Ask The Trades - Print Page

Ask The Trades
https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Plumbing Questions >> Baxi Combi boiler: no hot water https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1518020439 Message started by Phil the Fluter on Feb 7^th, 2018, 4:20pm

Title: Baxi Combi boiler: no hot water
Post by Phil the Fluter on Feb 7^th, 2018, 4:20pm

I have a Baxi Duo Tec 28 E combi boiler and it has an intermittent fault that has baffled a number of Baxi engineers.

It was installed new in August 2015 and after a few months in January, it developed a fault. I turned on a hot tap one day and the water ran cold (as it normally does until the hot water arrives at the tap from the boiler) but, on this occasion, the cold water did not become hot. It became tepid for a few seconds, then ran cold again for a few seconds, then tepid for a few seconds, then cold, then tepid, etc, etc.

The boiler had a two-year warranty so I called out an engineer but, within a very short time of the problem showing itself (hours or less) the boiler was working perfectly again and was still working perfectly when the engineer arrived. He had no idea what the problem was and so was unable to fix it.

Some months later, exactly the same fault recurred and disappeared again before the engineer arrived. He too had no idea what the problem was and so was unable to fix it.

Some months later still, exactly the same fault recurred and disappeared again before the engineer arrived. This engineer, however, immediately suspected the diverter valve and told me that the valve cartridge was probably plastic and needed to be replaced by a solid metal version. He carried out the repair.

The boiler worked perfectly for several months but, a few days ago, exactly the same fault recurred and disappeared again the same day. The boiler is now out of warranty.

If anybody has any ideas, I would be most grateful for their advice.

Title: Re: Baxi Combi boiler: no hot water
Post by thescruff on Feb 8^th, 2018, 12:00pm

Duo Tec boilers installed before Dec 2016 came with a 7-year Warranty, later than that 10-years subject to conditions.

Is the system clean.

What type of taps do you have and how old are they.

Title: Re: Baxi Combi boiler: no hot water
Post by Phil the Fluter on Feb 8^th, 2018, 12:39pm

Thanks for the reply.

My boiler was installed under some sort of special scheme for older people and came with a ONE year warranty which was extendable to two years if I phoned up and asked for the extension (which I did).

One of the Baxi engineers that turned up to try to fix the problem told me that, when these boilers are installed in a new property, the warranty is seven years. It looks like I got a good deal on the boiler (it was free) but quite a poor deal on the warranty.

The guys that installed the boiler remarked how clean the water in the existing system was.

I have a mixer tap in the kitchen (about two years old), normal taps everywhere else (about 10 years old), and a shower (about 10 years old).

The intermittent "cold – tepid – cold – tepid – cold – tepid" problem, when it happens, affects all the hot water outlets.

Title: Re: Baxi Combi boiler: no hot water
Post by thescruff on Feb 9^th, 2018, 1:08am

The cold tepid cycle is usually attributed to the Plate heat ex or Diverter valve, or an external problem with 1 or more outlets especially, the shower

Title: Re: Baxi Combi boiler: no hot water
Post by Phil the Fluter on Feb 9^th, 2018, 10:38am

Thanks for the reply.

Can you explain how the problem might be caused by the shower or one of the other outlets?

A bit of extra information: during the tepid-cold-tepid cycle, the "domestic hot water on" light and the "burner on" light remain illuminated, and I can hear the boiler roaring away even though there is very little heat in the water that is being produced at the tap/shower

Title: Re: Baxi Combi boiler: no hot water
Post by thescruff on Feb 9^th, 2018, 1:07pm

Some questions and maybe answer.

Any mixer or shower valve is likely to have backflow problems if it doesn't have a working check valve and or the outlets are originally designed for low pressure systems.

How high is the mains pressure?

Any Combination boiler will have a maximum flow rate, lets say average 11.5Ltrs/min & Δ=35° (your boiler). What this means is if the mains is say 5°C at 11.5 Ltrs/ min the water should be 40°C, a faster flow rate will reduce that significantly, as the water is passing through the boiler to fast to heat it.

Check the flow rate. Time how long it takes to fill a measured bucket.

Also try turning the flow at the outlets down to check what difference it makes.

Title: Re: Baxi Combi boiler: no hot water
Post by CWatters on Feb 10^th, 2018, 9:20am

Google suggests the plate heat exchanger is blocked or the diverter valve faulty.

Title: Re: Baxi Combi boiler: no hot water
Post by thescruff on Feb 10^th, 2018, 10:34am

CWatters wrote on Feb 10^th, 2018, 9:20am:

Google suggests the plate heat exchanger is blocked or the diverter valve faulty.

Agreed CW, but it could also be backflow or user error, too much flow through the boiler will cause the problem and I have to assume the engineers checked the diverter and HX.

Title: Re: Baxi Combi boiler: no hot water
Post by Phil the Fluter on Feb 11^th, 2018, 8:22pm

I don't know whether or not the kitchen mixer tap and the bathroom shower have check valves and, even if I could see the check valves, there is no way for me to tell whether or not they are working.

I have no idea what the water pressure is in the mains. All I can say is that a number of Baxi engineers have seen the mixer tap flowing in the sink and not one of them has suggested low-pressure.

Those engineers have also seen the kitchen mixer tap delivering hot water while fully open and none has suggested a low flow rate.

The fact is that none of those engineers was present while the boiler was operating other than in a perfectly efficient fashion. Every time the fault appeared and I phoned for an engineer, the fault disappeared long before that engineer arrived on the scene.

The only engineer that did any dismantling was the one that replaced the original plastic cartridge in the diverter valve.

Interestingly, the problem has occurred four times: twice in January 2016, once in January 2017 and again in January 2018. Each time, the problem appeared and disappeared within a matter of hours. Is it possible that January has anything to do with this problem?

Title: Re: Baxi Combi boiler: no hot water
Post by thescruff on Feb 11^th, 2018, 9:23pm

We are talking high pressure, not low pressure if the flow is too high the temperature will be reduced.

January could be interesting but not something one would rely on to fault find.

During the winter people don't use the same volume of water, eg Gardens, car washing etc. the pressure is likely to be at it's highest during that month. At the same time, the mains will be at it's coldest so the boiler would have to work harder to reach the designed temperature. Could be relevant or not.

Another thought is the heating would be working hard and the diverter valve could stick. Why just January is anyone's guess otherwise.