Old
Curious to understand where you're coming from on this and what's prompted you as replacing the central heating system is not for the faint-hearted as I'm sure you already know.
Is it that certain rooms are colder than others? Or is it that the radiators aren't heating up enough? You see where I'm coming from is that it might be that a decent power flush and system clean might be a quicker solution and much cheaper!
As dibs-h has said it depends on the accuracy that you want to achieve. For example, there are several calculators on the web that simply do a whole house calculation but don't go down as far as individual rooms. To go down to the individual room heat requirements you need a calculator that takes into account the six elements of each room (four walls, floor and ceiling), the u-values of those elements and that of any openings like windows, and the heat losses and gains to/from those elements from adjacent elements. Then add in a factor for ventilation change and another for domestic hot water. i did create a spreadsheet but have no idea where it went to sadly. i could dial in individual design temperatures for each room/element to simulate real life and then see what the effect would be in terms of heat requirement on adjacent rooms. For example, running the sitting room at 20 degrees dropped the heat requirement for the bedroom above quite considerably as that room gained from the heat coming from underneath.
But I do come back to my earlier question as to what is prompting you. Reason being that I've just gone through/am going through the same exercise. Maybe I/we should make it a feature in the project section :wink:
Then to do it properly (apologies if this is grandma and eggs time) is to work out how you're going to run the pipes - perhaps there is a better way than was done previously - calculate what the heat load is of each pipe run, factor in the losses due to right angle bends and couplers etc and that will then give you the size of pipe that you need to feed the radiators connected to them. Many central heating installers don't bother to do that.
When I installed my current ch system, I started with 35mm coming out of the boiler then gradually reduced to 28mm feeding a rads and a few branches of 22mm which then fed more rads and then more branches of 15mm for some of the runs.
Then you need to decide if you want an S plan or a Y plan.
I'm in the process of replacing my 18 year old boiler with an external one, replacing the hw cylinder and thermostat (because it's probably furred up after 20 years) and I want to put in place an 'intelligent' secondary return for the domestic hw as there are too many deadlegs. The latter was another design decision and on balance I screwed up on the economics and should have gone for a combi external that fed hw to the downstairs taps from the combi supply but left the rest of the system vented with the hw cylinder.
Sorry to have rambled on a bit.
Roger
I know and the more detail the more you believe the figures ! While trying to calculate the size of boiler I needed for the replacement, I did the whole house calculations (two different versions) and both came out with around 26kW. I then added up the actual heat that the radiators I put in could actually pump out and then added in a figure for dhw. Guess what? 21kW because I undersized the rads as I'm a bit of a cheapskate at times. Also explains why some people find our house cold!
