I have 4 expansion vessels on my system ... each has a different 'charge' pressure. (all set to the design documents)
Just interested as to 'in general' what the pressure should be .... should it be set to match the operating pressure of the pipe it is attached to, is it set higher, lower ?
It needs to be lower than the static (cold) pressure in the water system so that the air gets compressed a bit when you fill the system. Otherwise, you'd never be able to bleed a radiator.
Setting the air pressure to 0.7 Bar (10 psi) and the static water pressure to 1 bar works for most systems.
Not sure why you've got *four*! Are they all in the same system, or are any for a mains pressure hot water system? Assuming they're all in the same system, you could pre-charge them to different pressures so that they get used progressively as the system pressure rises.
on a related question...
It is never clear to me how you are supposed to 'top up' the air in an expansion vessel without draining the water pressure. Maybe you can't do this. Isthere some trick to it?
If you drain the water you can then pump air in until the pump reads 1 Bar (or whatever the official precharge value is), but can it be done without draining the water?
Robert
It can't be done while the air in the expansion vessel is partially compressed by water, because that would give a false air pressure reading. So you simply need to *de-pressurise* the water system - not drain it completely. Better still, if the expansion vessel is connected via an isolation valve and drain c*ck, you can easily isolate and drain the water in contact with it without affecting the rest of the system. [This won't actually have much effect on the *volume* of water you have to remove - unless you've got lots of radiators full of compressed air!]
Don't have any radiators :-)
One on boiler primary loop, (whole of thermal store volume cycles around boiler) One on DHW circuit - pumped loop One on Domestic heating circuit One on incoming mains
without radiators? Tell us more.
At a guess all underfloor heating. Best way to heat with a thermal store in the system. The temp of the store doesn't half drop quick when a radiator based CH system kicks in an lots of water at 20C gets shoved into it. Radiators heat up quick mind...
Not quite sure why the mains needs an expansion vessel though.
17 zone underfloor heating system .. each with own thermostatic control .... all fed from Thermal store. Having lived with this for a few years now ..it is far superior to rads. Especially on ground floor where it is pipe-in screed, so they whole floor becomes a massive thermal store for the heat. Obvioulsy I built with insulation in place to suit this.
Even heat, no convection currents, no hot spots, can place furniture wherever.
The pressurized Thermal store is like a DHW tank in reverse .. boiler primaries circulate full volume of store around boiler .... which means long efficient burns .. not continually cycling. There are 2 heat exchangers in the store .. the upper feeds a domestic HW loop ,, mains in .. DHW out... i.e. 22mm pipe out with take offs at all taps, bath, showers etc., then at last point a 15mm pipe runs back to store. On this is a very low speed (timed) circulator ... this means when you open any tap you get instant mains pressure hot water. No delay, no waste of water running it to get hot. It's on timed (about 20 time segments) so it only works this way at set time .. remainder its normal system.
This means due to the large volume of heat in the store you can fill a bath with water really fast (22mm pipes on mains pressure) ... we never ever run out of HW.
3 showers in house also benefit of having fully balanced mains pressure hot & cold water ...... tons of flow & pressure.The lower part of store has heat exchanger for heating cct ... this is set at low temp (compared to rads) and is a closed pressurized loop feeding manifolds on each floor via 28mm pipes. These manifolds have zone actuators that take a feed off to each zone.
Great system .... and a diy install.
>
Presumably stopping hammer when a valve closes sharply somewhere rather than to allow for expansion. I'm not sure why there's one on the mains _and_ one on the mains pressure hot water loop though.
(The overall system design sounds very similar to my house, but better implemented, and without the radiators replacing the upstairs underfloor heating pipes that rotted (Nu-Heat Contraflo). But I've only got the one expansion tank (well, actually two because new boiler has one inside too, but it's on the same circuit).)
Correct.
The hot water loop will have a check valve to prevent expansion down the mains pipe, so an expansion vessel to prevent hammer when a hot tap is turned off.
A condensing boiler re-heating the store operates at high efficiencies when this occurs.
I think you mean expansion "vessel" not "tank".
If there is no central room stat and all rads have TRVs and CH is off the store via a Smart pump, this would only occur on morning switch on. In normal operation the TRVs would close up somewhat, or even all of them closed right off, and only trickle heat out of the store.
Recovery rate should be quick as all the boiler heat is dumped into the store.
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