In an installation the size of this onbe, it might be worth creating
several programmable thermostated zones AS WELL as TRV's..the radio
stats mean less wiring..just that you would have to parallel the zone
valves outputs back t the boiler somehow.
TRV's are nice for living areas, with master stats in corridoors etc.
On 11 Aug 2006 14:59:02 -0700 someone who may be "Andrew Haylett"
If installed and maintained properly they are reasonably safe. Not
100% safe, but nothing else is either.
Annual servicing costs money (and I suspect the costs will only ever
go up) just to store some hot water. A nice little earner if one can
I agree with the ticks and crosses at the bottom.
Were I doing what you are doing at the moment I would look very
seriously at the Conus 502 supplied by multiple heat sources
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
This is a very clever design to maintain stratification within the cylinder.
It is good for solar applications. I would go for heat bank (thermal store
with a plate heat exchanger) with a more efficient plate. Range, Nu-Heat,
DPS, etc do them. I would have a blending valve (UFH models) on the flow
and return of the boiler to ensure condensing operation and swift reheat. A
plate heat exchanger as opposed to a coil is far more efficient and delivers
higher flows than a coil (that gas appliance guide does not mention plates
being out of date). Also plate heat exchangers can operate at higher
pressures, hence higher flows than unvented cylinders. Also have a
Magnaclean filter on the CH return to the heat bank cylinder. This ensures
no sludge build up.
Wow, thanks for all the help. I'm a CH neophyte so all information is
helpful. A few (quite a few) follow-up questions to help my learning
Mains pressure. Yes, there are some concerns about that. When the
plumber first measured the flow rate at a tap it was about 15l/min.
They replaced the stopcock at the entry point (it was locked solid) and
maybe they fiddled with something, but he said he can now measure
20l/min, which he professes to be adequate. What he wants to do is run
22mm from the entry point (currently 15mm) all the way up to the boiler
to improve the flow rate/pressure. (The boiler is to be on the first
floor, in the airing cupboard.) Is that a helpful thing to do, given
that the rest of the system is 15mm which will presumably limit the
flow rate at the tap?
What are the implications of the mains water pressure falling below a
certain level? Sub-standard performance at the taps/shower head, or
Location. He offered to put the boiler in the loft (makes the flue a
bit easier if mounted on the side wall). I demurred since it seemed we
weren't going to have much room for clothes in the airing cupboard
anyway due to the size of the cylinder, so might as well stick the
boiler there, and because we don't have a 'proper' loft ladder so I was
concerned about servicing access. But it has been suggested AIUI that
both the boiler and the cylinder could be put in the loft, with safety
advantages. I'll ask him about that.
Shower pumps. He didn't mention about getting rid of the one we have.
I guess I don't fully understand the regs. I thought as it was pumping
out of the cylinder then it would be OK - or does having a pressurized
system mean that the whole house effectively becomes an extension of
Unvented/heat store. I think I understand how the latter works. I
guess it's a bit like a combi where the HW to the taps is heated on
demand, except that the heating is done via a pre-heated body of water,
right? How large is a typical thermal store compared with an unvented
cylinder? How common, relatively speaking, are the two types of
system? And are there any ease-of-installation issues either way?
Could the Vaillant boiler already specced drive a vented system? I
guess the boiler just sees an indirect system either way, whether
heating a pressurized body of water or a thermal store.
Safety/maintenance. That's obviously a concern with 6 people in the
house - but how prone are unvented systems to blow up? Do they have a
bad safety record (assuming appropriate maintenance)? I'll check with
the plumber about servicing costs.
He hasn't said anything about building control approval for an unvented
system. Whose job would this be?
TRV/room stat. I'll skirt clear of this debate. ;-) He's fitting TRVs
throughout, plus an RF room controller. Hopefully that will give us
enough flexibility. I haven't considered zoning - didn't even know
about it - but will ask anyway. I would *think* that TRVs will give
enough flexibility to save energy in lesser used areas, e.g. spare
room, utility room, etc.
many thanks again,
The 22mm needs to go to the cylinder, not the boiler. Remember that if
each tap is fed by 15mm you want 22mm to allow full flow from several
taps simultaneously, or to stop the flow dropping suddenly when someone
turns on a tap elsewhere.
If you don't have any mains pressure you won't have any water at all.
There aren't really any safety advantages IMHO. You need a proper loft
ladder, boarding below and safety rail around the boiler area.
Not very, but they can be spectacular if they do.
Yours, ultimately; you should specify in the quote that the plumber is
responsible for compliance with Building Regs and the obtaining of all
permissions required, and that fees for such are included in his quote.
The drawback to TRVs is they don't adjust themselves and can be left on
'full' inadvertently. Zoning and a programmable thermostat would allow
you to have, eg, your bedrooms come on warmer in the morning for
waking/dressing, but be cooler in the evening for sleeping.
The extra plumbing is minor - just another zone valve located in the
It is if you just want to run one shower at a time or fill one bath -
not if you want to do more than this.
Also, it would be a good idea to check this at peak water usage times
and the times that you are likely to want to shower or bath.
