Vokera compact 28
- Thread starter bellairaphon
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To cbw
Yes I agree. But before I strip the pump down I wanted to ask the op what his solution was in case he could save me unwanted effort. The flow pressure differential implies no blockage of substance so the pump or the bypass is the logical place to go next but he might have a completely different solution of an obscure nature. I just thought I'd check with him first. Plus I was curious to see what he found.
Yes I agree. But before I strip the pump down I wanted to ask the op what his solution was in case he could save me unwanted effort. The flow pressure differential implies no blockage of substance so the pump or the bypass is the logical place to go next but he might have a completely different solution of an obscure nature. I just thought I'd check with him first. Plus I was curious to see what he found.
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How are you measuring the differential? Where is the PF machine connected?
A lot can happen in 13 years
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To dp
the power flusher was connected via two 22 mm stubs in the flow return about 400 mm below the boiler and under the magnetic filter and fill point. The downstream flow and return is isolated with butterfly valves just below that. This insured that the pf is only blasting through the boiler. A manometer on the pf can give flow pressure or differential pressure between flow and return. It reads only 0.09 bar when flushing which is not a lot more than the same length of 15 mm pipe would give.
so with little or no restriction the next place to look will be hoses and pump but I didn’t want to break in there until I’d got an o ring set.
I was hoping that the op might have found a funny condition elsewhere that I could investigate first before breaking into the boiler circuit.
the power flusher was connected via two 22 mm stubs in the flow return about 400 mm below the boiler and under the magnetic filter and fill point. The downstream flow and return is isolated with butterfly valves just below that. This insured that the pf is only blasting through the boiler. A manometer on the pf can give flow pressure or differential pressure between flow and return. It reads only 0.09 bar when flushing which is not a lot more than the same length of 15 mm pipe would give.
so with little or no restriction the next place to look will be hoses and pump but I didn’t want to break in there until I’d got an o ring set.
I was hoping that the op might have found a funny condition elsewhere that I could investigate first before breaking into the boiler circuit.
start your own post after you have changed the flexi hoses, they are completely chocked 
Take it the boiler is giving good hot water
It is the heating that is the issue I assume. If yes, have you checked the temperature of the flexi pipes to the heat exchanger? If same or similar, bypass within the boiler is stuck open. Your PF machine might not be sending any water through the flexi pipes, circulation instead through a stuck open bypass cartridge (if it is there as have seen a pump change with no bypass- that has to be installed at replacement)
As suggested ....have these been checked? Notorious for clogging up, especially when the system has been filthy.
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I went back today and removed the flexi pipes to and from the heat exchanger as you guys suggested. They did not appear to be too blocked. I have a small endoscope which I put through both pipes and could see no blockage. However the walls of the pipes were scaled. I flexed the pipes and could hear the scale cracking and bits came out eggshell thin. I then cleaned through with bottle/pipe cleaner brushes dosed with citric acid. Given how many hours the power flush was on the boiler blasting through fernox f8 I was surprised to see scale at all. Makes you wonder how good these deep cleaners are. Unless of course the bypass valve is stuck open and my power flush is not actually passing through the pipes and HE (as suggested). The amount of scale as lime and magnetite was not too much but every bit helps.
I then took the pump head off. All looked well behind there at first glance. Pump rotor good turning ok with the motor shaft. However when I looked inside the cavity behind the rotor (where the AAV is) I saw half a dozen blobs about the size of a pea to a kidney bean. I removed these to identify them. They are malleable and softish like a mastic, but a viscous mastic like that used in expansion joints in a brick wall. They were black from sludge. Goodness knows what they were, clay from boss white, mastic from a poor joint, old borax flux?
I wondered whether these would just sit in that cavity, when I power flushed, bobbling about on the incoming water flow, but when drawn in by the pump rotor they stuck to the rotor centre and impeded flow. That might explain why a PF saw little impediment to flow but the pump on its own struggled to circulate.
However with pump cleaned out and cleaned hoses put back on the same problem occurred, overheat and flame off. This is with my flow to return short circuit so not a flow impediment in the radiator run.
I now have to check the bypass valve (possibly more new o rings needed) or consider whether the heat exchanger is the problem.
Trouble is I was booked for a system power flush not a boiler repair. A new heat exchanger is 230 parts plus Labour. I don’t think the customer was expecting that.
Although the compact 28 is a combi it is only used for central heating. The hot water pipes through the heat exchanger are empty. I wonder if this lets a local hot spot happen near the overheat thermostat
I then took the pump head off. All looked well behind there at first glance. Pump rotor good turning ok with the motor shaft. However when I looked inside the cavity behind the rotor (where the AAV is) I saw half a dozen blobs about the size of a pea to a kidney bean. I removed these to identify them. They are malleable and softish like a mastic, but a viscous mastic like that used in expansion joints in a brick wall. They were black from sludge. Goodness knows what they were, clay from boss white, mastic from a poor joint, old borax flux?
I wondered whether these would just sit in that cavity, when I power flushed, bobbling about on the incoming water flow, but when drawn in by the pump rotor they stuck to the rotor centre and impeded flow. That might explain why a PF saw little impediment to flow but the pump on its own struggled to circulate.
However with pump cleaned out and cleaned hoses put back on the same problem occurred, overheat and flame off. This is with my flow to return short circuit so not a flow impediment in the radiator run.
I now have to check the bypass valve (possibly more new o rings needed) or consider whether the heat exchanger is the problem.
Trouble is I was booked for a system power flush not a boiler repair. A new heat exchanger is 230 parts plus Labour. I don’t think the customer was expecting that.
