Lukewarm Water from Combi Boiler Solved!

Combi Boiler System Diagram

Lukewarm water from a combination boiler, also known as “combi” boiler, can be a real pain for most people. No more hot showers to soothe those tired aching bones after a hard day of labouring. I managed to fix this problem for Father Clarke whilst looking at his boiler and figured this solution might help others who are experiencing a similar boiler fault.

Most boilers have a primary heat exchanger that will heat the system water to extremely high temperatures. This high temperature water, or “plasma” as I call it, as I am a Star Trek fan, circulates throughout the system by a pump. As you can see, the plasma also passes through the Domestic Hot Water (DHW) heat exchanger.

The DHW heat exchanger is a component that heats the domestic cold water to provide a hot water supply that you usually enjoy at the taps. For this heat exchanger to work properly there are four other subsystems that should be operating properly, otherwise there will be no hot water from the hot water taps.

Obviously, the pump should be operating to circulate the hot plasma around the system. If the pump fails, then there will be no hot plasma going into the DHW heat exchanger and therefore the DHW heat exchanger will not be able to heat up the domestic cold water.

When the pump is operating, you can hear it make a whirring noise, including a vibration sensed through touch.

In addition, the diverter valve should be operating correctly, because it is responsible for diverting the hot plasma into the radiators for the central heating system. If the diverter valve is faulty then the hot plasma will flow into the radiators heating them up, instead of the DHW heat exchanger. It is easy to check this because all you have to do is to open the hot water taps, and if you find that, the radiators are getting warm then you have found the fault.

Obviously, the primary heat exchanger, which creates the hot plasma, needs to be operating correctly. If blocked by limescale, there will be no water flow through it.

The Heat Exchanger Principle

DHW Heat Exchanger Diagram

Then finally, the DHW heat exchanger itself should be operating correctly. The simplest way to check if it is operating correctly is to compare the temperature of the plasma inlet and outlet pipes. Both pipes should get very hot very quickly when the hot water taps are opened.

On a blocked DHW heat exchanger, you will find that its plasma inlet pipe will be extremely hot -- too hot to touch -- however, its plasma outlet pipe will be just slightly warm. This simple test proves that the hot plasma is entering the heat exchanger but not returning out because its path is blocked.

Heat exchangers consist of multiple plates, and if the front plates are warm, but the back plates are cold, then this component is very likely blocked.

The Cheap Solution

Before you run out and buy a new heat exchanger, sometimes a cheaper solution of flushing the system works, and it can save a fortune in repair bills.

Flushing the whole system a couple of times -- venting the dirty plasma -- should clear some of the iron oxide in the system. Then if you can afford it, get a good quality cleaner to inject into the system, and then follow the instructions on the cleaner manual.

Gently tapping the heat exchanger with a rubber mallet also works, because the vibrations dislodge the blockages in the capillaries, but this only works for a light blockage. Sometimes a combination of tapping and flushing alternately works and the heat exchanger starts to work again.

My Repairs

After a hard day’s work, I sat down with a soothing hot cup of Earl Grey and a couple of chocolate peanut bars, which I dipped into the tea. For us simple folk that is luxury. Suddenly there was a loud thumping sound from the front door; it was My Friend Father Clarke...

Over the years I have performed various miracles for him such as repairing the USB Memory Stick Error, repaired the PBM8-500 MKII church amplifier, not to mention countless other miracles for which I am supposed to get payment upstairs. However, I am still waiting. Those lousy NS & I Premium Bonds have not come through, and my debt is so obscenely large that my bank manager has a hit man out looking for me. God must have a strange sense of humour... Oh well, no good deed goes unpunished after all.

I was considering hiding but unfortunately, he had already started peering through the front windows and seen me.

“Hello! Father Clarke! It is surprising to see you. How are you?” I said sarcastically.

“Jesus, Mary, Joseph! The church boiler is broken and there is no hot water any more.” He says, forgetting the niceties as usual and straight to the point in a loud Brian Blessed voice. This was one of his nicer qualities, as I seem to remember seeing Brian in those early 1980s sci-fi films such as Space 1999.

“Well, if you want the boiler fixed you’re speaking to all the wrong people; I don’t think Joseph was Corgi qualified. If I remember The Bible correctly, he was most probably a carpenter. Have you got any broken chairs?” I say jokingly.

“I’ll show you broken chairs! “ He replies angrily.

“Honestly we don’t know what to do. You do not suppose you could have a look at it for me. It’ll take you just 5 minutes I know.” He pleads.

“I have absolutely no idea how your boiler works, and even if I did I am not qualified to fix it. I can find you an engineer from the Internet who will have a look at it though. There are plenty of people out there who can have a look at it for you.” I explained.

“No. It is no use. I have already had three people visit and they all say it is knackered and needs replacing. The only problem is that we bought it only 5 years ago and the church doesn’t have the budget for another boiler”

“Well, have they actually told you what was wrong with it? Which component has failed?” I ask.

