New build boiler size

dyslexicwinker

Recently moved into new build house and have been given an ideal logic2 max heat 12kw boiler. I discussed prior to moving how I felt this was too small and the site manager said they’d specced the home at 11.9kw (not sure how that’s done) but everything I look at suggests this isn’t big enough. Phoned ideal but they won’t help as not installer. Can anyone advise as to who might be able to independently suggest what size we should get. We have 13 radiators, 1 bath, 2 showers and 5 sinks. It is a 4 bed detached house of about 1500sq ft. It’s also the only boiler int he range with 2 years and not 10 years warranty which would be better also. This is a system boiler with 150l water tank. Can anyone advise as just think I’ll end up going round in circles with developer - who in the main have been very good. Ta

FreeBear

Measure the size of the radiators and look up the rated heat output at 50°C. Add up all the numbers, and it should be less that 12KW. You could use an online calculator to see if the radiators have been appropriately sized - https://www.stelrad.com/basic-heat-loss-calculator/ - You may even find the information contained in the documentation that the developer hands over to you.

A 12KW boiler should be plenty big enough to heat the property. It might struggle to heat a tank of water at a rate you find acceptable though. I used to have a Baxi back boiler nominally rated at 12-13KW, and it would take around 45 minutes to heat a 120l tank from cold.

dyslexicwinker

Thanks. Assume that 11.9kw they gave me is based on the radiator sizes. Developer build 100’s of our houses so am sure that’s right but like you say with the tank also I just don’t know. A lot of things suggest using 1kw per rad plus 3kw for tank which take some to 16kw. 

The info I’ve found about cylinder (evicts) says ~22 mins to heat from cold but couldn’t find what size boiler that’s based on? We’ve set water on for 1.5 hrs morning and evening and am never sure boiler turns off? Assume it should once up to temp?

FreeBear

dyslexicwinker said:

Thanks. Assume that 11.9kw they gave me is based on the radiator sizes. Developer build 100’s of our houses so am sure that’s right but like you say with the tank also I just don’t know. A lot of things suggest using 1kw per rad plus 3kw for tank which take some to 16kw. 

The info I’ve found about cylinder (evicts) says ~22 mins to heat from cold but couldn’t find what size boiler that’s based on? We’ve set water on for 1.5 hrs morning and evening and am never sure boiler turns off? Assume it should once up to temp?

Don't know where you are getting the 1KW per rad from. Here, my radiators range from 650W all the way up to 2KW (in total, about 12KW @ ΔT50°C). Hot water will (usually) take priority over central heating, so the two wouldn't be on at the same time. So your 12KW boiler will either be heating up the radiators or dumping heat in to the DHW cylinder.

If you know the volume of the DHW cylinder, it is fairly trivial to work out how long it would take to heat up from cold. Failing that, post the model & size, and we can look up the specs.

dyslexicwinker

FreeBear said:

dyslexicwinker said:

Thanks. Assume that 11.9kw they gave me is based on the radiator sizes. Developer build 100’s of our houses so am sure that’s right but like you say with the tank also I just don’t know. A lot of things suggest using 1kw per rad plus 3kw for tank which take some to 16kw. 

The info I’ve found about cylinder (evicts) says ~22 mins to heat from cold but couldn’t find what size boiler that’s based on? We’ve set water on for 1.5 hrs morning and evening and am never sure boiler turns off? Assume it should once up to temp?

Don't know where you are getting the 1KW per rad from. Here, my radiators range from 650W all the way up to 2KW (in total, about 12KW @ ΔT50°C). Hot water will (usually) take priority over central heating, so the two wouldn't be on at the same time. So your 12KW boiler will either be heating up the radiators or dumping heat in to the DHW cylinder.

If you know the volume of the DHW cylinder, it is fairly trivial to work out how long it would take to heat up from cold. Failing that, post the model & size, and we can look up the specs.

It’s just an average quick guide I believe to save time. Not too far off with 13 rads 11.9kw. Cylinder is 150l thermaq evocyl I believe. Says it’s 16.3kw here. https://www.wolseley.co.uk/product/therma-q-evocyl-he-150l-indirect-pre-plumbed-twin-zone-cylinder/

FreeBear

dyslexicwinker said:

FreeBear said:

dyslexicwinker said:

Thanks. Assume that 11.9kw they gave me is based on the radiator sizes. Developer build 100’s of our houses so am sure that’s right but like you say with the tank also I just don’t know. A lot of things suggest using 1kw per rad plus 3kw for tank which take some to 16kw. 

The info I’ve found about cylinder (evicts) says ~22 mins to heat from cold but couldn’t find what size boiler that’s based on? We’ve set water on for 1.5 hrs morning and evening and am never sure boiler turns off? Assume it should once up to temp?

Don't know where you are getting the 1KW per rad from. Here, my radiators range from 650W all the way up to 2KW (in total, about 12KW @ ΔT50°C). Hot water will (usually) take priority over central heating, so the two wouldn't be on at the same time. So your 12KW boiler will either be heating up the radiators or dumping heat in to the DHW cylinder.

If you know the volume of the DHW cylinder, it is fairly trivial to work out how long it would take to heat up from cold. Failing that, post the model & size, and we can look up the specs.

It’s just an average quick guide I believe to save time. Not too far off with 13 rads 11.9kw. Cylinder is 150l thermaq evocyl I believe. Says it’s 16.3kw here. https://www.wolseley.co.uk/product/therma-q-evocyl-he-150l-indirect-pre-plumbed-twin-zone-cylinder/

16.3KW is the maximum rating of the indirect heating coil. Actual energy usage will depend on the water temperature in the tank and the flow temperature from the boiler. Assuming the Ideal Logic can dump 12KW in to the cylinder, reheat time from cold would be around 40 minutes. Less if it is just topping up after taking a shower for example.

In a modern well insulated home, 12KW should be more than ample to keep you warm - In my (not so) draughty old 1920s semi, I have the boiler dialed back to a max of ~8.4KW, and it heats the place up without any trouble.

dyslexicwinker

Thanks. Less is better obvs as it’ll cost less just want to make sure it’s right. Barely have the heating on tbh and when our lass sneaks it on she doesn’t moan so hopefully all good. Main prob now then is tank whistling when downstairs heating is on? and boiler is firing up even when no demand for HW or CH 🤷🏼‍♂️🙏🏼👍🏼 

Albermarle

Barely have the heating on tbh

That is the key point really. If it was an older, colder, less well insulated house, then you would have to have it on more. Especially on freezing cold mornings. 

 Main prob now then is tank whistling when downstairs heating is on?

Have you adjusted the controls since you moved in? Often the installers leave the pump at full speed setting which is not necessary. 

FreeBear

Albermarle said: Have you adjusted the controls since you moved in? Often the installers leave the pump at full speed setting which is not necessary. 

Other things to check and adjust Maximum boiler temperature - Often set to 70°C or even 80°C. Flow temperature could also be set to 70°C, but if the radiators are big enough, you can drop this to 60°C or even 50°C. Doing both will reduce gas consumption and improve efficiency.

Snuggles

dyslexicwinker said:

Main prob now then is tank whistling when downstairs heating is on? and boiler is firing up even when no demand for HW or CH 🤷🏼‍♂️🙏🏼👍🏼 

It shouldn't be doing that. We had this same problem a few years ago (boiler firing up when no demand). It was a a valve microswitch sticking if I remember rightly. If you've just moved in the developer should sort this. 

dyslexicwinker

Thanks all. Have heeded most of that advice. Will note valve micro switch. Developer has been excellent so far and will sort no doubt 👍🏼