Noise on HW only - open vented

[Jimmy5]

I've just fitted a new zone valve as the old one was not closing so I always had CH if just wanted HW. The problem is now that if I put HW on only I get HW only but there are noises from the pipes / pump that dissappear as soon as the flow starts on CH. The noises are initial banging if starting on HW from cold and a general low level rumbling. I notice that the feed to coil is hot and return from coil is cold so you would expext the boiler (baxi solo) to be fired up but it's mostly not when HW only is on. The water is taking a very long time (3-4 hrs to heat up). The feed / return to the coil is 15mm. The other thing I notice is that the 22mm flow pipe leaving the boiler almost immediately joins the 22mm vent to f&e ciistern then the pump. I read that the feed from the f&e cistern should be within 150mm of the vent to avoid problems and it's not it joins the return from the HW coil as it exits the cylinder, which in distance is about 10 metres. I read that if in this position it would cause a negative pressure and a sucking effect as it is behind the pump. I'm not sure if I need a flush or should move the cistern feed to next to the vent? Any ideas. I ran x400 for a week and now have x100 and x200 in the system and have bled it. The coil is not scaled on the outside as I looked through the immersion hole with a torch, very little scale. I did have a short spell of pumping over, but before the last draining. Not sure if it's because the thermostat on the boiler is on 3 instead of 4 or the chemicals helped but it's stopped.

[Set Square]

Which zone valve did you replace - HW or CH?

It sounds as if you've got a restriction in the HW circuit which is allowing only a small amount of flow - hence long heating times and boiler cycling when on HW only. Is there a gate valve anywhere to balance the HW vs CH flow? If so, open it fully and see whether that makes any difference.

[Ed Sirett]

Check that there is water in the circuit - do the topmost radiators give out water or air or suck in? If there is a HW zone valve: My strong suspicion is that the the HW zone valve is broken and it just letting enough through to eventually work. If there is no HW zone valve then there is either no water or there is some blockage restriction in the HW part of the primary circuit.

[Jimmy5]

Thanks for your comments: To my knowledge there is no gate valve and there is definately only one zone valve, the one I replaced, it's a 2 port and it just opens the flow to the rads when energised. The HW flow is T'd off before it. I did cut a section of pipe earlier in the year in the return from the cylinder coil and it was a bit restricted. When filling the system you get water out the bleed valve that sticks up about 6 inches above the coil feed so it is filling OK. If you think that moving the feed nearer the vent is a non starter, maybe I could try and replace the feed / return pipes to the coil in case they are blocked. Seems a bit expensive to have a full flush when the CH is piping hot on all rads.

[Ed Sirett]

It's probably time for an overhaul which will upgrade the HW coil to fully pumped. There maybe a lot of crap in the gravity circuit which is making the reheat times so poor.

[Jimmy5]

I'm no plumber but as far as I can see, it is fully pumped. There's one retun and one feed on the boiler. The feed comes out, the vent t's off to the roof, then the pump then a t off to HW coil then a zone valve that opens for CH feed. Then the CH & HW return together. Is this system a bit weird? Doesn't gravity fed HW have a seperate feed & return with 4 ports on the boiler?

[Ed Sirett]

Ok so there's no xone valve on the HW in which case there must be a blockage in the HW primary circuit - or just as likely there is a gate valve for balancing the HW circuit and the valved is nearly closed. Quite possibly that valve is defective and can't be opened even though it's knob moves. Such a valve may also have attracted a lot of crud causing a near blockage.

[Set Square]

This system is more than a *bit* wierd, if it's as you describe it! What other controls are there, and how are they wired up? Is there a cylinder stat? Is there a room stat? What actually switches the boiler and pump on - and what switches them off again when the HW and CH demands are satisfied?

[Jimmy5]

When i started looking at it there was no cylinder stat and 1 room stat, but the room stat only energised / de-energised the valve to switch on/off the flow of water to the CH. Also the timer always gave a HW output if CH was requested. It was not exactly controllable! I have added a new timer, a cylinder stat and wired in the microswitch on the zone valve so the room stat actually turns the boiler / pump off if HW is not being called for and just the flow to the rads if HW is in demand. The cylinder stat now turns off the boiler / pump if HW and CH are not required. CH always gives HW. As it's now wired it does everything I want except for this final flow issue. I think I'm going to take up the bedroom floor, cut out and replace the pipes to the Cylinder coil and it should be the end of it. I can't find a gate valve for balancing the system.

[Jimmy5]

Fixed ! 2 inch block in return from coil