Repeated Air in Central Heating System - Help appreciated

[Vessy_1]

Hi, I'm hoping for some suggestions as to where I go next in my endless mission to stop my system taking on air.

The house has a gravity fed system, 'S' plan, Heat only boiler supplying CH & HW, with separate F&E and vent just before the pump in the typical H layout, pump & valves are all located in the upstairs Airing cupboard.

Boiler and pump have just been replaced as the old boiler was proving unreliable and underpowered for the house (4 Bed, 11 Rads) new Ideal Logic + 24kw installed with Grundfos UPS3 pump. This issue is not new, I have battled with air previous to the boiler and pump replacement and I was hoping this would be solved following the clean, flush and a fresh dose of inhibitor that was done during the work.

So, the problem, once I have all the air out of the system it will run perfect indefinitely, as long as the water is cold!! the air only starts to circulate once the return temperature is in the 50's, as the return temperature rises the air gets worse to the point that the pump will eventually cavitate and stop moving water around the system, this happens on CH, CH+HW and HW only, the air is however much worse on HW only as the return temp is a lot higher especially towards the end as the cylinder approaches set temp (60 Deg). Again on CH only it only slightly makes a small amount of air as the return is around 50 Deg.

I added some further x100 as I had 50% emptied a rad a few times (air gets trapped in the rad return line) and this has made no difference to the issue, I have tried the pump on all speed settings and this makes no difference, I tried the glass of water on the vent line and all seemed good (water dropped about an inch when the pump turned on and came back up within a few seconds) header tank level slightly drops on pump turn on so looks OK.

Checked all the accessible unions on the valves, Pump, HW Balance valve and looked over everything, no evidence of leaks. I even checked the return line at the boiler with the heating on and definitely had positive pressure as the filter could still be bled (Mag filter above boiler on return line) so no sucking in of air.

I am all out of ideas, other than corrosion somewhere in the return line, is that possible and will it make air so quickly? After I have bled the system the pump will cavitate after 5 mins once the return is into the 55-60 deg region.

Currently I can keep the system going but reducing the boiler output temp to around 62, this keeps the return on CH well below 50, I start to build air when HW only on demanded but not enough to cavitate the pump, only downside is the HW takes an age to recover and the boiler short cycles as its minimum output is too much for the small temperature difference.

Any ideas or suggestions would really help, as I mentioned above I can leave the system running for hours with the boiler temp set to minimum so cannot get my head round this being a leak.

 

[steadyon]

1. How did you bleed the air from the hot water cylinder?
2. Photographs if possible might help diagnosis, as follows:
2a. Vent over F&E showing height of inverted "J".
2b. Hot water cylinder showing inlet and outlet for coil, together with any bleed valve(s) fitted.
2c. Pump with indication of pumping direction.
2d. Motorised valves (least likely to be useful, so if difficult, don't bother).

 

[Vessy_1]

Hi Steadyon

I will get some pictures soon, I can provide some answers your questions,

1. With the boiler off I ran the pump with HW demand only, opened the balance valve fully, not much air was in there, after a min or so there was noise of water moving but no air noise, As I get this issue when on heating only (once the return temps are up) does this not rule out air in the cylinder?
2a I am getting no pump over into the tank, it is about 500mm above the top of the water level, cannot go higher due to low roof rafters, I do however get a very small dribble when the pump turn off.
2c Pump is fitted the correct way, arrow towards the two valves, away from the F&E pipes.

 

[Vessy_1]

Pictures As requested.

 

[Vessy_1]

Sorry, system is open vented, not gravity as I put in the first post!!

 

[steadyon]

1. Many people call their systems gravity when the are open vented. Not an issue.
2. There appears to be no means of bleeding air from the hot water cylinder coil. You could try running the system (HW and CH) up to a reasonable temperature, then turning the boiler off. Wait 15 minutes or so, then manually open the HW motorised valve, and slacken the compression nut nearest the pump until all air is out and water starts to appear. Then re-tighten the compression nut. Old towels advised.
3. The vent pipe should rise up then fall over the F&E. The volume in the rise of the inverted "J" should normally be enough to accommodate system water expansion. From memory it should be able to accommodate 4% of total system volume, and should rise a minimum 450 mm. Would you have scope to:
3a. Raise an inverted "U" between the rafters, dropping the existing vent pipe so that it only has a slight rise from the boiler end giving more room for a rise between the rafters?
3b. Rerun the vent up towards the apex of the roof to achieve a vertical height of 450mm, then drop back to over the F&E tank?

 

[John.g]

Even though you have a "accepted" system with boiler, then vent, then cold feed, then pump, (VCP), I strongly feel that the combined vent and cold feed like mw own almost 50 year old system is a far better arrangement as its in a "static" part of the system, see below. If your vent passes up close to the F&E tank or you can modify it to do so, then cut and blank the H at cold feed tee in and tee in the cold feed directly like mine then I would think that would solve your problem.

The UPS3 looks as if its on CP1 mode/setting, is that correct??

Edit: It is also acceptable (but NOT with solid fuel) to have no vent as long as there is no isolating valve on the cold feed, I personally don't like the idea and wouldn't dream of it except there is a a 3 bar PRV fitted at the boiler. I see there is a compression fitting on the end of your vent, have you been blanking this off?.

