Thermal store - hot water quantity has reduced

[leckie12]

Hi

I was asked to look at the electrical installation of a heating system that packed up. No hot water or heating.

The system is comprised of a gas boiler, a thermal store cylinder (I hadn't seen this set up before), a pump, a 2 channel programmer, a room thermostat, a cylinder thermostat and a 3-port diverter valve with a spring return.

Ist thing I noticed was the programmer has a supply but no output to either channel, I replaced the programmer with the same model and it fired up. I started to look at the operation. The 3-port diverter valve is normally allowing water from the boiler into the HW cylinder, when the cylinder stat is satisfied the 3-port valve gets a supply and opens up the heating port (A) and shuts off the HW port (B). However, the cylinder thermostat was permanently closed so the heating didn't come on. I thought it a bit odd that the timer and the cylinder stat were both no good.

I replaced the cylinder stat, its a type that goes into a pocket on the side of the cylinder. Its a Honeywell Aquastat with no external controls and a temperature range of 70 - 140 C with a dial inside the cover (no over temperature cut-out). I fitted that and left it at the lowest setting as per the previous unit. It fired up, got to temperature and went off I turned up the room stat and the valve opened and the heating came on. It all seemed fine, so off I went.

Got a call the next morning saying that the everything was OK except the shower ran out of hot water after about a minute. It has previously worked fine before the breakdown. I thought I would turn up the cylinder stat a bit, so I did and the pump start sending the boiler water into the cylinder but it didn't last long and turned off. I thought it couldn't have increased the water temperature that quick? As an experiment I turned it up a bit more but the same again. Anyway I left it overnight, the occupant said it was not much better the next day. As an experiment I cut out some of the insulation on the cylinder (it the foam covered type) and fitted a strap on cylinder stat and this seemed to operate as you would expect. So now I am wondering if the probe type stat I fitted is not calibrated correctly, but that seems a bit of a coincidence. I have read that this type of stat is not considered suitable but I thought it would be ok for one night. I set it at 60 degrees C. They phoned the following day to say it was better but still didn't last long enough in the shower. I'm not sure why the surface cylinder stat is not considered suitable but I was thinking of setting it at 70 degrees C for tonight to check for an improvement?

Any thoughts would be much appreciated. I wasn't sure how or why the probe stat wasn't working properly but I suppose it could be just faulty, but it seems odd that the programmer went kaput, and the last cylinder stat and now this one seems to switch off early?

 

[oilhead]

If as you state, this is a thermal store, then how is the hot water obtained? On a thermal store, the primary water is heated and there is an internal heat exchanger that is fed through a mains supply, and combined with a mixing valve before delivery to the taps. There could be a fault in this area. Is there a separate heating circuit (Under floor circuit)? Is your 3 port valve just a diverter? If so, where does it divert to? Could it just be a by pass back to the boiler?

 

[Dean744PiN5]

i think a picture of the set up would be a great help in this situation if possible.

 

[leckie12]

Ah well, I did say I wasn't an expert on thermal store. The Plumber told me it was but he's just gone on holiday so I cant quiz him.
So, its just domestic HW, and radiators. the heat source is the gas boiler. The valve is just a diverter, the normal position is AB to B open (To cylinder) and when the motor is energised its AB to A open (to radiators). The programmer was definitely kaput and the original cylinder stat was definitely calling all the while as it didnty divert to the heating even if I turned the stat down. I' m not up enough on the plumbing side to explain the full details of the mixing valve but I will pop in a bit later and have another look and take down the cylinder details. It a tall thin foam covered cylinder with a header tank built in on the top I thought it was a fortic tank when I first looked but the plumber said it was thermal store.

 

[leckie12]

i think a picture of the set up would be a great help in this situation if possible.

I'll get a pic in a while and post it later on.

 

[leckie12]

 

[leckie12]

 

[leckie12]

Sorry, Im a new poster, but I have posted two pictures that might help.

No sure if this shows any blender? I can see something that looks like a mini expansion vessel (silver thing level with the motorised valve) and the brass fitting that is shown on the 1st pic that has three connections, but I don't know what it is.

I have put the cylinder stat up to 65 degrees to see what it does when they use the shower tomorrow. As this surface mounted stat is not recommended I didn't fancy putting it up to 70 degrees. Any idea why the surface cylinder stat is not recommended instead of a probe type stat?

 

[45yearsagasman]

That does look like a thermal store to me. The blending valve is in the top pic and has a brass cover. What temp is the boiler stat set at?it needs to be near to max.

 

[leckie12]

The boiler stat is set at max, no numbers, its not been changed.

I'm starting to think the replacement cylinder stat that went in the cylinder pocket is not calibrated properly. I have, as I said, fitted a temporary clamped on standard type cylinder stat on and its now set at 65 degrees (I think the recommended setting is more like 70 degrees but Im a bit worried about setting the temporary stat at that temperature as Im not sure why they are not recommended).

 

[picasso]

There is a blending valve on your thermal store, you could have a cylinder full of hot water but faulty blender will stop the hot water, do you have any way of checking how hot the water is coming out of the cylinder ?

 

[oilhead]

I'd be inclined to await your plumber's return. This certainly looks like a thermal store, and the hot water will be provided through an internal heat exchanger. If the store is up to temperature, and you have little hot water, I would suspect the blending valve, Which is the three port valve adjacent to your temporary thermostat. Normally the heating would be provided from the thermal store, and not direct from the boiler and I would think it probable that the 3 port valve is only a bypass for when the store is up to temperature.

 

[leckie12]

OK, I can see the blending valve. Picasso I'm not sure how to check the hot water coming out of the cylinder; how would you do that?

Oilhead, obviously I cannot see how the pipes are run, but once the cylinder stat is satisfied, the 3-port valve gets a supply via the Number 2 terminal on the cylinder stat and opens, letting the water through to the radiators. So it's hot water priority, when the how demand is satisfied it diverts to the heating system.

Don't know if this helps but the supply to the indirect coil HW also bring on two radiators (bathroom and en-suite). So I think the rest of the radiators are fed direct from the boiler - but maybe not! I am after all a humble electrician, not a heating engineer.

I will wait for the plumber to return from his holidays, but I am interested in what the possible causes are.

I think it would appear to be a faulty blender valve or the cylinder stat is not correctly calibrated. I think I will be getting a thermometer out and do a bit of measuring! Thanks for your replies, any more ideas are very welcome.

 

[leckie12]

Oh,by the way, would the two bathroom rads be a bypass for when the water gets to temperature? I'm not good enough on the plumbing side to know how they would come on with the hot water. They wouldn't be supplied via the blender would they? I bet that a really stupid question, but hey ho!

 

[holty]

looks like an old thermal store done by mcdonald engineering. could try the immersion to rule out external controls etc. and see if blending valve is a problem. immersions on these are usually set higher than the average cylinder immersion.