pumping over!

[anotherdiyjob]

Anyone had this:?
'Y' plan single feed/return potterton boiler(fairly recent), 3-port motorised valve(recent), sunvic programmer (recent), pump (very recent), hw cyl (fairly recent).
Original symptoms: downstairs rads cold, upstairs hottish, hw supply hottish.
Powerflush was recommended, company contracted (poor job done, with the result of burned out pump, leaking joints, broken-off rad valve stems, damaged pump isolator valves, etc). Fault traced to boiler cold feed blockage, cleaned out and chemicals added. Pump replaced. Company did not return to flush/check/set/test after a couple of weeks. Pumping over (pump was left on high), was told 'It will do that for a while'.
Inmate (tenant!) complained heating only came on with hot water, and was sporadic. Sunvic programmer was not sending command to open 3-way to rads. (internal relay replaced (couldn't geta programmer at 2a.m.!). heating restored, but severe pump-over evident (in all modes).
system was perfect before initial problems. pump is placed after vent(22mm) & feed(15mm), and goes straight to 3-way diverter (pumping INTO valve). even with pump on lowest speed, pump-over occurs. All rads get hot, as does hwcyl.
Could pump be too powerful?
Only other thing I can think of is a 15mm isolator (ball type) was fitted on the cold feed from the header/expansion tank. Possibly causing a resriction?
HELP!
thanking you in anticipation.

 

[newbie1]

poss causes
1/boiler thermostat gone
2/cylinder coil split causing hot water to go up vent pipe on cylinder
3/cylinder stat gone causing water to boil and expand up pipe

you need someone there to check all scenarios

 

[anotherdiyjob]

Hi, thanks for getting back to me!
Boiler stat switches on/off as normal, water isn't boiling when in operation.
cylinder stat switches boiler on/off when adjusted (about 70 degrees C)
I don't think there is any evidence of contamination in the stored hot water. I suppose I could see if the pump-over stops/reduces if I turn on all the hot taps.
The pump-over is a steady flow, so it is not boiling/spurting, nor is it at high volume or a trickle!
Only slight 'kettling' noise from boiler when working, no heavy boiling sounds (clonks/bangs).

 

[newbie1]

cylinder stat is too high should be set at about 60deg
determine is water pumping over through the vant pipe to feed and expansion tank or through vent from hot water cylinder

 

[anotherdiyjob]

Hi Newbie1.
Thanks again for your response.
Definitely from the vent into the expansion/feed tank (small one)
Do you think the restriction in the feed (due to the small ball-isolator valve) could be upsetting the 'neutral balance' on the negative side of the pump?

 

[Davefridge]

Check the vent height. I had same problem & found the vent was only 100mm above the water level---boiler instructions said minimum of 450mm above water level. I raised the height of the vent with some more 22mm pipe & elbows. It's now fine.

 

[anotherdiyjob]

Ooooh! I shall check that! But why should it suddenly become a fault? it worked before.
thanks

 

[Winston]

I think it a combination of the system being flushed and the new pump have you tried turning the pump to a lower setting? Was any pipework changed or altered?

 

[anotherdiyjob]

Hi Winston.
The pump is a Grundfos 3 speed, set to the lowest value.
The ONLY changes were that a 15mm ball-type isolating valve was fitted in-line with the boiler feed (from the expansion/feed header tank).
Some of the bars on the towel warmer rail either take a long time to get hot, or are cold/cool. The towel rail is fitted in parallel with the primary circuit.

 

[Davefridge]

Ooooh! I shall check that! But why should it suddenly become a fault? it worked before.
thanks

Don't know. I'm not a heating engineer, just a expert DIYer. I just check the instructions!

 

[anotherdiyjob]

Perhaps when I have exhausted all other avenues.............I shall read the instructions!!!!

 

[secret squirrel]

I never read any instructions for anything ever that is until I started this career, now, I read all instructions , sometimes they even help....