Replacement Gas Boiler Installation – What Size Boiler?

fezster

The problem with most domestic installers (and this is very much driven by consumers wanting to pay the lowest price possible) is that I've yet to come across one that does proper calcs to work out what the heat loss of the house is. The fact all 3 of yours have not even made a ball park attempt is pretty poor. You can do this yourself with some guesstimates and half an hour of your time here: https://www.heat-engineer.com/

I would also highly recommend this website for some very good advice, and consider paying a fee to have a system properly specced up for you: https://www.theheatinghub.co.uk/

I've not used them but I had to go through a very steep learning curve and spent more money than necessary because of a poor initial installation, and wish I'd had someone like them to help me.

Your house sounds quite large and not a typical domestic installation. This can lead to problems if the existing installation is not designed properly. Here are some things to consider (I'm giving you a list of things which applied to me, but may or may not apply to your setup):

- The boiler needs to be sized to match the heat loss of the house when it is -1 outside. Ignore all advice which tries to match a boiler to the number and size of radiators.

- It is better to have oversized radiators to allow your boiler to run at lower flow temps (and condense more) 

- Older boilers (not sure how old yours is) were designed to run at an 11 degree delta. Modern condensing boilers run at a 20 degree delta. This means your radiators will output less heat (you can check radiator spec sheets to see how the output differs compared to average flow/return temps)

- If you don't already have zoned heating, it's worth considering, esp if you have a large property. 

- You may want to consider a low loss header and/or plate heat exchanger setup, especially if you have undersized pipework and require a larger than domestic pump. 

- Try and get weather compensating controls for more efficency and if you have an unvented cylinder, go for a PDHW (priority direct hot water) setup.

That's all I can think of for now. Will add more if needed. 

Another_Level

Bendy_House said:

From what I can see, the 27kW Greenstar Ri can have as low as a 9kW output when required. It's actually quite hard to 'oversize' a boiler these days, due to the fact they do 'modulate' as mentioned before (eg, you'd usually choose a combi boiler for it's DHW performance, and not on the number of rads you have, so most could well appear 'too big'.)

Interesting point raised before too - since it's a heat-only boiler, it also has to heat up your DHW cylinder. For that, the temp would need to be 60oC plus. I'm guessing, therefore, you'd have to have the boiler's output going at over 70oC to enable the cylinder to heat up quickly enough. Does this mean that, when outputting to CH, it'll also be at 70+ degrees?

The answer is - I don't know. But I think it's worth an email to WB to ask.

Certainly, as the 'demand' is near to being met (ie, rads nicely warm/DHW cylinder nicely topped-up), then the boiler will modulate down to suit, as it won't be able to shift its hot flow fast enough. But, it does still suggest that for a lot of the time, it'll have to output at 70+ degrees, which isn't the most efficient.

Another thing to consider in the whole caboodle - I'd personally take the opportunity to lose the F&E tank in the loft, and make it a sealed system. And if your system currently is Y-plan (ie a three-port diverter valve splits the flow betwixt CH and DHW), then I'd change that to 'S' (which has two, 2-port valves). These are small changes to make, but will give you a better system.

Tbh, there are many advantages of installing the correct power rated 'heat only' boiler. 

Its possible, just needs some thought and calculations.

  

tacpot12

WB are confusing the matter with their terms "high temperature regime" and "low temperature regimes". They are talking about non-condensing and condensing regimes (temperature ranges). If you have a hot water tank to heat and a large house, you are likley to be in the high temperature 'regime'. If you have weather compensation and hot water priority, you can be in the low temperature 'regime' when DHW is not required and/or the weather is mild.