No hot water? – blocked Truma Trumatic C3400

A website visitor (Nils Söderberg) was recently reading our article on Truma Trumatic C3402 troubleshooting and posted about an issue he was having with his Trumatic C3400 in a 1996 Hymer B544

He had no flow of water from the system, but the water pump etc. was working OK. After a bit of troubleshooting he found the problem to be a blocked water vessel in the boiler itself – presumably from limescale/sediment build up over the years.

Here is his original comment from when he diagnosed the problem:-

It was the actual truma boiler that was blocket. Somewhere between the frost guard and the boiler.  That was why I could not get any water through the hot water tap. I disconnected the frost guard and the pipe between the frost guard and the boler I could se a sluggish paste in the pipe and since no water came flowing back i suspected that there were something blocking the inlett to the boiler.  I bought a pipe cleaner for kitchen sinks and pulled it back and forth until the water and bits of gray paste started flowing back from the boilers inlett. I connected the pipe back to the valve and drained the boiler from water. Now I have filled the boiler and freshwater tank with a mixture of citric acid and hot water (500gr pr – 50l ). I have flushed it trough the system and are going to leave it in in the boiler overnight. Tomorrow i will use the valve to empty the boiler the bottom so the deposits of scale will hopefully pour out that way and not clog upp something ells. And if more people have the same problem I hope that this description could help them as much as your tear down helped me.

If you are experiencing the same symptoms it is worth following the approach above. Even if you are having no problems it’s a good idea to descale the boiler annually, to help prevent issues like this from occurring.

Many thanks to Nils for the information!


  1. Will citric acid do any harm to the boiler or pipes?

    • Hi Tony – are you planning to you citric acid to remove limescale in the system? – I can’t see any reason it would damage either the stainless steel tank in the boiler or the plastic water pipes, but I guess for safety it would be wise to use the weakest solution you think will still be effective. Let us know how you get on if you try it!. Cheers, Dan

  2. Yes thanks Dan I will use a week solution. just wanted to make sure it won’t damage the heater or pipes.

  3. Hi Dan,
    We have just bought a 1993 Hymer b544, and are having the same issue of no hot water from either taps. I have removed hot let pipe from heater and blown through with taps open on hot and water spluttered out, so the pipe is clear. I have also removed the clear inlet pipe on the right hand side of the frost switch (between switch and boiler) nothing came out of the pipe, should it? I have also blown and sucked on this and again nothing, when blowing it wasn’t free flowing.
    I then turned on the hot tap and water came through the frost switch/valve.
    I believed the issue could have been a faulty outlet elbow, but could it be a blockage of the inlet? I have spent two weeks trying to figure this out as new to motorhoming and learning as I go. Would appreciate your thoughts please.
    Many thanks Adam

    • Hi Adam – if you blow into the pipe from the frost protection valve to the boiler with a hot tap open you should in theory be able to blow all the way through to the hot tap (note – you will need to turn the electrics off at the control panel to prevent the water pump from running while the tap is open, otherwise it will pump water out at the frost protection valve as you have seen. If you can’t get any flow through the boiler with the tap open and you have checked the pipework either side of the boiler it sounds like you may have a blockage in the boiler itself?
      Regarding the clear pipe – I believe this is just a pressure relief system so I don’t think you should be able to blow through it under normal circumstances. Hope the helps!, keep us posted with progress 🙂 Cheers, Dan

      • Hi Dan.
        I was able to rod from the cold manifold through the frost stat with complete ease up to the bottom inlet of the boiler… then I think touched the actual bottom inlet to the boiler with a 6mm spring..

        Any thoughts on how to get the spring into and into the actual boiler.

        I have the citric acid ready but I need to «  open up” the boiler first.

        I have tried to blow into the boiler from the top outlet on the boiler down into the road via the frost stat… No joy!

        Any thoughts welcomed.

        • Dan

          December 7, 2023 at 9:46 am

          Hi Edward – is your boiler completely blocked (so you can’t just pump the citric acid into it via the water inlet?) In that case it might be worth connecting a pipe to the hot water outlet at the top of the boiler and holding the open end above the boiler. This way you can add citric acid to this pipe and it will hopefully flow downhill until it reaches a blockage, at which point it can start to dissolve it. Keep adding the acid until the boiler is full – I’ve never tried this and it might take a while but it sounds like it should work? What do you think? Cheers, Dan

          • Hi Dan.


            1. I turned off the 12 volt so taps would not pump water.
            2. I used a 1.2m long 6mm rodding spring for handbasins . I gently pushed the spring up the cold water pie from the cold water manifold for about 800mm. I then had someone turn/rotate the rodding spring while I tried to get the spring inside of the boiler by pushing /easing the rodding spring inside of the actual boiler. The spring got inside of the boiler by another 150 mm the the blockage crumbled with white gunky water and bits of calcium came out. It was like de-scaling a kettle.

            I filled the water tank and Turned the 12 volt back on. I removed the shower head and turned the hot tap on for 8 minutes to let water flow through the boiler. I avoided using the kitchen tap and shower head as I did not want the calcium to clog up the sink tap or the shower. After the initial draining down of the milky water, gunk and calcium bit nothing else came out.

            Plan to dissolve the calcium remaining with citric acid powder (2 X 250g with 20 litres of warm water with a £12 another eBay water pump once circulated i will use the boiler to warm well ( 2 hours) citric acid. Leave for’24 hours. Then flush out with clean cold water. Will then flush every year.

            Dan , I have a picture of the rodding spring and a video but unable to up load. If you could send me a email I will forward to you.

          • I did not want to get citric acid into the boiler if I could not be sure of removing the acid…Just in case I could not remove the blockage.

  4. Thanks to Dan and others.
    I am nearly bald from scratching my head to get hot water out of the hot taps.

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