvoHome the TRV heads tell the thermostat (hub) what heat is required which in turn tells the boiler what temperature water is required so every room is the reference room not just one.

It is expensive, so there are cheaper methods, Nest uses a follow command so the TRV heads are connected but not as good as EvoHome, Terrier i30 heads have no wifi and just control each radiator independently.

The idea is the lower the temperature of boiler when it switches off the less heat goes out of the flue, so eBUS control saves money, but the major problem is boilers tend to be far too big for the house so they can fill a bath in a reasonable time. Likely this house only needs around 15 kW for heating, but boiler can go to 28 kW so it can quickly fill a bath, in the main it's the minimum setting that matters, today 6th Nov my boiler is hardly switching on, clearly 6 kW is much more than required so it is cycling, my son when renovating his small three bedroom house only had 2 x 13A sockets to do everything, and two oil filled 2 kW radiators heated whole house, on time clocks off while at work, living room in evening bedroom at night, and my last house as built had one 4 KW gas fire in living room with open plan house to heat whole house, and that with no cavity wall, loft insulation or double glazing. We do tend to go OTT on boiler and radiator sizes, we have stopped all the heat going out, but still use same size radiators and boilers.
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