Netaheat 16-22 - shutting down except on minimum setting

[alban]

(Re-submitted from another thread as probably confusing to mix the two problems although they seem similar)
Hi
I have a problem with Netaheat 16-22 (20+ yrs old). Started shutting down allowing rads to go cold. Heating limps along on minimum setting firing up OK but for short periods only but if I crank up thermostat on boiler it shuts off completely very soon and outer casing gets very hot to touch. Then rads go cold. All rads warmish on min. setting. Tried draining system and now have put Superfloc in via F&E. I presume there's a problem with boiler overheating causing shutdown? Could this be scale in the heat exchanger leading to reduced water flow? However problem showed itself fairly suddenly which is against that. Getting some noise from boiler but not very loud.
Noted today that it failed to come on again this pm even on minimum setting. Started up on re-setting - which it always has done during problem.

Pump seems to be working inasmuch as system didn't drain down without pump but drained well with it on and all rads get warm on min setting. Haven't checked it otherwise - dunno how. Don't know about motorised valve or where it is or how to judge if its working OK. Evidence of sludge as rads are cooler at the bottom. But on allowing water to run through from F&E tank it ran clear quite quickly.
I was going to leave Superfloc for 7 days then try to descale boiler with DS-40 for 12 hrs.
Any ideas? I feel the boiler works OK but it is in need of a declog or something.
Cheers for any help
Grahame

 

[ChrisR]

Some sort of circulation problem - hard to guess, but if the pump impeller is clogged up it would do just about what you describe.

Close the pump isolating valves (they'll probably leak a bit for a while afterwards) and undo the 4 hex screws holding the pump together. Tap a knife blade in the join if it doesn't come apart. Check it aht - there should be a clear slot between the two discs of the pump impeller.

Or drain it down first,
or buy a new pump anyway, sludge knackers them and they don't last forever, so if yours is as old as the boiler..

But also check for air in the system - bleed any vent point in the airing cupboard where the boiler pipes go into the cylinder.

 

[alban]

Hi
Thanks for replying Chris
Do you have to apply a fair bit of pressure to prise open the pump casing? It doesn't want to shift despite undoing 4 screws and trying to undo central black plastic cap/screw. Also does it matter if the elec supply is not necessarily off? There's a switch next to pump but goes to a jumbled junction box rather than straight to pump. Difficult to see which it goes to.
I have a 5" black plastic cylinder stuck on top of a junction of two pipes next to HW cylinder with a red thimble-sized plastic screw cap on the top. Is this a vent bleed point? No water comes out when red cap removed. There is a brass cap-off with a screw in it a bit further down and water comes out of that but no air.
Cheers
Grahame

 

[ChrisR]

Yes turn the mains off first.

Do you have to apply a fair bit of pressure to prise open the pump casing?

Yes.

already answered!

Tap a knife blade in the join if it doesn't come apart.

And be prepared to catch some water. Should only be a cupful, unless the valves haven't turned off properly!

If you turn the boiler off and mains on I think the pump will still pump - if not leave it just on and feel the torque on the rotor. You should be able to stop it with one finger but it should be quite hard.

 

[alban]

Chris,
Prised pump open but the copper coil that drives the rotor comes away too with the housing, so the rotor doesn't move when pump switched on. I can move the rotor so it's free to move. I can't seem to get a look at the business-end of the rotor which impells the water directly either as I can't seem to retract it. I note that the pump seems to get very hot without heated water going through it AND it seems wet at the coil end (even with isolation valves turned off there's a drip through to that end). Is this right - water and elec together?!!

Later!! - When I put it back together and switched the power on the vibration that normally comes on didn't and it got very hot indeed with burning smell coming out plus steam. I switched off. Have I boobooed? I don't know how I could have kept the coil dry tho as the 'leak' goes to that end of things. Does all this sound like a knackered pump? Trouble is I don't know if it was the original problem or one I freshly caused!!!
Aaarrgh! One thing leads to another.

 

[fireman22]

i have same boiler, if you set boiler thermostat to your normal setting eg 3? set prog to HW if tank of hot water heats up to normal temp but takes longer ( as its now working as a gravity system, i`m assuming its fully pumped ) its telling you its the pump. One or two rads may warm up on CH setting again suggesting its your pump.

 

[ChrisR]

Pumps (cheap ones mostly!) can come apart like that, but with a bit of persuasion the internal bits will come away so you can get to the rotor. I reckon you do need to see if it's blocked. Also if you reassemble the business bits of the pump it should be possible to check it goes.

They do get hot, especially with no water going through them. What make pump is it and how old - might be time for a new one anyway.

If the water wouldn't come out with the pump off, it must be pretty blocked. Sometimes the water does run clear, the deposits sit where they are until you get some high speed water (poss mains) or some active chemistry going.

 

[alban]

Have replaced pump! No change with boiler performance however.
What should I go for next?
I tried system with mot. zone valves both manually open but no difference either. Boiler shuts off prematurely before heating has cranked up enough and front gets too hot to touch.
Boiler doesn't make a lot of noise, so is scale likely to be a problem?
Thanks

 

[Zdz]

After 29 years I am going to replace my Netaheat 16-22. The main burner won't light despite the pilot igniting and I am assuming it is the mercury vapour switch, but everything else should be good if old.
If anyone is interested in having it please call me on 07768 832 844.

 

[ChrisR]

Eeeeh, another 29 years in it yet. You can still get the mercury vapour switch.

 

[speedysteve]

Perhaps you have fixed it by now but, it could be the overheat thermostat playing up - i.e. tripping much too early.
They are easy to replace and about £25. The genuine replacement I got was not calibrated right so it was similar to yours - tripping too early. I calibrated it in some hot water so that it responded the same as the old one.
I replaced my thermo due to excessive radio interferance on bolier shut down.
I wrote a thread on here about my experiences that you could search for.