Conventional boiler problem

I have a gas boiler and a pretty normal cylinder upstairs in the hotpress. There is a weird problem in that when I fire it up, the CH heats up great, but there is no hot water - the cylinder is tepid even after an hour of running the boiler. Boiler appears to work normally - heats all the radiators and then maintains the temperature.

There's a thermostatic valve on the hot water pipe going into the cylinder coil, at the centre of the cylinder, and a regular valve coming out at the bottom. Hot water is definitely passing through the input valve as I can feel it piping hot on both sides of the valve, but the cylinder itself is cold immediately next to where the pipe enters.

The boiler itself is very simple, one pipe goes in and another comes out. There is no motorized valve, there is only one heating circuit that does both CH and HW.

Is this kind of problem common or rare ? Are there any tricks that can be tried before the plumber drains the whole system to try to figure out what the problem is ?

Regards

GWC

Reply to
Geronimo W. Christ Esq
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This is a "gravity" system where the water for the cylinder coil circulates naturally by convection as the boiler heats it - i.e. the pump is not involved.

Generally, these systems heat he DHW quite slowly anyway in comparison to a pumped system. Even so, it should be better than it is.

The valve that you describe is probably for balancing the heat delivered to the cylinder vs. the CH. Is it fully open?

Otherwise, I would suspect that the coil inside the cylinder is heavily scaled. Are you in a hard water area?

Another telltale would be that when the boiler is running DHW only, does it cycle on and off a lot? This would indicate poor transfer of heat through the cylinder coil. Is the other pipe from the cylinder back to the boiler also hot?

If this all fits together, then the solution will be to change the cylinder.

While this is being done, it would make very good sense to use either a part L1 (this is pretty much the standard nowadays) or a fast recovery cylinder and to convert to a fully pumped arrangement with diverter valve.

Reply to
Andy Hall

Andy, thanks for that very comprehensive reply.

I am not sure that this is the case. The cylinder coil seems to be connected to the same circuit as the radiators are, and there is of course a pump there which appears to be working normally.

I'd expect the cylinder to be full of hot water after an hour or so. Even if I run it all morning it's still cold. It takes about half an hour for the gas boiler to finish heating the water in the system.

I think you're right. Yes it's fully open.

This is the interesting thing. The house is only four years old and has been occupied by the previous owner for two of those (I have just bought it) so it would be really surprising if the coil was gunked up. Also, the water is soft - according to a Calgon water testing stick I had to hand when I checked a few days ago.

The pipe from the cylinder back to the boiler is cold, so this does indeed suggest that the water is not passing through the cylinder coil.

Thanks, I will keep your comments in mind. The cylinder that is there seems basic, ie it is not one of the ones covered with spray-on insulation - there's just a frankly slightly crappy looking jacket - I was surprised that such a fairly new house would have the less efficient cylinder, but there you go.

Reply to
Geronimo W. Christ Esq

OK, so are you saying that there are only two water connections at the boiler? A "gravity" system has four, with two generally being 28mm (go to cylinder coil) and two being 22mm for the CH. In that scenario, you wouldn't have a motorised diverter valve (3 ports) but might have a zone valve (2 ports) for what is called Honeywell C plan.

If by "connected to the same circuit" you mean that the water is common between the CH circuit and the cylinder coil and fed from a small header tank in the loft, then yes that is normal.

If there really aren't any motorised valves, then it implies a "gravity" system unless there is something very strange.

The test is whether there are two or four water carrying connections at the boiler. What is the boiler, BTW?

That seems reasonable.

It would be surprising in that time, but possible in a very hard water area.

Sounds like it is because connection into cylinder is hot, but very slowly. I'm wondering about that valve near the cylinder.

Are you absolutely *sure* that there is no motorised diverter valve. The symptoms would happen if you had one and it was stuck in some way.

Can you do a sketch of the system in ASCII art and post it or some photos and put them on a web site somewhere? Specifically around the cylinder would be interesting.

Reply to
Andy Hall

Thanks again, Andy. I'm finding this an instructive conversation.

I think it's a strange one :) There are only two pipes coming out of the gas boiler (which is a basic Baxi appliance - just a white box with a thermostat control and some LEDs showing that it is operating).

I do not think there is a motorized valve. I can't see one, and I imagine if there was one it would require an electrical supply - the only wired devices that I can see are the pump and the immersion heater.

It is possible that the valve is stuck, yes. I'm as sure as can be that there is no motorized valve. There is nothing to control it. The timer clock on the boiler is very simple, just a rotary dial with little "notches" all the way round where you can set the ~15min periods you want it on or off.

