Boiler - help - unvented system?

[Koala2]

Hi we are planning to replace an old boiler in our 5 bed house, 2 bathrooms, 13 rads. Two floors. I am keen that we can run both showers at once with the new system. Both say that a large combi - 29kw - should do the job. But if we prefer we could go for an unvented system (flow / pressure etc seem to be fine for an unvented system).

However they have different ideas about the unvented system. One says a 27kw boiler with a 210l cylinder, with the tank located upstairs. Also recommends a powerflush for the system. The other says a 24kw boiler with a 170l tank in the garage (i.e. next to the boiler). No powerflush but chemicals in the system.

The larger boiler / tank quote is for all Worcester products. The smaller is for Main branded products (I think these are Baxi). Price difference overall is £690 overall on the unvented quotes (the combi quotes are very similar to each other). I have excluded the powerflush from these costs as I suspect we would need this regardless of fitter.

Can anyone advise on the best option(s) please? I am totally confused. Thanks for reading this long post!

An unvented cylinder is better than a combi, if you have adequate incoming flow and two bathrooms.

I presume you have verified that you get about 20 litres per minute from the incoming main. You will probably need to run 22mm pipe to the cylinder and the bathrooms.

You do not say how old your system is, or if it is currently open vented, but I would be suspicious of anyone who wanted to omit a powerflush. It would probably void the manufacturer's warranty. If the system is old I would want a system filter as well.

100 litres of water is about right for a bath, not all of it will be hot. So 170 litres would give you about 2 baths. The cylinder would weigh about 230kg when full, and be about 6ft tall. Either would do. The boiler power is adequate in either case unless you have an unusually large or badly insulated old house. If you have a large airing cupboard the cylinder will keep it warm, though heat losses from a modern cylinder are very small. It is more economical to have the cylinder close to the hot tap you use most often, which is probably the kitchen sink.

If the boiler and pipes are in an unheated garage, you will have a small extra cost because it will have to run intermittently to protect against frost.

Worcester Bosch is noticeably better than Baxi. Look at the manufacturers' websites and see how long the warranty is, and the conditions. Vaillant and Viessmann are two other good makes. Look for the local Approved Installers lists

Thank you Piglet - that is really helpful. Do you have any recommendations for brands of hot water cylinder?

The Megaflo is well thought of and the name carries a premium. It has an internal air pressure bubble which is neat but may be tiresome to replenish once a year or so. Other brands have an external air pressure vessel which looks less elegant but is probably less tiresome and can easily be pumped up or changed by any brand off the shelf if it ever goes wrong.

The boiler companies often offer their own branded unvented cylinders.

Santon, and Sadia (who own the Megaflo name) make various models and are sound British companies, you will find it easy to get them supplied, inspected and serviced as they are so popular.

Get one with an upper and a lower immersion heater so you can use electricity when your boiler is out of action or if you get solar panels one day.

Thank you

We went for a 250 litre Stelflow cylinder with a 35 kW Worcester Bosch boiler and are very happy with it so far (4 bed 2 bathroom house). Boiler may be more powerful than needed, but we were extending with underfloor heating so wanted to err on side of caution.

Megaflo would have been much more expensive than Stelflow; we didn't want to pay for the name. We thought it worth paying (very slightly) extra for the larger cylinder as it takes up no more floor space (just slightly taller) and would kick ourselves if we started running out of water on mornings when all 4 of us were showering.

with a bigger cylinder, you need to run the boiler less often. Using the timer/programmer to heat it once for an hour or less will probably last you all day, which is more economical than letting the boiler run for multiple short periods (so more economical on gas than a combi) though you will only notice the saving in summer, when the heating is off.