New boiler sizing

shiraz99

I'm replacing an ageing Worcester Greenstar 37CDi and just trying to work out what size and make of model I should get. It's a relatively small, 3 bed semi with one bath and mixer shower (not separate). For heating I think my old boiler is probably a bit overkill but I measured my mains water and it's around 15l per minute so probably why it was originally installed.

Any recommendations?

sheenas

Sizing is normally based on the number of bathrooms and radiators. So 1 bathroom and around 10 radiators you talking a 24-28kw boiler. So yes your current boiler is on the large size.

Albermarle

With a combi, it is heating the hot water that needs the high power, so in nearly all cases that means the boiler is overspecced for the heating system, For heating only a small 3 bed semi, you could probably manage with just a 12 Kw boiler, 15KW max, but you have to have the higher power for the hot water. Exactly what you need for just hot water to one bathroom, I am not sure, but probably not as high as 37KW.

A more modern boiler will probably be able to modulate down more, once the heating is up to temperature. So once it is 'ticking over' it might only be using around 3 KW of power. Something you need to check when choosing.

FreeBear

https://forums.moneysavingexpert.com/discussion/comment/81969123#Comment_81969123

That is a very rough way of estimating boiler size.

For a combi, you need to look at the flow rate of cold water. With 15l/min, a 32Kw or 35Kw would be about right. And maybe fit a flow reducer to limit max flow rate to ~12l/min. This assumes you want hot water at 60°C. If you are happy with a lower temperature and/or a lower flow rate, cap it at 10l/min and install a 28Kw combi.

For a system or heat only boiler, work out the total power of all the radiators - You will need to measure the size and use a table to find the output (this one provides a handy table → https://simplifydiy.com/plumbing-and-heating/radiators/power ). That will give you the maximum required power with a 70°C flow temperature. It is unlikely to be more than 14kW..
Then look at the DHW tank - If you want a fast reheat time, you will want a higher power output at an elevated temperature. A 14kW boiler would give you a 60-90 minute reheat time depending on cold water and final hot water temperatures. This will also depend on the size of the coil inside the tank. Unless you have really cold incoming water, want very hot water out, and a fast reheat time, 18kW is likely going to be the biggest boiler you need.

I replaced an old ~13kW Baxi back boiler a few years back with a new combi. Cold water flow rate was ~10l/min, so I fitted a Viessmann 30kW (actual output is 28kW). Way more than I really need for DHW, and a heck of a lot more than the heating system requires. Have a heat meter on the output, and even on the coldest of days, the system rarely requires more than 7kW. Occasionally, it has peaked around 10-11kW if the flow temperature is wound up high. Much of the time, 4-5kW is sufficient to keep the place warm.

With a combi, the most important number to look at in my opinion is the minimum output. The Viessmann will modulate down to ~3.2kW which means it rarely short cycles. If I had gone for a cheap boiler with a minimum output of 6-7kW, it would be constantly short cycling which is bad for efficiency and long term reliability. Also helps to have a new boiler controlled by a smart thermostat that can change flow temperature on the fly and allow for different target temperatures throughout the day. Limit the maximum flow temperature to 60°C or less, and you can achieve 90% or higher efficiency (I run my system at 45-55°C and get 92-94% efficiency).

But with all that said, it is worth looking at replacing the gas boiler with a heat pump - You'll probably need new & bigger radiators all round, but running costs should be lower IF you can take advantage of low flow temperatures and get a suitable electricity tariff. Even if you don't go for a heat pump, it is still worth fitting larger radiators and running at a reduced flow temperature.

shiraz99

https://forums.moneysavingexpert.com/discussion/comment/81969123#Comment_81969123

Actually with a combi you should size it for the hot water output to match your mains pressure as the heating side can be rated down to match your requirements.

FreeBear

If you are looking for specific model recommendations, I'd suggest a Viessmann 050 or 100 - Both will modulate down to 3.2kW, and depending on model, will cope with your cold water flow rate. Viessmann also come with an OpenTherm interface which allows you to use a wide range of thermostat/controls (I use Home Assistant with mine along with a DIYLess shield).

WB Greenstar 4000 and 8000 models also modulate down to around 3.2kW, but have their own proprietary interface. Not so easy to find suitable thermostat/controls, although there are a couple of open source interfaces available.

Intergas is another manufacturer to look at, but not as well known in the UK. Baxi is about the only UK made boiler that I'd suggest, but you need to watch the modulation range. Their base models are pretty poor in that respect.

WIAWSNB

https://forums.moneysavingexpert.com/discussion/comment/81969239#Comment_81969239

I'd take that a bit further. I'd only size a ~15lpm combi to a ~15lpm mains supply if I didn't have any wish to blend in cold...

You almost certainly will.

Most combis will be set to deliver at least 50oC, and may need to be even higher to make a thermostatic shower operate correctly - my current thermo requires a 60oC DHW supply.

A 30kW combi will deliver this - a DHW flow rate of 12+LPM - and that leaves a few LPM of cold to do the thermostatic blending.

If your mains supply was in excess of 20lpm, then I'd consider a 35+kW combi, for the indulgence of powerful showers, and possibly to be able to supply two gentle showers at the same time. But at 'only' 15, I personally wouldn't.

shiraz99

Any thoughts on this deal I've seen for a WB 8000+ 36kW , it seems a reasonable price given that it's cheaper than the 32kW model that I was also looking at.

Worcester Bosch 8000

FreeBear

WB 8000+ 36kW modulation range is 1:10 - Typical for higher end boilers and means minimum output is just 3.6kW. Depending on controls and minimum flow temperature, you may find it short cycle from time to time.

The Hive control only does basic (crude) on/off, so you lose out on being able to modulate the heating system. To get the best out of the WB 8000, you need a thermostat/control that "talks" EMS. Something like the WB EasyControl or something that can do load compensation (a.k.a. modulate output).

sheenas

https://forums.moneysavingexpert.com/discussion/comment/81969239#Comment_81969239

Not the way i would recommend size, thu if you have low water pressure consideration.The logic I was using as the flow rate is stated as 15l/min (which is very good). That a decently economical show only needs around 7l/min, so unless you run two shower at a time then you don't need 15 litres minute. A 15l/min boiler would be around a 35kw unit while a 24-28kw would provide around 12l/min which would be perfect for most house holds, but best suited to 10-15 radiators max.. There is no value in having an oversized boiler, as other have said the boiler will modulate to match demand, so no adjustments required.

shiraz99

https://forums.moneysavingexpert.com/discussion/comment/81969647#Comment_81969647

I've decided not to go for that deal now as BG only offer a 5 year warranty. If I go for a WB I may go directly to Worcester for the 32kW version which will include WB controls.