If I want to buy a conventional gas boiler for a 4 bed 2 bath house....

[Coldtoeswarmheart]

...what should I be asking for?

I'm holding a quote for an 18kw Baxi, how do we tell if it's the right size for the house?

If's it's a regular boiler with a separate hot water cylinder then yes it's big enough. Would question a standard cylinder feeding two bathrooms - unless you have a huge cylinder or electric showers.
Imo Worcester are a better boiler though. Take a look at the Worcester Greenstar 18ri regular - get a quote from accredited installer and you get the extra guarantee.

Just to add, always best to get 3 quotes ;-) and go from recommendation if you can.

Yes, it's a standard cylinder, at the moment we can get a bath and a shower out of it one after the other, so I'm not too concerned about that for now.

We have just had a valiant eco28 fitted, the boiler cost 975 and it was 400 for labour, a brilliant job all round at a competitive price. We are in the north though.

vicki, is that a combi or heat-only boiler?

Baxi is a somewhat budget brand.

Vaillant is a good mass-market brand, though I have Viessmann which is less common.

18kW is probably OK if you have a cylinder (it would be too low for a combi)

You can estimate the heat loss of your house here calculators.baxiknowhow.co.uk/boiler-sizing/

It can improve comfort if the radiators are, added together, larger than the heat loss calculator suggests, as you can run larger radiators at a lower and more even temperature, but they can also heat a cold house faster. It can be very annoying, on a cold night, to have a heating system that is "only just" enough and takes a long time to warm the house. The radiator outputs you will see in catalogues are calculated under ideal, and rather optimistic, conditions.

Larger radiators - could that include fitting doubles where we currently have singles?

could do. Or ones with fins on the back. Or longer ones. If you have lived in the house for a winter, you will know which rooms are chilly or slow to warm.

As an example, a typical single rad, no fins, 600mm high and 1000mm long, might have a nominal output of about 1kW, whereas I think even a smallish bedroom ought to have about 1500W which you could get from a double www.screwfix.com/p/kudox-premium-type-22-double-panel-double-convector-radiator-white-600x1000/65888 nominally about 1750W. Most of the time you will only need about half of the available power and the boiler will be ticking over quietly.

The doubles do stand out from the wall more, which matters if you have to squeeze past e.g. the side of a bed or you walk into them on coming through a doorway.

500mm is another popular height and I think a long low rad looks better than a short high one, and it spreads the heat better. In small rooms with little free wall space, like bathrooms, you might fit a short rad into an alcove or gap. You can get a datasheet of nominal outputs for the vast range of sizes from a manufacturer website.

If you are buying a new boiler and changing radiators, fit TRVs and new lockshields all round. The RAS C2 is a good one, and very neat.

btw modern boilers generally run less hot than old iron boilers, so the radiators will not be scorchingly hot, and they will give out less heat and need to be bigger. If your old boiler was undersized and the radiators never got too hot to hold, that will not apply.

Modern boilers are at their most efficient around 60C, though you can turn them higher if you need extra heat in unusual cold weather (I put mine up to 70C for the recent frosty nights, and it will go to 80).

Thanks - looking round our bedrooms, we will need to replace the radiators. The rooms are quite chilly in winter.

We replaced the shower room radiator recently and that is a single with box sections on the rear, I guess those are fins?

Off to look at the Baxi link now.

Can I butt in??

we are just about to move to a 4 bed one bath house with an "Ideal Mexico 2" heating single radiators throughout. The whole house is in it's original 1970's state and is practically uninsulated so I will be doing that first. Do you think we will manage with the old boiler for a while??

We are used to the Viessman we installed in our current doer upper...am a bit nervous about the cold.....as am doing this one on my own with 4 DC...needs must.......

You can improve the insulation and draughtproofing, and that will make the house noticeably warmer. You may be able to get a free or subsidised work, start by looking on the BG website.

An old system is likely to have a lot of sediment and corrosion, and you will do nothing but good by giving it at least a chemical clean and fitting a system filter. The clean is a cheap and easy DIY job but you will need a plumber to fit a filter (it does not need a gas engineer).

There is a risk that your old boiler might have been neglected and break down needing a part that is difficult to come by, or it may soldier on for years. Try to get a recommendation for a local independent engineer to service it.

If you are moving in this winter, look for plastic glazing film that you can tape over windows as temporary double glazing. It is not elegant but makes a quick improvement.

Try not to drape wet washing inside the house as it will cause damp and mildew.

Electric heating is much more expensive to run than gas, but would get you through a cold spell. An older house probably has a hot water cylinder with an electric immersion heater that you can use when the boiler breaks down. Fit two red insulating jackets round it, and foam lagging on all pipes. An experienced old plumber can easily swap a faulty immersion heater. If it is old it needs some skill to take out the old one without damaging the cylinder.

Thank you...that is really helpful

I've been up in the roof lagging the pipes today.....in preparation for the loft insulation that will no doubt otherwise cause them to freeze...and realised I'm going to have to ventillate the roof space aren't I? Argh.

good idea with the film...lovely ice patterns on the windows today :(

it is a conventional boiler, also 4 beds and two bath

Thanks Vicki.