Let's try the ASCII art. I will keep it simple. The gas boiler is downstairs in the kitchen, in a cupboard, and the cylinder is of course upstairs. The pipes reach up into the bathroom, and under the bathroom floor to the cylinder. I'm not in the house at the moment so will have to get photographs if more detail is required.

"TRV" is the thermostatic valve going into the boiler (there is a sensitive strip running down the centre of the boiler which is connected by a piece of thin wire to the TRV). "V" is just a regular tap that you turn on and off. It is currently turned up full. The water is cold on the pipes either side of V, but piping hot on the pipes either side of the TRV.

Cylinder

+------- (up to roofspace) oo o o

--TRV---o o Hot press o o

-----V--oooo ========================= | | | | two pipes going up | | ooo o o o o ooo boiler | | gas pipe |

Reply to
Geronimo W. Christ Esq

I am - London - and my cylinder is pushing 30 years old. Still works fine, though.

Reply to
Dave Plowman (News)

With only two pipes this would say to me you couldn't have hot water only in the summer - unless there are motorized valves somewhere.

Can you trace the pump wiring back to a junction box? And see if there are 'rogue' wires going off to a three port valve somewhere? It's just that I can't imagine a 'gravity' hot water system being fitted this recently - they were obsolete many years ago.

Reply to
Dave Plowman (News)

It may not be near the cylinder. Can you find the point where the CH part of the circuit splits from the feed to the coil?

Could be under a floor?

This "TRV" is an interesting one - especially as it connects directly to the boiler. Can you describe in more detail - shape, make? It sounds as though the boiler controls it is some way, but without more detail of it and the actual model of boiler, hard to say. Photos of the cylinder area would be a help.

Do you know where the pump is? Inside the boiler?

Reply to
Andy Hall

That is correct, the system cannot do hot water only. However in the summer you can just go round and knock off all the radiators - not as efficient obviously.

If there is a motorized valve then I don't understand how it is being controlled, as the time clock does not have any switch or device that allows for independent heating of CH or HW.

The pump is connected to a fused switch on the wall which allows you to turn it on or off. There are no other wires that I can see.

It's possible that it was done on the cheap ? House is a terrace and was built 2000/01.

Reply to
Geronimo W. Christ Esq

Yes, that is very near to the cylinder (there's an array of pipework there). No valve or wiring is in evidence other than the pump and the immersion heater.

Possible, but I'd have expected to see wiring. And a motorized valve would need something to tell it to open or shut shurely ? - no switch or clock to do this is in evidence.

I'm a bit worried about the system I have, going by the reactions of yourself and Dave it seems to be a bit non-standard. I'm in Northern Ireland so I wonder how the usual regulations and quality standards line up compared with the rest of the UK.

The valve looks just like the type of thing you'd see on a radiator :

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is inline like the one in the picture. The difference is the piece of wire that obviously conducts heat from the cylinder so that it knows what the temperature of the cylinder is. It is also marked with temperatures, eg 40, 60 etc, and then fully open.

My limited understanding of plumbing suggests that the cylinder is connected up as if the coil were another radiator, so the entire system basically does not know or care whether it is doing HW, CH or both. The water is pumped around through everything without regard to any of these factors.

I've perhaps misled you by making the diagram too simple, a lot of the extra pipework was purposefully left out of my drawing. I will acquire photographs and post them up somewhere for you to see.

Yup I'll organize those.

No, it's on one of the pipes outside. Looks like this :

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the system is activated from the timeclock, the pump operates continuously (a soft whine is audible, and sounds healthy enough) and it gets very warm as the water passes through.

Greatly appreciate your help so far.

Reply to
Geronimo W. Christ Esq

There are different Building Regulations in Northern Ireland, AFAIK, but I don't think hugely different.

Do you mean the cylinder (not the boiler) as the place where this wire goes?

If so, it would now make complete (and horrible) sense.

Basically the "wire" would be a capillary sensor for the TRV. These are sometimes used on radiator versions where it is required to sense the temperature remotely.

If I'm right, and I have a horrible feeling that I am, then the sensor is on the cylinder and would be used to turn off this "TRV" which is being used to control the flow to the coil.

In essence, the coil is being treated as though it's another radiator.

Certainly there was some penny pinching with this job.

For this application, the "TRV" is probably not a room type because the temperatures involved are very different, obviously.

The valve going to the cylinder is to balance the flow to the coil against the radiators.

Check one more time that this really is open fully. Check whether the return pipe out of the cylinder gets hot when the radiators are all turned off and the pump and boiler are running. You could also turn up the pump setting.

If none of that works, then I think the culprit has to be the "TRV" - simply allowing a very slow flow which is enough to keep the pipe hot but not deliver anything notable to the cylinder.

The question then is what to do. You could try replacing the "TRV", bearing in mind it is probably a special one.

