air in boiler primary cct

[Rick Hughes]

I have a Baxi Barcelona boiler on primary loop to a Thermal Store. In this configuration .. the store water contents are circulated around the boiler (rather than just a transfer coil)

Primary loop has a pump, flow control valve and expansion vessel, vessel charge is to 1 bar and static pressure on primary loop (via filling loop ) is also 1 bar. Had to change pressure vessel due to failure ...

Issue I have is that is I set static pressure to 1 bar cold ... then it hits 2.5 bar hot (too high) ... this would suggest that there is air in the system, expanding when hot. There is an AAV on top of store which is working and if I manually operate only water coming out .. (this was actually removed entirely during filling) There are air bleed screws on highest point of F & R pipes ... checked and only water comes out of both.

Phoned system manufacturer he concurs it is air in system ... he suggested releasing all static pressure, setting vessel to 2.5 bar and then running pump (boiler off) and then refill system to 1 bar static charge. Done that .... same result.

Anybody any tips for getting air out ? If you need a sketch due to my description being unclear ... :

[Dave Liquorice]

Is that drawing correct in regard of the relative vertical layout of the bits?

If so the highest point is the filling loop, have you tried venting there once the system is "full".

[harryagain]

I would start by checking that there is in fact air in the expanson vessel. The air in the expansion vessel is squeezed as temperature and pressure rises in the system so stopping pressure from rising too much. OR The expansion vessel may be too small. You are going to need a big one. If you have a stored water system, that is a huge amount of water compared with a "normal" system. You need to work out the water volume in the system, , temperature rise and consult the P/Vessel manufacturer's data.

[John Rumm]

ok so the thermal store itself is also pressurised by the sounds of it...

Not sure I concur with the train of logic. First if we assume that 2.5 bar is too high (and that is also questionable[1]) It sounds more like you have either too little expansion room, or too high an expansion charge pressure.

What pressure do you *expect* on a hot system?

Do you have an estimate of the actual water content of the primary side?

It strikes me that air in the system would *reduce* the ultimate pressure rise you observe - after all that is exactly what an expansion vessel is - a bubble of (readily compressible) air waiting to be squeezed but the incompressible water when it expands.

[1] Many sealed primary systems will be designed to operate with say 1 bar cold pressure, and say 2.5 bar hot system pressure, with the PRV set to operate at 3 or 3.5 bar.

Would a more plausible explanation be that previously the system did have extra air in it, resulting in lower than expected hot pressures while the expansion vessel was working. When it failed and swapped it out, you bled out the extra air that was once there and are now observing the expected pressure rise?

[Rick Hughes]

No highest point is the 2 vents screws on pipes ... and only water comes out of these.

[Rick Hughes]

The expansion vessel is correct one ... sized for system ... I bought the whole lost as a 'package' and been in use for more than 10 years .... just failed so replaced ... in fact replacement is 1L bigger as they no longer make a 24L.

[Rick Hughes]

I didn't come up with the 'air in system' idea, that was system manufacturer. maybe its something else.

Checked air charge in pressure vessel .. set to 1.0 bar.

I removed AAV filled system slowly until store was full, refitted AAV and continued to fill ... air was heard vesting from AAV. Checked bleed vents - only water coming out.

Used filling loop to fill to 1.0 bar static pressure.

If I run system without boiler firing (i.e water being pumped in loop/store) pressure drops a little but returns to 1 bar if I stop pump.

I start system and as temp increases in loop/store pressure goes up ... reaching 2.5bar

System is a complete designed package ... and pressure vessel is correctly sized.

[Tim+]

Well, as John says, having air in the system would reduce the pressure rise due to temperature rather than increase it. It is after all what your expansion vessel should be filled with. Many pressurised tanks actually have an air bubble in the tank for pressure control rather than an external expansion vessel.

My first thought is that your new vessel is faulty. Does water come out the air valve on the expansion vessel?

Tim

[John Rumm]

Which sounds pretty reasonable in the absence of any other information like a manufacturers statement that the hot pressure should never excess x etc.

Yup was not suggesting otherwise. In fact it sounds like its doing what it should!