 

[John.g]

1. Many people call their systems gravity when the are open vented. Not an issue.
2. There appears to be no means of bleeding air from the hot water cylinder coil. You could try running the system (HW and CH) up to a reasonable temperature, then turning the boiler off. Wait 15 minutes or so, then manually open the HW motorised valve, and slacken the compression nut nearest the pump until all air is out and water starts to appear. Then re-tighten the compression nut. Old towels advised.
3. The vent pipe should rise up then fall over the F&E. The volume in the rise of the inverted "J" should normally be enough to accommodate system water expansion. From memory it should be able to accommodate 4% of total system volume, and should rise a minimum 450 mm. Would you have scope to:
3a. Raise an inverted "U" between the rafters, dropping the existing vent pipe so that it only has a slight rise from the boiler end giving more room for a rise between the rafters?
3b. Rerun the vent up towards the apex of the roof to achieve a vertical height of 450mm, then drop back to over the F&E tank?

I have often wondered about this 4%, I know that water expands by 4% at 100C, but stlll can't figure out its relevance as the cold feed takes the majority of the expansion volume and the level in both the vent and the F&E tank will only rise by a few mm.(u tube effect), Maybe the correct description is vent + cold feed and expansion.

 

[Vessy_1]

Even though you have a "accepted" system with boiler, then vent, then cold feed, then pump, (VCP), I strongly feel that the combined vent and cold feed like mw own almost 50 year old system is a far better arrangement as its in a "static" part of the system, see below. If your vent passes up close to the F&E tank or you can modify it to do so, then cut and blank the H at cold feed tee in and tee in the cold feed directly like mine then I would think that would solve your problem.

The UPS3 looks as if its on CP1 mode/setting, is that correct??

Edit: It is also acceptable (but NOT with solid fuel) to have no vent as long as there is no isolating valve on the cold feed, I personally don't like the idea and wouldn't dream of it except there is a a 3 bar PRV fitted at the boiler. I see there is a compression fitting on the end of your vent, have you been blanking this off?.

Hi John,

I’m up for doing anything proactive to fix this, but with no pumping over and no suction of air on pump start would this be of benefit? The air only builds in the system after everything is hot and the return line from the rads or cylinder is 55 deg +. This is why it only seems apparent when HW only is being demanded and the cylinder is almost to temp, it really has me baffled.

Regarding the pump setting, yes at that time it was on CP1, my logic for this was that once I had balanced all the rads for a 12 deg drop any upstairs rads shutting down on the TRV’s shouldn’t change the drops on the downstairs rads as the pump would reduce and maintain the same pressure in the system, similarly if HW and CH are both being demanded the pump will speed up to keep pressure so the rads should still have a 12 deg drop, is this setting not right?

I have today been running the system on fixed speed 3, although maybe slightly better it is still producing air when the return is hot.

Could I still have trapped air somewhere, maybe a capped off pipe somewhere that filled with air when the system was drained, then as the system temp increases the air expands and enters the flow/return pipes, feasible?

Just would have thought after 2 weeks the air would all be gone!

Do these UPS3 pumps cavitate easily?

 

[John.g]

There are no guarantees with any OV system, most problems seem to arise in systems plumbed like yours where the pump is mounted high up on the flow from the boiler and very near the motorized valves (all for convenience). Mine (oil fired boiler) has the pump located in the boiler heatpac downstairs, on the boiler return.
I can run it at any pump heand or temperature I like with absolutely no problems.

You say that a increase in return temp > than say 50/55C triggers the air ingress problem.
Is the boiler up to its target flow temperature before 50/55C is reached, if not then its the flow temperature (pump inlet) which is probably triggering this, difficult to see how the increase in return temp without a increase in flow temperature can trigger a problem on the flow side at a steady temperature, the flow temperature will increase by 5 to 7C each time the boiler cycles as the burner cuts out at target temperature+5/7C, again don't know if your problems start with steady target flow temp or only when the boiler starts cycling.
Also don't like the thought of that dribble from the vent, I would think that each time this happens that you are drawing in some air each time. Of course, if the problem arises under steady, continuous pump running then another reason for the problem.
Can you ensure that the circ pump keeps running during all boiler cycling as it should only stop when both CH and/or HW is satisfied.
Also check that the pump start/stop is from the motorized valve auxiliary contacts (end switches)

You might just try (again?) running o n PP2 mode as it should run at ~ 0.5M lower head and still give sufficient flow with maybe no dribble on pump stop.

Re UPS 3 cavitation, havn't seen the NPSHr (Net positive suction head required) for this model but I would think that ~ 1.5M (like the selectric) would be OK,, the NPSHa (Net positive suction head available) however is/should be way above this as :
NPSHa is the Atmospheric pressure (absolute) + the static head - the liquid vapour pressure - the suction losses.
vapour pressure at 70C=3.1M, 60C=1.9M, & 55C=1.2M. so assuming suction loss at 0.2M (between cold feed and pump suction) and static head at 2M (use your own numbers for static head and suction losses) then the
NPSHa at 70C = 10+2-3.1-0.2, 8.7M and 10+2-1.2-0.2, 10.6M at 55C, so unless some very big problem with the suction loss at the pump (or my calcs are crap) then can't see how flow temperature itself should cause any cavitation problems so possibly points to air ingress/release at higher temps, there was a long running thread on here abut someone with problems like yours but with the circ pump located low down on the system (flow) and again couldn't be run at anything > say 65C due to noise, the system was then sealed but problems still persisted, unfortunately, the OP never told us if he ever fixed his problem.

Edit: Just saw that you had/have been running on fixed speed3? hope not for long as this is a disaster IMO with your type of system, CP1 at 3M should be a very good setting.