Frankly though, you would be far better off fixing the problem properly and installing a diverter valve, room and cylinder thermostats.

If you are OK with a bit of plumbing and electrical work then this is certainly in DIY scope. Post back if so and I can give you some links. Note that there are wireless thermostats available for both room and cylinder if you are concerned about the impact of running wiring.

Yep. I think the penny dropped.

Reply to
Andy Hall

It almost sounds as if someone has plumbed the whole system without

*any* DHW circuit at all. Then just strung the HW cylinder between flow and return pipes on the heating circuit as if it were a giant radiator. The TRV being used to shut off flow to the cylinder when it is up to temp (although temp of what I am not sure - in this case it could be the temp of the cupboard!).

You can see that it could work (sort of) if the system was balanced such that the cylinder got the lions share of the flow until it closes its TRV. That would give similar behaviour to a three port diversion valve based system with DHW priority. Needless to say HW reheat will be slow in the winter since you can't remove the radiator load and devote the boiler to the HW cylinder, and a right PITA in the summer since you won't be able to turn the heating off unless you turn off each rad or rely on TRVs on each of them to keep them off due to high ambient temp).

Reply to
John Rumm

See my other post, Dave.

I have a feeling that this is some kind of a "TRV" controlling primary water through the coil.

Reply to
Andy Hall

Similar to this .Ravi Thermostatic Cylinder Control

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Shift THELEVER to reply.

Reply to
Stuart

That's exactly the one - not sure if mine is Danfoss but it's that type of thing.

Reply to
Geronimo W. Christ Esq

Reply to
Andy Hall

d'oh sorry. Yes it is the cylinder. stupid posting error on my part.

It sounds like you're on the money. You've confirmed my suspicions then that this is an El Cheapo setup. That's one thing at least ;)

I have tried knocking off all the radiators and running it - this did not help. The pump is running at it's lowest setting, but I'm a bit worried about what problems could occur if I turn it up - I guess it's been left that way for a reason.

Going by what you are saying, this should just be a matter of rejigging the plumbing in the hotpress to add the valve and using wireless thermostats to control it. That would be ideal as I don't want to rip out my kitchen units and tiles in order to add a wire and replace the timeclock. However I'm not sure how well that

For now I will probably get someone in to patch up the problem and then look into making the system work a bit better during the summer.

Thanks again for all your help.

Reply to
Geronimo W. Christ Esq

The reaction to this has been interesting. I think all the houses in this street were done this way when they were built in 2000/01. It looks like it was done in a planned/deliberate way, ie not bodged by someone who was incompetent. I guess it is just the cheapest possible way to deal with the problem :)

The radiators all have TRVs so I assume the system is intended to be used in this way. It's straightforward enough to go around and switch off each radiator in the summer all the same but I agree that it's nicer not to have to.

Reply to
Geronimo W. Christ Esq

From reading the other posts in this thread, I'd say that you have a conventional boiler with a very *unconventional* hot water system!

It seems to have been established that the coil in the HW cylinder is connected in parallel with the CH radiators and is, in effect, just another radiator.

Has it ever worked properly since you've been in the house? The thought occurs to me that the TRV on the cylinder may be one intended for a radiator, and having a remote sensor which is supposed to sense room temperature. If so, even with the valve on max, it will close when the sensed temperature reaches the upper 20's degC. So if it is strapped to the cylinder, it will close when the HW gets to 25-30 degrees - which is no use to man or beast!

To test this theory, either pull the sensor away from the cylinder so that it's just sensing air temperature, or remove the head from the TRV. If the hot water then gets hot, you'll know that the TRV is the problem.

One thing I don't understand: You said that the pipe going back to the boiler from the cylinder was cold. But from your description of the system, this pipe is shared by the central heating system (presumably connected under the floor somewhere) - so I would expect the return pipe to be hot - certainly at the boiler end - whenever the radiators are hot, regardless of what's happening to the cylinder. Am I missing something?

Reply to
Set Square

I've seen a few of these setups - horrible. Its not the case that the TRV pin has stuck? (Can this happen on a remote sensor type TRV)? Unscrew the head and you should be able to gently push the protruding pin in and out with something hard (e.g. side of a spanner). If it goes in & out, it should be ok, if it doesn't move, then it is stuck. Now, I was trained to simply knock seven shades of poopoo out of the TRV (not directly onto the pin) to unstick the pin!

ALSO, you should never turn off ALL your TRVs unless you have a bypass built in (or there should be a rad without a TRV - building regs now stipulate that the rad in the same room as the roomstat shouldn't have a TRV, failing that it is traditionally the Bathroom rad without one).

Angus

Reply to
Fentoozler

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