System or Regular boiler

[akist]

I am being quoted by a few plumbers to install this system boiler, Worcester Bosch 8000 35kW. I have added up all the radiators, total 23, and they work out to about 31kW, so the 35kW seems to be OK. There is also a lot of pipes in the house, large runs are 28mm, small runs 22mm and 15mm to radiators.

However a few of these plumbers have suggested I need an external/additional expansion vessel and a very "large" pump. Some have suggested I need two separate pumps. As the boiler has its own pump and expansion vessel, I am confused. How do we establish whether the pump and/or expansion vessel are large enough for the job?

Concerning the choice of a pump, unfortunately I have designed the installation badly and cannot separate downstairs from upstairs so I cannot have a dedicated pump for each floor. It seems I will have to use just one pump to do all the radiators.

I am thinking, would it be a better idea to buy a conventional boiler and then have freedom to add whichever pump we need?

 

[oldbuffer]

1. I'd use a conventional (heat only) boiler. You then have your own choice of pump and expansion vessel.
2. Make sure the boiler can be fitted to a sealed system (nearly all can).
3. The expansion vessel needs to be able to cope with the expansion of the whole volume of water in your system, including radiators, pipework and the boiler itself. Reckon on 4% of the total system volume as a minimum. At a guess you will need at least an 18 litre EV, probably better to go for a 25 litre one or even a 35.

 

[MNW67]

There are some technical specs in the installation manual. I don't know whether any of this helps.

Greenstar 8000 Life System Installation and Maintenance Instructions | Worcester Bosch

central heating boilers, combi boilers, gas boilers, oil boilers, gas central heating, heating engineers, boiler installation, boiler repair

www.worcester-bosch.co.uk

 

[BlueLoo]

Std.

You can maintain the pump and gubbins without calling in the cowboys.

System boilers comprise much to get everything in one box which is the point of them tbh.

If you have ample room and it's not on show, std boiler.

 

[gas112]

The internal expansion vessel will only be about 8 or 9 litres which isnt big enough for 23 rads

 

[BlueLoo]

The internal expansion vessel will only be about 8 or 9 litres which isnt big enough for 23 rads

Ha.
Quite so.

"Quoted by a few plumbers.."

Says it all.

 

[fixitflav]

I am being quoted by a few plumbers to install this system boiler, Worcester Bosch 8000 35kW.

Does it have to be a sealed system? My preference is open vented, so if you have, or can fit a F/E tank in your loft that's another option.

 

[omph]

Does it have to be a sealed system? My preference is open vented, so if you have, or can fit a F/E tank in your loft that's another option.

What are the pros for open vented in this case ?

 

[JohnD]

Simplicity

Less go go wrong

External parts that can easily be replaced at low cost

 

[MNW67]

WB told me that you can't do PDHW on the regular version of this boiler (at least not a manufacturer authorised way of doing it). Only on the system version.

 

[JohnD]

I wonder why. How does the boiler know?

You have a hot water cylinder?

Why do you need HW priority?

 

[oldbuffer]

1. You can have a sealed system on an open vented boiler, provided the boiler MI's says its OK. Most do.
2. On the sealed v. open vented debate, I prefer sealed. Because:
2.1 The only extra things to go wrong in the long run are the expansion vessel and pressure gauge. In my opinion this outweighs the chances of air causing corrosion if the inhibitor is not maintained.
2.2 You can get rid of the F&E tank, and its normal collection of nasty gunk waiting to be washed into the pipework.

 

[oldbuffer]

1. PDHW = Priority Domestic Hot Water

 

[MNW67]

I wonder why. How does the boiler know?

I think the extra electrical connections are missing from the regular boiler.

 

[JohnD]

I suppose this would matter if you have a huge cylinder with a big coil and want fast recovery after multiple baths.

In my case the boiler has enough power to heat cylinder and rads at the same time. Neither creates full demand unless switched on after being allowed to get fully cold. If they did I would time HW go start half an hour before CH.

So from my POV it wouldn't matter.