My Baxi conbo Boiler is used for central heating and hot water.The heating comes omwhenever we want it to.The hot water either comes on or it does not. Sometimes hot water from the tap will run cold and not heat up at all.An engineer has replaced the heat exchanger and that improved things for s couple of weeks. But it has gone back to being unreliable.The engineer now says that there is a faulty hot water tap in the house causing the problem and I need to get a plumber to look at the taps.Being hard up I don't really fancy that.Has anyone any ideas?

The engineer now says that there is a faulty hot water tap in the house
That cannot be correct.

You could have a faulty balancing valve in a shower but no where else.

If you just had a faulty balancing valve in a shower you would still have hot water at other faucets.

I don't know much about combination boiler/water systems but you have a mixing valve on the unit. This valve mixes the very hot water in the boiler with cold to provide domestic hot water to the faucets. Obviously that valve is periodically failing.

You need someone else to examine the system and replace that valve.

Visit this web site and try to locate manuals for your model Baxi combo boiler.
http://www.baxiboilermanuals.co.uk/


At this link you will see Page 40 of the manual for a Baxi-Combi-Instant-80e

You will see that there is a sensor for heat and a sensor for domestic hot water. Perhaps your Baxi combo is similar.

http://www.manualslib.com/manual/1020354/Baxi-Combi-Instant-80e.html?page=40#manual

Thank you very much for your reply and the links you have supplied.I will definitely be looking at them today.You say "That cannot be correct.".And that is what I thought. How can a faulty tap stop hot water getting through every time.But I can see that as a possibility now because a plumber was at my house this morning to deal with a leak from my microbore central heating system which was installed when the builders threw my house up 25 years ago.I have the system insured under a maintenance plan with D+G. Anyway he told me that mixer taps can develop a problem where the cold water supply to a tap can go down the hot water supply pipe and thus feed the system with cold water.He told me that he always starts with a shower when he deals with the problem.So that is what I am going to do this pm.But I am also going to look for the manuals on the sites you listed.Thank you once again for being so kind as to give me the info you uhave done.Very best wishesJohn

Hi John,

First,you are very welcome.

Next my prospective is from pressurized water system, usually at a pressure of between 30 and 60 lbs. I don't think a gravity flow system exist in the US. I am interrupting your term "mixer tap" as a faucet or spigot that has a hot and cold water supply which can be manually adjusted to provide cold, warm or hot water as the user desires. Some of our shower valves have what is called a pressure balancing valve. This is an internal, self adjusting valve. The water pressure on the cold and hot lines is always equal. The water volume may or may not be equal but the pressure is always equal.
The pressure balancing valves are only used in shower or tub/shower valves. Its purpose is to prevent scalding. Let's say that your are in a multi-family dwelling or that the cold water piping to a bathroom is of an inadequate size. Let's also say you are in the shower and some one flushes a toilet or opens a cold water tap, causing a sudden reduction of cold water pressure. The pressure balancing valve automatically reduces the hot water volume, there by keeping the mix and temperature equal.

In a "standard" hot water system, cold water is fed to a tank where it is heated by some means. The cold water input pressure is what pushes the hot water from the tank to your faucet or mixer tap when the hot water side is opened. Therefore the pressure is always equal. If you had an obstruction in the hot water side, cold water could pass from the cold side into the hot side piping but only up to the obstruction. This could only happen if both sides of the more than one tap were open.

Let us say, for the sake of argument, that you have a 3/4" line from the water heater to the kitchen where it branches of to the kitchen tap with a 1/2" line and continues to the bath with another 1/2" line. Let's say that you have an obstruction in the hot line before it reaches the kitchen. If you open the hot side of the kitchen or the bath tap you would simply not have any hot water. If you open both the hot and cold of both taps you could theoretically get cold water passing through one and coming out the other.
I say theoretically because first, obstructions don't occur in pipes. Obstructions only occur at valves. Second if you did somehow have an obstructed hot water pipe, the pressure and volume of the cold side would have to be more than could come out the spout in order for the cold water to move into the hot side piping. The cold water could only move into the hot water piping only if one tap is open both hot and cold and another tap hot only.

In your post you implied that the lack of hot water was sporadic, intermittent and unpredictable. Obstructions or blockages in pipes don't come and go. If a pipe is blocked, it stays blocked until you physically remove the blockage.

In the US we have stop valves on both the hot and cold lines at all or almost all faucets or taps and toilets. These give you the ability to service or remove the taps or toilets with out turning off the water to the entire home. Assuming you also have these stop valves you can test my logic.

The next time you hot water goes out. You simply close the cold water stop valve at all taps. That will insure that the water coming out a tap on the hot side is indeed coming from the water heater. It cannot be coming from the cold side because you have blocked all entry points.

If you remove the cover of your water heater you should be able to locate the domestic hot water sensor and mixing valve. Even if it is a different model than the one shown. I don't know how you would determine with certainty which is faulty. Because it is intermittent I am incline to believe it is the sensor. Since this is low voltage you might try cleaning all wiring contacts.

The mixing valve itself could be at fault if there is a build up of mineral deposits. You could try tapping on the mixing valve if you are incline to believe that the mixing is stuck. Might help, might not.

All this is of course based on the assumption that you have pressurized system. If you have a gravity flow system it is out of my league.

I find your problem quite interesting. Please post back with what you find as you resolve this problem.