Upgrade to system - Unvented or combi

[loggs]

I was looking for some advice on this probably popular subject. Iam doing my bathroom so with to upgrade my shower from the old Mira power shower to a 'normal' mixer shower. I have a gravity vented hot water system (160l) just now so require a method of boosting the pressure for the shower. I had originally thought of a shower pump or digital mixer system that would boost the pressure but understand these can be noisy. I also have another en suite (not presently used but will be in due course) there are 2 adults and 3 children so its conceivable that more than 1 person will require a shower at the same time in the future. I then started to think about the other options of unvented or combi as i have also thought of replacing the boiler soon as its 20 year old and obviously not very efficient.
Along with others i guess i have then received different advice/opinions of which would be best for me and the claims for each made:

High flow combi (Worcester 440, Valiant 937 store, Baxi duotec 42KW (or heat saver but very exspensive) - cheaper to install, claimed cheaper for hot water as only on demand.


Unvented - More efficient for potential high usage as when heating on only like an extra radiator and system boiler will run at a lot lower rate than a combi, better for multiple usage.


Did a crude measurement of the tap in my garage that tees straigh off the main as it enters the house, gettimg about 24 l/min but from what looks like a 12mm pipe for this section does this matter? Also measured off the kitchen tap at 10 l/min but have heard they are normal low anyway. The guy championing the unvented says the stated flows for the combis are based on a 35 degree rise in temperature so when the inlet temp drops in the colder months the boiler cannot produce the same flow rate as stated. Measured just now as its snowing and its 8 degc and guess it could easily drop to 5 degc so hot water of 40 - 43 degc would not be hot enough i guess. Awaiting one guy to come back and properly measure the flow & pressure.
What would be the acceptable limits for these for either system?
While looking at boilers have seen the Baxi gas saver not cheap at £700 but cheaper than solar thermal and claims to reduce hot water heating requirements, anyone used one?

Forgive the long winded text that is probably a bit of waffle but very confusing when speaking or reading all the different views. The high flow combis look good in theory but do they work as well as claimed and anyone used the VAliant 937?

 

[Centralheatking]

Combis were designed for flats and small houses with little space for a header tank and a cylinder. Because they are simple to fit some elements of the trade saw an easy solution as they are literally a box on the wall which requires no fancy planning.

The flow from a combi no matter what the book of words says is only as good as your mains water supply - which can be fine but can drop in volume and pressure on a regular basis.

If your home has more than one w.c. then a conventional system is the best way

seen tooooo many really disappointed home owners who listened to the installer promise the world from a combi only to find in the real world poor performance, slow bath filling times and dodgey shower experience.


Yes they are an easy option but beware...........centralheatking

 

[Millsy 82]

I agree with Centralheatking combis are great in flats and small houses with only 1 bathroom.

Combis are more efficient in small houses but I would go unvented it will work out more efficient in the long run as most cylinders are high recovery now so they dont take too long to heat back up. The other downfall of a combi is they can only do 1 thing at a time if you have 5 people taking 10 minute showers 1 after the other your heating will be off for 50 mins.

 

[loggs]

Thanks guys, I was leaning towards the unvented route, just waiting for the pressure/flow check. Oneonce says he has the Worcester 440 and can run 2 showers at once. Does the cylinder have to be serviced as one guy says basically its just a visual inspection. I believe they should run the inlet and outlet to/from the cylinder as 22mm. Is it likely to affect anything if I put in the garage? Does the gas pipe to the boiler have to be any bigger .