Although the compact 28 is a combi it is only used for central heating. The hot water pipes through the heat exchanger are empty. I wonder if this lets a local hot spot happen near the overheat thermostat
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Unless I've missed it no mention has been made of the pump torque.
Pull the head off and whilst holding the impeller with a cloth/glove start the pump. Check the starting torque and running torque is sufficient. Compare with a new pump if you're not sure.
AFAIR these boiler use standard Grundfos...so get a cheap Worcester part as a replacement if necessary.
Pull the head off and whilst holding the impeller with a cloth/glove start the pump. Check the starting torque and running torque is sufficient. Compare with a new pump if you're not sure.
AFAIR these boiler use standard Grundfos...so get a cheap Worcester part as a replacement if necessary.
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Ok went back to the church Friday afternoon to see if I could fix it before their Sunday service. And I finally resolved the issue. I checked the pump torque (as suggested) and that seemed no different to another 15-50 pump head I had. I swapped them just be be on the safe side and no improvement.
So with no blockage in the boiler isolator valves, the AAV ok, the pump rotor free, the pump upto torque and pressure, the bypass valve ok, the flexi hoses clear and clear flow from flow to return (via a short or my pressure wash machine) the only thing left is the heat exchanger.
But this showed no clear evidence of blockage. At just 600 mbar pressure it was passing way more than the 14L/min flow, well within spec. So why overheat.
After removing the exchanger and a bit of investigation consulting with vokera we found the problem.
The heat exchanger has an inlet on the rear left and let’s the water pass into a manifold where the flow splits into two flow tubes which pass through the flame fins from left to right. Within these flow tubes, coaxial style, is the hot water circuit. So the flow through the flame section is restricted to being between the outer and inner pipe, quite a narrow cross section compared to the overall pipe diameter. On the right hand side the two flow tubes pass into another manifold where the water moves to the front of the heat exchanger and then flows into another two coaxial flow pipes from right to left through the exchanger. On the left the two pipes combine the flow into a manifold (similar to the inlet manifold that spilt the flow). From the manifold the water exits via the outlet on the front left.
This works fine if there is no sediment in the flow water. But a relatively small amount of silt can accumulate in the front of the right manifold. This starts to inhibit flow through the front tube of the pair but the rearward tube easily carries the flow. The reduced flow in the front tube means more silt accumulates, and so on. Eventually the front tube becomes complete blocked but good flow continues in the rearward tube.
With the front tube blocked it kettles and cooks the scale onto the narrow walls. So under flame the front quarter of the exchanger gets very hot even though the rearward tube is taking through the hot water. However the overheat thermostat is mounted on the exchange at the front left of the left front manifold and senses the overheat. That front tube hotspot will always trigger the thermostat
That is the fault condition I have, good apparent flow overall but an overheat and flameout.
It’s hard to flush out even with the power flusher as the water just passed through the rearward tube and no descaler touches the scale in the front tube. And once it’s completely blocked and baked no flushing will touch it. Maybe a long term pickle might but basically the exchanger is dud.
So a quick visit to Wolsey for a new one, 230 pounds to the customer (plus Labour) and all is well.
But it shows the importance of keeping sludge and lime down.
One can see why heat exchangers are better without coaxial piping and using full diameter pipes and a plate exchanger for hot water instead.
So with no blockage in the boiler isolator valves, the AAV ok, the pump rotor free, the pump upto torque and pressure, the bypass valve ok, the flexi hoses clear and clear flow from flow to return (via a short or my pressure wash machine) the only thing left is the heat exchanger.
But this showed no clear evidence of blockage. At just 600 mbar pressure it was passing way more than the 14L/min flow, well within spec. So why overheat.
After removing the exchanger and a bit of investigation consulting with vokera we found the problem.
The heat exchanger has an inlet on the rear left and let’s the water pass into a manifold where the flow splits into two flow tubes which pass through the flame fins from left to right. Within these flow tubes, coaxial style, is the hot water circuit. So the flow through the flame section is restricted to being between the outer and inner pipe, quite a narrow cross section compared to the overall pipe diameter. On the right hand side the two flow tubes pass into another manifold where the water moves to the front of the heat exchanger and then flows into another two coaxial flow pipes from right to left through the exchanger. On the left the two pipes combine the flow into a manifold (similar to the inlet manifold that spilt the flow). From the manifold the water exits via the outlet on the front left.
This works fine if there is no sediment in the flow water. But a relatively small amount of silt can accumulate in the front of the right manifold. This starts to inhibit flow through the front tube of the pair but the rearward tube easily carries the flow. The reduced flow in the front tube means more silt accumulates, and so on. Eventually the front tube becomes complete blocked but good flow continues in the rearward tube.
With the front tube blocked it kettles and cooks the scale onto the narrow walls. So under flame the front quarter of the exchanger gets very hot even though the rearward tube is taking through the hot water. However the overheat thermostat is mounted on the exchange at the front left of the left front manifold and senses the overheat. That front tube hotspot will always trigger the thermostat
That is the fault condition I have, good apparent flow overall but an overheat and flameout.
It’s hard to flush out even with the power flusher as the water just passed through the rearward tube and no descaler touches the scale in the front tube. And once it’s completely blocked and baked no flushing will touch it. Maybe a long term pickle might but basically the exchanger is dud.
So a quick visit to Wolsey for a new one, 230 pounds to the customer (plus Labour) and all is well.
But it shows the importance of keeping sludge and lime down.
One can see why heat exchangers are better without coaxial piping and using full diameter pipes and a plate exchanger for hot water instead.
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