Hot water ice cold and expansion tank overflowing

Hi,
I need your help.
I've got a conventional oil fired boiler. The central heating is working fine, but I have two problems, which I don't know -incidentally- whether they are related or not:
a. I have no hot water at all. The flow is fine, but the water is cold -rarely, if I have kept the central heating on for a long while, it might come off lukewarm if at all. b. The expansion tank overflows every now and then.
Sincerely,
Josi
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Josi wrote:

We need a bit more information...

How is the hot water heated: Convected or "fully pumped"?
(i.e. the boiler will have a flow and return pipe for the central heating, but does it also have an additional two separate pipes for the hot water circuit? (if just the former it is a fully pumped setup, and if the latter it is convected of what commonly known as a "gravity" circulation)

Again hard to say for sure, but I would guess you have a fully pumped system and a valve that would normally direct water through the heating coild in the hot water cylinder has failed in some way.
The pictures here may help you identify how your system works:
http://content.honeywell.com/uk/homes/systems.htm

Does it ever refil from its ball valve, or does it seem to just acquire water anyway?
--
Cheers,

John.

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Thanks John,
I've checked according to your description and the pictures, and you're right: my system is a fully-pumped one. So, it seems that there's a problem with the valve directing water through the cylinder -at least this was your guess. Now, is there anything I can do or I need to call a plumber to change / overhaul it?
With regards to the expansion tank, how can I check whether it refills from the ball valve? Simply by looking where water comes into it from?
Cheers,
Josi
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Josi wrote:

There is a fair bit you can do with it - depends a bit on how brave you are feeling!
Chances are you have a three port valve - i.e. there are three pipes connecting to it. There are two common types of these - a diversion valve and a mid position valve.
The diversion one is the simplest - it can be in either one position or the other. Typically it is arranged such that when the cylinder stat is calling for heat (and possibly HW is enabled on a programmer if you have one) the valve switches and directs all the flow from the boiler through the heating coil in the HW cylinder. Once the cylinder is hot enough it reverts to the heating.
The mid position valve is similar except it also has a middle position that allows water to flow to both the CH and the HW at the same time (this happens when both room and cylinder stats are calling for heat).
So that gives you four typical places for it to go wrong (in rough order of most to least likely):
1) The motor that drives the valve can fail
2) The valve body itself can seize - this is not usually that common unless there is lots of air getting into the system since this causes corrosion (see below on this).
3) The cylinder stat can fail such that it never calls for heat
4) The programmer / wiring may have a fault
1 and 2 can be investigated without too much difficulty. The motors and valve bodies are separate and you can replace one without changing the other.
Have a look and see if you can find the valve. Look to see if it has any switches on it. The mid position ones often have an override switch that can force it to the mid position even if the motor has failed. That will at least give you heating and hot water (but with less efficiency and control than normal).
It should be possible to remove the motor unit from the valve body. This exposes a shaft that sticks out of the valve that is normally driven by the motor. You should find you can rotate this either by hand or with light use of a pair of pliers. If it is seized solid the the valve body needs replacing. If it is very stiff you may find that working back and forth a few times by hand is enough to free it, again if it remains stiff then replace it. If it moves freely then it is probably fine.
Next check the motor. With the system running you should be able to power up the valve motor simply turning the cylinder stat down to the point where it calls for heat. If you can not get any motion from the motor then that suggests either 2, 3, or 4.
To diagnose these you need a multimeter and be happy to probe about in the electrics... let us know if you want more information and we can post more on the next steps. It would also be worth using the google "groups" search on this group since quite detailed diagnostic procedures on just this sort of problem have been posted in the recent past.

It should fill from a valve on one side rather like the main water tank or even a loo cistern.
When the system overflows you loose water from the heating circuit. This will get replaced by some means. There are a couple of possibilities here: It ought to come from the header tank, but there is a failure mode where you get a hole in the heating coil in the cylinder that allows the water in the cylinder to flow into the heating circuit (something that usually it can not do). Either of these are bad news since it means fresh water is getting into the system and that will contain a certain amount of dissolved oxygen. And this can lead to corrosion (pinhole leaks in rads, cylinders, and stuck valves etc.
The important question is why does it overflow. It could be because of a fault in the cylinder as described above. It could simply be that the water level in the tank is set to high (by the position of the ballcock) so that when the water in the system heats up and expands it pushes some out of the overflow in the tank.
--
Cheers,

John.

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