This may well improve the flow rate depending on the supply.
It depends on the length of the runs. Unless they are particularly
long - i.e. one side of the house to the other - that should be OK for
showers. The bath plumbing may already be 22mm - it usually is if
there is a roof tank system originally.
Substandard performance and variation in flow as other things are used.
A thermostatic valve on showers will maintain temperature reasonable
well unless there are gross changes in available flow.
You would have to check the construction in the loft if the cylinder is
to go up there. Should be OK if it goes in the position of an
existing storage tank but needs to be checked - it's a lot of weight if
you have a large cylinder. If the boiler goes up there, the area
from the access hatch to it must be boarded and there must be a safety
rail around the hatch.
In effect, yes. The pump would have to go.
A thermal store can be run at 75-80 degrees as opposed to the 60
degrees of stored HW. The implication of that is that for an
equivalent amount of HW production, the thermal store cylinder can be
3/4 of the size of a storage cylinder. However, there are
additional plumbing components such as valves, pump and a heat
exchanger. By the time those are added, there is not really any
saving of space. Also a thermal store needs some kind of header tank
to supply the water in the main part of the cylinder because this is
vented. Either this can be a small tank in the loft, or if the
cylinder goes in the loft, there are thermal stores with the header
tank integrated on the top. Again by the time you add this, there is
no space saving. On the other hand, space may not be as critical in
If you want an airing cupboard, you can always put a small radiator in there.
If you use an indirect store, there are, in effect, three lots of water.
- The primary, which runs through the boiler, the heating circuit and a
coil in the cylinder. This can be sealed and pressurised or open
vented according to boiler requirements and preference. There are a
lot of advantages in going for sealed and no real disadvantages.
- The water in the store cylinder. This is open vented and is
circulated, on HW demand between store and heat exchanger. Having it
open vented gets around the regulatory requirements of a pressurised
- The HW being heated from the mains. With a thermal store, there is
again no large volume storage in this part either.
They are widely fitted in many countries outside the UK and do not have
a bad reputation or safety record.
He needs to have a particular training and certification to
install/service an unvented system. Ask him about that. Assuming he
does, then he can self certify the installation and there is no need
for other approvals.
You could get him to fit TRVs on all the radiators - even the one where
the room thermostat is going. Normally, the radiator in that location
does not have a TRV in order to prevent the two controls conflicting.
However, you may find that the initial location for the room thermostat
is not ideal for one reason or another - for example a hallway and
someone leaves the door open or living room and you want a separate
log/gas fire or something. Either can affect the behaviour of the
heating in an unwanted way.
If you fully open the TRV on the radiator where the room thermostat is
located, then that room becomes the "controlling" room. You cn even
take the thermostat from room to room if you want.
Pressure equalisation valves should be used, or mixers with them
Can be as low as 60C.
Some thermal stores/heat banks have the plate heat exchangers inside the
cylinder. Some can be sealed to 1 bar, like a CH system.
Not if it is sealed. One 24 litres expansion vessel will act on the
cylinder, CH pipes and boiler. But you have the same explosion points of an
unvented cylinder, but the boiler and cylinder have independent relief valve
giving two blow-off points.
Abroad they do. I read one case in the US where they think a nasty
neighbour broke into a house while they were on hols. He put a compression
cap on open vent relief pipe and by-passed the electric immersion stat. He
then switched it on. Many hours later while he was elsewhere - kaboom, the
side of the house gone.
There is no debate, it is just me explaining it the amateurs.
Just have a Grundfos Alpha pump when using a thermal store/heat bank. TRVs
all around, and no wall stat just a simple programmer.
Until it gets lost down the back of the sofa with the rest of the remotes.
Not at all. A thermal store is a great neutral point and buffer for CH and
the boiler. Worth having for that alone. Also TRVs on all rads with all
zones going back to the store (the neutral point) is an advantage. All
using the same water, no heat exchangers to reduce efficiency, with little
chance of sludge build up is an advantage. A DHW only unvented cylinder
does not give you what a pressurised thermal store/heat bank does in any
way. Also a pressured thermal store using a plate heat exchanger can be
DIYed, no Building Control notification and no annual service. I would have
two pressure relief valves just to be sure - the boiler usually provides one
In Germany a heat buffer is becoming pretty standard on many installations,
just a small cylinder in the CH line, the thermal store/heat bank does this
for free, it also does boiler buffering for free too...and it goes high flow
Here is one German system. They use a pressurised cylinder with fresh water
in the boiler and through the heating circuit. Look at the system as
primary water and add an external plate heat exchanger for DHW take off and
it is all there for the UK market.
"Compact systems with just one tank [thermal store] that also acts as buffer
storage for the boiler dominate the German market. In conjunction with
wood-burning boilers there remain no alternatives since such a buffer volume
is urgently needed for their use. However, combination tanks with integrated
gas or oil burners use a large storage tank to replace the boiler and its
tank for heating domestic water [integrated thermal stores]."
The Germans are way ahead of us.
I don't live in America. Has yours gone kaboom? Understandable.
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