“No. The first engineer changed something called the diverter valve but we still get lukewarm water. The second person came and changed the NTC resistor thing-a-me-bob, but still no hot water. I now have another person coming in from the Gas Company, and I was wondering if you could speak to him. He is supposed to be a specialist and I’m not technical enough to understand him.” He explained.

“Well, it shouldn't matter if he’s a specialist. He still puts his trousers on one leg at a time like the rest of us. He should be able to explain to you what is wrong. But OK lets go and see what he has to say” I explained as we walked towards the car.

The Diagnostic Expert

As we approached, the engineer was already there with a large briefcase. He was a Roger Moore look-alike wearing a magnificent three-piece suit, with neatly combed hair. He looked like a city banker, and he spoke in a perfect English accent.

“Nice to meet you, I shall have you know that one will have this boiler repaired within the day.” He says, in the most perfect Queens English I had ever heard.

“I have brought with me The Maxwell One Thousand Analyser. We have used this at Sandhogs College in Cambridge for our intensive training. It is state-of-the-art technology which has never been wrong.” He said, whilst Father Clarke just stared at him as if he had just landed from Mars.

“Oh Great” I thought to myself, another techno-monkey, who is about to dazzle me with some grade-school science. There must be a very busy technical college out there... I can tell already that this is going to be a huge waste of my time, but I figured I might as well hang around.

He opened up the suitcase and it was full of wires with crocodile clips, and probes, and a large computer. He then began to connect all the wires to various parts of the boiler whilst simultaneously referring to his manual.

After thirty minutes, he had all the wires connected and began a NASA style count down to press a large red button to switch ON the computer. As he pressed the button, the computer instantly surged into life, as dozens of lights began flashing, and beeping noises emanating from the machine. He then got another manual out and started to follow various diagnostic procedures, whilst simultaneously prompted by the computer.

After two hours, 4 cups of tea, 3 bathroom breaks, 12 business phone calls, 4 private phone calls, and a smoking break, he started shouting excitedly, “One has it! One has found the answer!”

Of course, we all gathered around him to find out exactly what he had found, as he quickly started to look through another manual.

“By Jove, I do believe I have got it. It is hash 1-8-2-8! Yes, I do believe it is that!” he spoke to himself excitedly.

“What is hash 1828?” I asked.

“Hash 1828 means that the domestic hot water temperature is below normal operating parameters.” He read form the manual, whilst raising one eyebrow like Roger Moor.

“So basically, after all this time, this computer told you that there is no hot water!” I say in a puzzled tone.

“Yes, but I do believe it is the motherboard that has failed. I shall have to return tomorrow with a replacement board. It shall cost a mere £150.00, not including VAT naturally.”

“Yes naturally...” I reply. Of course, I could tell that he did not have a clue.

The Solution

Three days later Father Clarke returned, and unfortunately changing the motherboard had not solved the problem. In addition, he lost his temper and fired the engineer who left without fitting any of the panels back on. Unfortunately, the engineer recommended to Father Clarke that he should buy a new boiler, which apparently sparked off the whole debacle.

Therefore, “muggings” here had to replace the three panels, which was simple as there were no screws to them and they simply hinged back on. However whilst I was there, I decided to look at the heat exchanger simply out of curiosity. The DHW heat exchanger was easily visible to the side.

I asked Father Clarke to turn on the hot water tap and leave it running. The inlet pipe to the heat exchanger became hot within seconds and it was so hot that I could not touch it for more than a fraction of a second. However, the outlet pipe on the heat exchanger was cool. Those pipes are the thicker ones so they are easy to identify.

Clearly, that did not seem right, because the hot plasma normally passes through the heat exchanger, so both the inlet and outlet pipes should get hot... Unless blocked by limescale... Of course it all made sense now. This was a 5-year-old boiler, in a hard water area, without any limescale reducing agents within the CH system. I seem to remember reading somewhere about boilers requiring replacement heat exchangers every 5 years in hard water areas.

I ordered a replacement DHW heat exchanger on eBay, as the sellers had brand new sealed kits consisting of the plate heat exchanger and connecting pipes for just £50.00, which was a real bargain.

I then phoned a local “Corgi engineer” to replace it for me. The labour cost was £60.00, and it was a two-hour job. Total cost to the church came to £110; however, my mother brought me up as a good Catholic boy so I decided not to ask the church for the money, and wrote it off as charity.

The last I heard, Father Clarke had turned a deeper shade of red due to the extremely hot water that his boiler was producing. He was over the moon enjoying the lovely hot water, and no doubt, he will be knocking on my door when something else breaks. As they say, “No good deed goes unpunished.”

This Article Continues...

Lukewarm Water from Combi Boiler Solved!
Lukewarm Water from Combi Boiler: Heat Exchanger
Lukewarm Water from Combi Boiler: The Diverter Valve