[Brian Gaff]

I cannot see the pictures, but I do remember the antics of a neighbour with similar air issues and in the end it seemed there was air in a point in the pipework where it went up a bit, then down a bit again. I think in the end they re routed the pipe to make it less of an issue. I'm glad I don't have a boiler in this house one hears so many tortuous incidents!

Brian

[Onetap]

Wot 'arry said.

Air charge in expansion vessel depleted or it's too small.

I don't see it could be air in the system, but check and recharge the expansion vessel as they say.

The boat winch thing looks very impressive, I'd missed any updates.

[Onetap]

PS Good practice is to have an air bottle on the air vents, i.e. a tee with an upright dead-leg with the vent on the end of it. The idea is to trap a few cubic inches of gas before the tee fills up and the bubbles just go round and round the circuit.

[Onetap]

PPS Also good practice to have the pump pumping away from the expansion vessel.

Google "pumping away".

The point of connection of the expansion vessel is the point of no pressure change, pressure there is the static head.

Pressure drops, due to frictional losses, as you go around the circuit to the pump inlet. The flow control (regulating) valve will be a big pressure loss. Pressure at pump inlet is lowest and pumps do not like low inlet pressures. See NPSH.

PPPS Also suspect a duff pressure gaug until you've confirmed its accuracy. Some are very cheap and nasty.

[Rick Hughes]

.

Released all pressure and refilled ... set static pressure to 1 bar ... again run pump without firing boiler ... get some air pushed out via AAV .. keep topping up until 1 bar steady.

Put boiler onto heat .. and as it gets hot pressure rises to between 2 and 2.5 bar

Called system supplier again this morning ... pressure vessel is definitely the right size for supplied thermal store. They are suggesting that pressure increase could in fact be acceptable as water heats up .... Seems a big change to me, and didn't do it previously.

Boat winch works a treat ... now just watch and press button on remote control ... been in use for almost 18 months now.

[Rick Hughes]

It has 'reduced a bit' ... Now with a 1bar static pressure when cold ... it gets to 1.5 - 1.7 bar when hot. Maybe that is what should happen ... just that now I have an accurate

0-2.5bar 100mm pressure gauge I can read it accurately ... whereas previously I only had the 30mm 0-10 bar gauge built into pressure vessel valve.

[Rick Hughes]

With hindsight should have soldered in short 15mm upstands on bleeds - instead of just putting the bleeds direct on 28x15x28 Tee. However there is a 150mm upstand on top of the thermal store to which is fixed the AAV.

So any air being 'pumped around' should find its way to this upstand.

[Gazz]

There's yer answer, your using a different gauge to before, and to make it 'worse' the new gauge has a totally different pressure range to the old un.

So basically, don't worry about it, the old gauge was cheap and inaccurate, the new gauge is just showing what the system has been doing all along, and what all hot water systems do as they heat up.

[Onetap]

It could well be so. Or it may be the new gauge that's wrong. Or both of them. Or both of them are right and the pressure really has reduced.

Besides all that, if there was air in the primary circuit, it would just a ct as another mini expansion vessel and it would reduce the pressure rise f or a given temperature rise. That is, the opposite effect to what you'd tho ught (think that's right). It's worrying that the supplier had thought the same.

After all, the expansion vessel is merely an air bubble in the primary ci rcuit, the diaphragm is there just to contain the air charge and delay it's wandering off.

[Onetap]

Are there expansion vessels & pressure relief valves on both secondary circuits? Might a leaking heat exchanger be involved?

[John Rumm]

To me that seems like a reasonable pressure rise for the amount of expansion capacity available and (my estimate) of the water content. (by way of comparison, my system boiler with a large primary circuit (21 rads) shows a similar rise as it heats to full temperature (although due to the weather compensation it rarely needs to get that hot)

My guess would be you are seeing a combination of two things: previously it had not been bled as well, so there was an additional air pocket in the system somewhere that was lowering the pressure rise observed. Also a better pressure gauge is showing you more detail than was available before.

Ultimately, if the store is rated to take the pressure, and the PRV valve is set correctly, then you have nothing to worry about.