New Pump in Combi - but CH still not working ?

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Norfolk
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Old pump in Combi was running slow and overheat stat was cutting burner off in DHW and CH. The downstairs rads were cold and upstairs rads just getting warm.

New Grundfos pump fitted has restored the DHW but the CH is not improved and boiler cycles.

When the CH is turned on, hot water flows down to the 1st floor initialy (boiler in attic). But after about 1 minute the auto by-pass in the boiler opens, returns the hot water back to the main heat exchanger and causes the CH stat to turn the burner off.

Following a suggestion on another post of a potential blockage, I have flushed the system using the fill loop at the boiler.

By closing the CH flow isolation valve under the boiler I can get a good flush down the return pipe, through the rads (5 mins each in turn) and out the drain-valve which is next to the TRV on the kitchen rad.

However, closing the CH return and opening the CH flow means the water is now going through the main heat exchanger etc before going down the flow pipe and out the drain. Result is a much smaller amount out of the end of the drain hose.

Definately a blockage in the flow pipe somewhere ?
 
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What make/model boiler do you have?, it seems like you have a diverter valve problem.
 
blocked return filter on blr? dependant on location of fill loop
 
What make/model boiler do you have?, it seems like you have a diverter valve problem.

Its an Alpha CD24C.

While drained for flushing I removed the DHW heat exchanger (back flushed it while off) and diverter valve as a precaution. The valve in the diverter returned to CH several times ok when I pushed it across to the DHW position. There was no sludge or build-up inside either unit.

While running the CH, the DHW heat exchanger stays cold until DHW is actualy demanded. The diverter valve seemes to move ok, trips the flow switch and operates the burner. I can now fill the bath with hot water :D
 
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blocked return filter on blr? dependant on location of fill loop

The CD24C has a cyclone on the CH return just before the pump, which then feeds into the main heat exchanger. The fill loop sits between the cyclone and pump.

This does mean when I flushed the rads with the flow isolated, the water was traveling down throught the cyclone first (in reverse direction) which wasn't causing a problem.

To be honest I don't know why Alpha bothered to fit the cyclone. I have had to remove the DHW heat exchanger several times in the past due to blockages and each time I have checked the cyclone it has never had anything in it !
 
The CD24C has a cyclone on the CH return just before the pump, which then feeds into the main heat exchanger. The fill loop sits between the cyclone and pump.


Automatic bypass stuck open??????????????.
 
Automatic bypass stuck open??????????????.

I have checked the by-pass again and now confirm as soon as the hot water reaches the by-pass valve position, it does immediately divert through the by-pass (not after 1 minute) and into the return pipe. Some hot water continues down the flow pipe toward the 1st floor, but the return pipework next to the by-pass heats up much more quickly.

Next job I suppose is to take out the by-pass valve and have a look !

I am puzzled at what normaly operates the auto by-pass. Is it back pressure when all the TRVs are closed ?
 
I am puzzled at what normaly operates the auto by-pass. Is it back pressure when all the TRVs are closed ?



Basically it works the same as a PRV with a spring loaded diaphram that opens by the pessure of the pump when TVR`s etc close, I would suspect that crud is stopping the diaphram from closing/seating correctly.

spraggo
 
By-pass was full of crud.

This one has a mushroom shaped valve held against it's seat by a spring. Once I had cleaned it out it was easy enough to push with my finger, but the spring was way too strong for me to get the valve to open when I blew into the pipe - No matter how much puff I used.

Even so, the by-pass is now opening fully straight away and turning the boiler off even quicker (must be very powerfull those little Grundfos pumps) ! Even less hot water is getting down to the 1st floor and the radiators are hardly warming up now ?

Don't you sometimes get one step forward and two back !?
 
By-pass was full of crud.

This one has a mushroom shaped valve held against it's seat by a spring. Once I had cleaned it out it was easy enough to push with my finger, but the spring was way too strong for me to get the valve to open when I blew into the pipe - No matter how much puff I used.

Even so, the by-pass is now opening fully straight away and turning the boiler off even quicker (must be very powerfull those little Grundfos pumps) ! Even less hot water is getting down to the 1st floor and the radiators are hardly warming up now ?

Don't you sometimes get one step forward and two back !?



The bypass valve should not open fully until you get a flow restriction in the heating circuit, ie, all TRV`s closing. Are you sure you do not have an air lock?.

spraggo
 
Are you sure you do not have an air lock?.

Not sure of anything !

How would I test for an airlock and overcome the problem ?

Would the effect of a blockage not be the same as the TRVs closed i.e. by-pass opens due to the pressure of the pump not going anywhere ?
 
when you flushed it, what cleaning chemical did you use?
 
Its had X400 in it in the past, but has been flushed and re-filled a couple of times since (always added X100) when the DHW heat exchanger clogged up.

Previously I allowed the sytem to heat up then drained it while still hot and repeated this a couple of times. A friend who fitted the boiler showed me how to do this, but he is now retired and has moved away.

I just used water via the fill loop this time, but this is also the first time I have back-flushed the rads in this way.
 
if you have reason to believe it had sludge or sediment in it, you need to use a chemical cleaner. X400 is very mild, but will loosen old black sludge (but not hard limescale deposits). Other cleaners are usually acidic and aggressive and can cause damage..


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I understand X800 is faster and will remove lime but I have not used it myself. It can be used even if you do not have a powerflusher, but must be rinsed out thoroughly.
 

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