"IMM" wrote in message ...
Pressurised thermal store/heat banks are available. These have the sealed
primary flow and return from the boiler directly connected to the cylinder.
They are basically grade 1 cylinders which are rated at 2.5 bar working
pressure and a test pressure 1 bar above that. When used as a thermal
store/heat bank connected to the boiler they operate at approx 1.5 bar
working pressure and a blow valve of 3 bar, so well within range. The makers
tend to stipulate that the thermal store/heat bank has its own blow off
valve as well as the boiler.

I would rather go with one of these than a thermal store/heat bank using a
an intermediate primary heating coil.

At the risk of being associated with 'dumb plumbers' I'll give my
reasons for recommending an unvented cylinder rather than a thermal
store. I've got nothing against the technology of a thermal store but
personally feel its had its day now that condensing boilers will soon
be mandatory to the mass market.

I agree that an unvented cylinder must be installed by a trained
person. IMM says that UV cyls can explode and while true it is rare
due to the safety features that are mandatory. UK regulations are lot
stricter than many countries were UV cyls have been the norm for many
years.

To the points in question;

1. Reinstating the air bubble in a megaflow is not a difficult task.

2. 1" copper overflows are not required unless the route to discharge
is extraordinarly long. The tundish has a 22mm output and this is
sufficient for most installations. However, the discharge pipework
must not be plastic.

3. KISS. Which is easier to understand; the UV cyl or a heatbank?

4. Solar. As far as I know OSO and Viessmann are the only UV cyls to
have dual coil models. Yes, the solar won't heat your radiators, but
since solar systems are designed to work at their peak in summer,
surely this a moot point?

5. condensing boilers. I commented this elsewhere in the NG. UV cyls
are a better option with a condensing boiler. With inefficient boilers
there is no such choice between an uv cyl and heatbank.

6. DIY. Yes I'll agree that a heatbank can be DIY-ed. This suggests
the owner has some interest in the heatbank technology, which is fair
enough. Future owners may not be so inclined. Still, future employment
for 'plumbers' when the next owners rip it out to put a megaflow in?

7. The marque. A 'Megaflow' has become something of a must-have.
Probably due to marketing and keeping up with the Jones. Still, the
educated customer wins out. It may be a plus-point on a future house
sale.

8. Boiler cycling. Any system is susceptable to this if badly
installed. I've not seen a modern UV cylinder which causes the boiler
to cycle before it reaches the desired temperature. I only install the
keston celsius 25, which has some features to avoid cycling.

9. All in one packages. Not something I go for myself but just a
personal choice. There are equally valid pros/cons for this type of
unit.

Martyn