Extended house, feels cold all the time - bigger boiler needed?

[Alloutofusernames1]

We have gone from having 4 beds 1 bath to 5 beds 3 baths. The boiler is 28KW and is a good one but looks quite old (we have only been here a few months).
The house feels cold and sometimes a couple of the radiators are not on at all even though we have not touched them.
Is it because the boiler can't manage the extra rads? If it is relevant we have a Megaflow system for water.
My problem is that if I do get a new boiler, options are going to be very limited for a powerful one as the space where it can go is only 53cm or at a push about 60cm high and most seem to be 65-70 cm tall.

Is there anything else we can do?

Make sure it’s bled, flushed, balanced, no valves turned off, pump adjusted, etc

OK so a good service visit by a heating engineer plumber might make a difference?

Yes get it serviced and they can tell you about its efficiency and life expectancy. Boilers are alot more efficient now.

Make sure you balance the radiators. You can find tutorials on You Tube. We needed to after extending and adding radiators.

Is the boiler running at full power, all the time? If yes, then the boiler might be underpowered.

But if not, then the radiators are for some reason not able to use the heat properly.

If you are lucky, they just need balancing.

But on extensions, a common bodge error is to add new radiators onto the pipes of the old ones, which are not big enough to supply the extra.

Turn off your hot ones. Do the cold ones now get hot?

How many of your radiators have TRVs?

Do you think the radiators were added by a skilled heating engineer?

28kw will almost certainly be ample unless it's Buckingham palace or has no roof.
If there's an extension it should be really well insulated, after a comprehensive extension on an old house you might actually have less heat demand, as we did.
I'd also recommend getting someone to take a look, there must be a circulation issue or blockage preventing all the radiators from warming up. They should all get toasty withing 15 minutes of turning on the heating if they are standard domestic radiators.

Thank you for the replies and sorry it took me a while to get back on the thread. After rebalancing it is ok. Phew!