I'm sizing a new boiler, I've got a rough idea of the kW required to heat the house but what should I add on for the hot water cylinder? Or should I ignore it since with the CH on it will get to temperature and not require much more heat input?
Ta
Pete
If you are going to make the system fully pumped (meaning zone valves or diverter valve for the cylinder coil), it doesn't really matter because the normal configuration is to direct all boiler output to the cylinder when it requires heat. Thus it's reasonable to choose the boiler based on CH requirements.
On older systems, the DHW coil was fed by convection ("gravity") from the boiler and so it was conventional to allow 3-5kW for that.
You can use a cylinder that was previously used for "gravity" as long as it is an indirect type with coil. However, you may find it advantageous to change the cylinder for a fast recovery type. These have a larger coil surface area and are able to transfer more heat more quickly into the cylinder and therefore the boiler is not taken away for heating the house for so long.
If you're having a conventional system with an indirectly heated hot water cylinder, then the boiler is sized for the C.H. requirements. If you're having directly heated hot water on demand e.g. combi then the boiler will be sized on the hot water flow desired with 30kw being probably a minimum.
What sort of hot water? Tank and cylinder? Unvented cylinder? Thermal store?
The current system is pumped but with no valve, so its either CH+HW or nothing (I use an immersion element in summer). I'm going to put a 3-way valve in for water heating in summer. Minor point - is it worth insulating the pipes to/from the boiler so that in summer more heat goes to the cylinder and less escapes under the floor?
The cylinder is only a couple of years old, I've got no idea whether it is a fast recovery type but over winter the heating is normally on constant, at a very low setting.
Presumably, with a room stat and tank stat, the boiler will fire for either one depending on the controller setting and adjust the 3-way valve accordingly.
Pete
Thanks for the answer, it's an indirect cylinder.
Didn't give much info did I? Answered in another post, thanks anyway
Since the average immersion heater is precisely 3KW even for a big house, and 10Kw is a small boiler, I would ignore it frankly.
Sizing boilers is a bit hit and miss anyway.
You tend to want a boiler that will heat a house up from cold in a given time, rather than one that will just keep it warm in a hard winters night..
So do yer calcs, and pick the next biggest ...
e> Ta
Que? My system has two zone valves, one for CH and one for DHW. Actually it has one for UFH as well, but lets not go there...so it actually is capable of heating all three zones simultanously, albeit at a fairly pathetic rate if they all happen to come on togther. And the DHW is the lowest 'resistance' so it hogs most of the boiler..but that is not implicit in the design - I could throttle it back..
The answer is to have the DHW come on half an hour earlier..
Boilers tend to be one size nearly fits all. A modulating boiler of around
25kW will heat your house up pronto and modulate down to suit. It will also heat the DHW fast too. So just get the best priced modulating boiler, most are modulating, above the size you need for CH only.Condensing boiler: get a one piece heat exchanger, top or side mounted pre-mix burner, and the flue exiting the bottom of the heat exchanger. The flue may exit the top of the casing, but come out of the bottom of the heat exchanger.
Get a 3-way "diverter" valve, not a mid-position 3-way valve. This means the system will be a diverter system with DHW priority. When DHW calls for heat, all the boilers heat is directed to the cylinder for a rapid heat up, which shouild eb a matter of minutes and then back to CH after. Mid-position valves can be troublesome.
Thanks to all who replied.
Pete
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