Larger radiator

littlemoney

I had a combi boiler installed to replaced 42 year-old non combi boiler but kept existing radiators. My house has always is slow to heat up which I attributed to the old system. 

However in my first winter the main room still took a long time to get warm. Late morning the room was still only 18 degrees despite heating on at 6.45am

Would installing a new double panel radiator, currently single panel, mean the room would heat up more quickly.

There's nothing more I can do to to improve insulation, already doubled glazed in 1988 and cavity wall insulated in 2005.

TELLIT01

Do the radiators all get hot quickly?  If so, a larger radiator may well help.  However, if radiators are taking a long time to heat up it may mean the boiler isn't big enough.

DullGreyGuy

How was the new boiler selected? You choosing one? Just a power match to the old one? An engineer doing calcs on the BTUs required etc?

A double radiator will increase the heat output (measured in BTU normally) as long as the boiler has the capacity to take on the bigger drain from the system. 

born_again

Is anything blocking radiator such as a sofa or curtains?

FreeBear

Had an ancient Baxi boiler - Whilst it did heat the house up, it wasn't particularly fast... Replaced some of the old single panel, no fins radiators with slightly smaller double panel ones (T21). Despite being a little smaller, they had a higher rated output and heated the rooms up a little quicker.

Have since replaced the boiler with a new condensing combi - In the process, I took the opportunity to replace the remaining flat panel radiators with bigger twin panels (most T21 and a couple of T22). Can now get a 1°C per hour temperature increase which I'm reasonably happy with. I can increase that rate if the flow temperature is boosted to 70°C or even 80°C, but doing so will severely impact on fuel efficiency. Much of the time, I'm running at 55°C max.

It is quite likely that your old radiators were specified with a design flow temperature of 80°C. Modern condensing boilers need to run at a much lower temperature in order to get good efficiency. It is quite possible that your boiler has been set to either 70°C or even 60°C. In which case, fitting larger radiators all round will help to heat the house up faster. If you go even bigger, you can reduce the flow temperature which will improve efficiency further, and reduce corrosion & the build up of scale (the latter two will increase boiler reliability).

To get some idea of a suitable radiator size, punch the numbers in to an online heat loss calculator. Use a Mean Water Temperature of 45°C and perhaps add an extra 10-15%. BUT.... If you have microbore plumbing, that will place a limit on how large your replacement radiators can be.

ButterCheese

@littlemoney in order to establish a baseline, have you checked that there is no air in the system?  i.e. all rads bled properly?

Lorian

Might just need the lock shield valve opening a little more

WIAWSNB

littlemoney said:

I had a combi boiler installed to replaced 42 year-old non combi boiler but kept existing radiators. My house has always is slow to heat up which I attributed to the old system.
However in my first winter the main room still took a long time to get warm. Late morning the room was still only 18 degrees despite heating on at 6.45am
Would installing a new double panel radiator, currently single panel, mean the room would heat up more quickly.
There's nothing more I can do to to improve insulation, already doubled glazed in 1988 and cavity wall insulated in 2005.

Welcome to your new, high-efficiency, super-condensing, boiler. That outputs CH water at a lower temp than yer oldie :-)

When we bought this house, it had a floor-mounted Mexico in the kitchen, with an enamelled flue pipe coming out the top. It was set by the previous elderly couple to 'kettle', so was outputting water presumably at around 90oC. Yup, the rads were HOT!, and the house was warm. Yup, when a plastic bag landed on the flue, it melted instantly on to it; it would have done the same to skin.

The GlowWorm that was fitted shortly afterwards is set to output at a typical ~70oC - still quite high by modern standards - and in winter the house takes ages to get up to temp. The boiler has plenty of 'capacity', but simply cannot deliver a water temp high enough to suit the single-panel rads (the mains ones which I've since replaced with doubles).

So, to answer your Q, yes, fitting a larger-output rad should do the trick. And this is the best solution, as it means you should be able to keep the boiler's output temp lowish, which means it runs with greater efficiency.

Some caveats, of course - are you sure the existing rads are getting as hot as they 'should'? Ie, are they at the expected temp, given whatever the boiler's output temp is set at?

FreeBear

WIAWSNB said:

littlemoney said:

I had a combi boiler installed to replaced 42 year-old non combi boiler but kept existing radiators. My house has always is slow to heat up which I attributed to the old system.
However in my first winter the main room still took a long time to get warm. Late morning the room was still only 18 degrees despite heating on at 6.45am
Would installing a new double panel radiator, currently single panel, mean the room would heat up more quickly.
There's nothing more I can do to to improve insulation, already doubled glazed in 1988 and cavity wall insulated in 2005.

Welcome to your new, high-efficiency, super-condensing, boiler. That outputs CH water at a lower temp than yer oldie :-)

When we bought this house, it had a floor-mounted Mexico in the kitchen

...

The GlowWorm that was fitted shortly afterwards is set to output at a typical ~70oC - still quite high by modern standards

The new super-condensing boiler will only be "high efficiency" if it runs at a low flow temperature (to be pedantic, a low return temperature) - The lower, the better up to a point.

Some plumbers are still using a design flow temperature of 70°C, and radiator manufacturers generally quote output based on a ΔT50°C (or a 70°C flow temp). Building Regulations for new heating systems (either a new build or a renovation) now require a design flow temperature of 55°C or lower* - This came in to effect back in 2023. It is unfortunate that this lower flow temperature doesn't apply to boiler swaps and a shame that most boilers have a maximum operating temperature of 80°C.

*) Building Regulations Part L (sections 5.8 to 5.10) contains a cop-out where there is insufficient space for low temperature emitters (either radiators or UFH), a higher flow temperature can be used.

Albermarle

Some caveats, of course - are you sure the existing rads are getting as hot as they 'should'? Ie, are they at the expected temp, given whatever the boiler's output temp is set at?

OP - As above - what is the boiler flow temperature set at and what is the pump speed set at?

How long from the heating coming on does the radiator feel hot to touch?

RavingMad

We replaced our lounge radiator from 800mm to 1200mm and that's made a difference to the room in my mind