Been thinking about this and want some opinions please.
You don't have to ask for opinions here - you'll get them whether you want them or not
I currently have an old baxi gas boiler with copper cylinder upstairs, I was thinking of replacine all with a new combi boiler which involves a lot of system changes....or should I just update with a new gas boiler...simpler, cheaper to replace and much more efficient I guess ?
Just not sure...
It does mean having to heat more water than needed but if I insulate the cylinder well...maybe ok ?
OK, just to make one thing perfectly clear, in the long term you do not heat more water than you use. It is true that you are heating a large cylinder from which you might only draw small amounts, but you only have to heat the small quantity of water which replaces what you've drawn off. Thus, if you had (say) a 100 gallon tank, and drew off (say) 10 gallons of hot water - you'd only need to supply enough heat to heat up 10 gallons of water. Many a time I've come across people who are convinced that you are heating 100 gallons from cold each time
So having go that out of the way, the "cost" of storing your hot water is down to how much heat it loses - or the "standing losses". This is down to how hot you keep it, and how good the insulation is - don't heat it hotter than you need, and have good insulation, and the standing losses will be quite low.
Also, for most people this isn't "wasted heat" for most of the time. Assuming the DHW cylinder is inside the property, then the heat that does leak out goes towards heating the house - so is only wasted during those times when you've got the windows open to cool down in summer.
Against that, don't believe anyone that tells you a combi doesn't have standing losses. Many of them have some form of "keep warm" function - either they fire up from time to time to keep the heat exchanger hot, or they have a small hot water tank that they keep warm. Without this, it's normal to get a delay (sometimes considerable - I've measured a whole minute on a not that old combi !) between turning on the hot tap and getting hot water out of it. With either function, there's a heat input required in order to keep things hot.
In favour of keeping the hot water cylinder :
You've got stored hot water, so the only delay when you turn on teh tap is down to the length of pipe and the time needed to flush the cold water through.
You've got a backup, so you can switch on the immersion heater when the boiler breaks down - which it will !
Because you've a much simpler boiler (there's a lot of complexity in a combi) then it should break down considerably less often.
You can size the boiler properly according to the heating load for the property.
The downside (which doesn't bother a lot of folks) is that your hot water is at low pressure - whilst a combi does mains pressure hot water (as does a thermal store).
A combi is complicated and will break down more often.
When it breaks down, you've have no hot water.
Because it has to have a huge capacity in order to provide a decent flow rate at decent temperature, it will be oversized for the heating load in most properties (which is inefficient) - however modern boilers are getting better and better about how low they can turn the burner down.
And the biggie - because they have limited flow rates, you can have a situation where an upstairs shower will stop completely if someone turns of a tap downstairs.
Developers love combis because it means they can build a rabbit hutch about 1m^2 smaller by leaving out a DHW cylinder. And because there's less plumbing, they can knock a few hundred quid off the build cost.
Personally, since you already have a DHW cylinder - I'd say keep it and stick with a heat only boiler.
I just had another thought...what about using an electric immersion heater and the gas boiler just for heating only ?
Is is cheaper to heat with electric or use the gas for all (as I do at moment)
From memory, gas is about 1/4 the cost of electricity, and still about 1/2 the cost vs a "cheap night rate" sort of tariff.