I use cheap overnight electricity to heat my DHW. I have a good size hot water tank and a very large buffer (750l or so). Normally this works well, but when it is cool, the defrost cycle switches the divert valve to take from the DHW, reducing the DHW temperature about a degree each time. I rapidly have no hot water! Essentially I do not want it to turn the divert valve, and instead to take from the buffer. But there is no obvious option to do this. I have a modbus unit that I haven’t connected yet. I was wondering if there was any way of overriding the divert value? Preferably an elegant way rather than a bodge!
Hi @CoP,
Your observation is somewhat at odds with the info I obtained from UK Tech Helpline. They told me that the defrost heat source was whichever system was in use at the time of the defrost demand (i.e. DHW tank during a hot water demand, buffer tank otherwise). In other words, a defrost demand does not flip the diverter valve, so as long as DHW isn’t enabled when the defrost is required, then heat should only be taken from the buffer/volumiser.
I suggest that you contact the helpline ([email protected]) - I may have misinterpreted their info, but otherwise is it possible that there is a fault in your setup?
[There is a setting on the Outdoor Unit panel - Defrost Operation K-buttons = 010x where x = 0 (basic) or 1 (option), but there is no information in the Installers Guide (p122) on what “option” does, so I’d regard this as a long shot…]
Thanks @SarahH ! I don’t think the information from the technical helpline can be right. I’m almost certain that the defrost cycle always switches the DHW valve and takes from the DHW, but I will certainly double check if there is a way around this.
I guess one easy way to check would be to completely disable DHW using the Schedule option on the remote controller one really cold night (when a defrost is likely), and see whether the DHW tank temperature has fallen noticeably afterwards. (It could be - if you have DHW heating permanently enabled on its programmer - that the cylstat just happened to be demanding DHW heating at the moment that a defrost was also demanded.)
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