Truma hot water tank not draining

[solaris]

First noticed this problem towards the end of last year. All the drain taps had been left open but on refilling the system the pump brought it up to pressure within a minute, it normally takes much longer to fill the hot water. On returning from the trip I put a jug under the drain before opening the automatic drain valve and about a litre drained out. The internal taps were all open. I changed the dump valve but it has made no difference. I can drain the tank by driving with the valve open but this dumps water on the road, anyone following would not necessarily know it was clean water. Had considered an airlock but this has occurred over several drain downs. Cold water tank drains no problem. Anyone got thoughts on this. TIA

 

[boxtraveller]

Had this happen once a couple of years ago, I switched on the pump for a few seconds and water started to flow.
Not happened since so assume it was an airlock.

 

[solaris]

Thanks. Yes if I turn on pump water flows out of drain valve but stops immediately pump is turned off. Would not have thought airlock would keep repeating but now we’re using van more regularly will see how it goes

 

[mapman]

I have had similar issues. When I drained down in November by opening the blue frost control lever allowing blue button to pop out I was convinced insufficient water had drained to fully empty the boiler. I also opened two drain valves under the sink on the advice of dealer workshop. Still uncertain I decided to put some low level heating in van as an added precaution. On preparing for a trip next week I filled up and followed usual procedures to prime the system. However unlike your situation it took time to fill, used many litres of water and therefore I concluded boiler was empty. I saw no evidence over winter of water dripping out but will be monitoring the system while we are away and on return.
Last night we had an unexpected frost and the blue button popped out (as it should, since frost valve is in locker) but once it had warmed up pushed button in and reprimed ..this time only took a minute or so. I recall being told that in order to avoid damage to the truma boiler frost valve dumps a small amount of water rather than the complete contents. This seemed to be the case today. This begs the question of how you get it to dump all the contents when you want a full drain down for winter.
The truma fitting has a blue tap/lever on the top in addition to the button at the base. When button popped last night obviously the top lever remained in closed position. It only released small amount of water. When I drained down in November I opened the top valve and button popped. Maybe this does release all the contents despite my misgivings?

 

[boxtraveller]

Thanks. Yes if I turn on pump water flows out of drain valve but stops immediately pump is turned off. Would not have thought airlock would keep repeating but now we’re using van more regularly will see how it goes

If I remember correctly I was draining the fresh tank at the same time as dumping the boiler and because the boiler was not draining I put the pump on and stopped the fresh tank dumping until the boiler started draining the re-opened the tap on the fresh tank to drain.
Maybe that above combination or sequence causes the airlock, I have no idea but it solved the problem when I turned on the pump and stopped the fresh tank emptying.

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[solaris]

Thanks for your help. Like mapman have put a low heater in van as cannot be sure water has all drained until I refill. Wondered about possible problems with automatic dump valve but problems persist with new manual valve. Pretty sure it must be an airlock but why it’s just occurred after 3 years is a bit of a mystery unless it’s programmed to wait for end of warranty. Suppose I could fit a Floe system or similar if it gets really annoying. Anyhow thanks again