Will I need two expansion vessels in a closed system design if the
boiler doesn't have an internal one?
One for the heating and flow to direct hot water tank
one for supply to tank
To explain further see schematics here
where two expansion vessels are shown
Also do I need to fill the hot water and heating sides seperately or
will they be connected together somewhere?
Yes, you need to one handle expansion in each closed (unvented) circuit,
one unvented circuit is the boiler flow and return circuit, and the other is
the unvented hot water cylinder. Note that if you do not have unvented hot
water (you have a cold header tank in the loft) then you do not require the
second expansion vessel. IIRC, the Eco hometec has an internal
expansion vessel, albeit a small one, which may meet your needs for the
Andy Hall has one of these in a system, so he may be able to add more.
That will depend on how the circuit is implemented and the boiler chosen. It
isn't clear from the schematic what the two different flow and return lines
from the boiler are. Are they isolated circuits? Are they just two outlets
from a diverter valve on the same circuit? (i.e. similar to a
Worcester-Bosch system boiler with the optional internal diverter valve).
You may need one anyway, as the water content of such a system might be so
high that a boiler internal expansion vessel wouldn't be big enough to
contain the pressure rise.
The expansion vessel for the DHW is to accommodate expansion of the
water in the cylinder when it is heated and so needs to have an
unrestricted connection to the cylinder; you've currently got a non return
valve between them.
Be aware that UK water regs prohibit amateur installation of unvented
DHW systems and that building regs approval is required.
My understanding that it is an "or", not an "and".
Be aware that UK building regs prohibit amateur installation of unvented
DHW systems unless building regs approval is obtained.
P.S. it is only above a certain storage capacity that this applies. (For
some reason 13 litres rings a bell, but I could be well out). This enables
unvented mains pressure water systems not using storage to be fitted without
approval (unless also required by Part L).
After investigating further, I've found it is in Approved Document G3. It
does seem more strictly defined than other such "competent person"
situations, but I think there is still a loophole. I am not a lawyer,
though, so if intending to do this, you should check with your local
building control before doing anything to ensure that they agree.
"3.8 The unit or package should be installed by a competent person i.e. one
holding a current Registered Operative identity card..."
It goes on to list various bodies. Although this indeed might suggest that
it is not permitted, I doubt that legally speaking the use of "i.e." could
be said to exclude other competent people. (Again, I'm not a lawyer, so
maybe it can so exclude alternatives). One thing is not in doubt though. You
certainly need to be in one of those bodies to be able to self certificate
the work. You would, therefore, need to get approval from the building
control department, which could be costly and involve an inspection.
P.S. It is 15 litres for the exemption, not 13 litres.
Regarding fitting, we are doing other work on the house as well so the
building regs people are already involved.
The inspector's opinion was that it was fine for me to do the work
providing it met the regulations.
The only thing he specifically asked about was the temperature/pressure
I'd be very grateful if people could comment on my system design which
as mentioned earlier is available here:-
The temperature/pressure relief valve will be supplied with the cylinder as
a package. The installation instructions that come with the package will
describe the pressure relief measures which must be followed to the letter.
There may be restrictions and requirements about where the pressure relief
line must run to. There will be a separate safety zone valve on the indirect
coil, which MUST NOT also be used for functional switching/temperature
control. The immersion heater must have two thermostats, one of which must
be manually resetting requiring user intervention.
You must also ensure that the system is designed to work without relying on
expanding water backwards into the water supply, like some US systems. UK
systems typically have pressure reducing valves or check valves that prevent
If you are doing this yourself and haven't yet bought the system, consider a
heat bank instead. I've just fitted one myself and it definitely works.
Installation couldn't be simpler. Connect the flow/return lines, mains water
in, hot water out. Connect to mains electricity. Fill it up with water.
That's it. No safety valves. No overflows. No vents. No additional tanks.
Nothing. Costed the same as an equivalent Megaflo.
The main possible problem is that the boiler might limit the flow
temperature to below the 80-85C you need to reheat it. This will depend on
the internal electronics of the boiler. If the boiler can select different
flow temperatures for heating the cylinder or running the
underfloor/radiator circuit, then it could be ideal.
On Tue, 11 Nov 2003 17:42:19 -0000, "Christian McArdle"
On the MAN Micromat, actually you have a lot of control via the
You can buy a water cylinder temperature sensor for it quite cheaply
from Eco Hometec for optimum control. This is normally intended to
monitor the temperature of the cylinder water and the boiler firmware
allows that to be set at up to 70 degrees.
Alternatively, for a heatbank, where 80 degrees for the bulk water may
be desirable, a conventional cylinder thermostat could be used and the
boiler can be programmed to use that instead.
The flow temperature can be set separately for HW (or heatbank)
service vs. the CH and boiler has electrical outputs to operate pumps
or motorised valves as appropriate. In addition, the boiler power
level is increased in the case of the EC31 up to 36kW vs. 28-31kW for
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