I'm in the process of working out what boiler to get for my new home. I'm doing most of it myself, just paying for MrGas to connect it up to the explosive stuff (we need a meter fitting anyway as the house has a supply but has never used it!!)... I would just pay someone to do it all but in order to get the house we're looking down the back of the sofas for lost pennies.... I'm determined to hang on to the Locost, however....

I've done my energy loss equations and worked out my BTU requirements for each room. And I've been quite generous with my airflow loses as well as with my fabric loses (it's an 1909 house so pre air-gap brickwork and it's Grade II so no double glazing allowed...)...

Trouble is when I add them up and convert them into kW I only actually need quite a small boiler (if I go on the kW rating of the boiler) which the manafacturers say is only big enough for a 1 bedroom flat... Which sounds dodge when it's running a 2 bed house....

why?

Is it more to do with the hot water flow rate rather than the CH? Or is it just standard to overspec the boiler quite a bit for optimum efficiency from the emitters?

What size roughly in metres area is your house and what size boiler have you calculated?

What U values have you come up with?

It's to do with warm up times from cold, as depends on combi or conventional boiler too.

Well each floor is 36m2 (it's a small house!) and I've worked out 24,000BTU or 7.032kw.

The U-values I've used from http://diydata.com/information/u_values/u_values.php

The boiler I'm looking at is Worcester 30CDi which has a good flow rate of 15.1 but its central heating output is 7.7-30.0 which seems massive overkill.... Trouble is most of their boilers have a minimum heat output in the 7s which is my BTU requirements a -1 degC.....

yes it is more to do with the DHW flow rate - i.e the larger kw rating of boiler the faster you can run DHW through the heat exchange. The 30 cdi is a cracking boiler, fitted 40 off them last year...have nothing but praise for them

Please promise me that you will not fit a Baxi or any product to do with that brand.

Oh no

Most new houses we design come in between 50-60 watts a meter, poorly insulated houses at around 100watts, I'm guessing yours is solid walls or rubble fill so on the side of caution lets say 150 watts. We usually design at an internal temp of 22 degrees with external temp at -3 degrees.

So 150 watts gives us 10.8kw say 11KW,then you need to consider water heating, most cylinders use a 3KW heater so add 3kw - total 14KW.

You now need to consider warm up time, lets say an hour this will increase the size of boiler to 20KW.

So a Worcester Bosch Greenstar 24Ri would be ideal link

Now you have a boiler sized you need to consider your radiators are they correctly sized no point having a good boiler if the rads don't do the job.

I would recommend a Megaflow hot water cylinder too if you can stretch to one.

If your on a combination boiler I would recommend at least 40KW for fast warm up of water and you will still have some capacity left to provide heating when filling a bath, obviously don't need a cylinder with a combi

[Edited on 21/9/09 by big-vee-twin]

Cheers! That's all intersting stuff. Wow- 40kw is one hefty boiler!! It'll sure get things toasty.... Cheers for the info

to bypass all the tech stuff, get something around 28kw or more. More the merrier in fact, you can always turn it down if your house gets too hot!

Yes, I suspect it's one of these things where you adjust the figures to confirm what you wanted to hear in the first place