F23 fault when boiler fires

15 Dec 2020
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United Kingdom
I’ve just had a new valiant boiler installed (sealed system). It faults (F23) a couple of times a day. Seems to be when the boiler fires up. The pump seems to be working ok in that all rads get hot in good time. It always runs fine after a single reset. Could it be that the pump isn’t working optimally? It’s maybe 4 years old. We did have a previous plumber who put a screwdriver in it and said it wasn’t resisting the screwdriver that hard but that seemed fairly unscientific! We have maybe 15 rads plus an underfloor heating circuit. I’m slightly at my wits end having paid quite a lot for a new boiler.
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What model of ecotec ie 418, 428 ect and how big is the pump (it should say on the front of the pump) ?
Hello, it’s an eco tech pure 412-435 and the pump says Grundfos UPS3 15-50/65 130 appreciate any wisdom you have. Cheers
Hello, it’s an eco tech pure 412-435
It can't be both of those. 412 is a 400-series 12kW, 435 is a 400-series 35kW. There are also others in between - 415, 418, 424, 428. Which do you have? How many radiators do you have?

Also, your post is slightly confusing. You start off by saying you've just had it fitted, then later say it's 4 years old. Was it second hand?
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Boiler is new pump is probably existing, 15/60 on 15 rads dont sound good.
Sorry. That’s what the manual said! It’s a 435. Plumber has turned output down a bit but hasn’t solved the issue. Boiler is new. Pump is 4 years old.
And it’s 13 rads plus underfloor heating circuit heating largish room.
The pump is too small , you need something like a 25/80 or another 15/60 in series with the pump you have now.
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Thanks Picasso. That is very helpful. So would that explain why after a single reset it always works?
The boiler will work to a certain degree but the software will protect the boiler from the flow and return temps having more than 35c difference.
Thanks Picasso. That is very helpful. So would that explain why after a single reset it always works?
The F23 fault is thrown up when the difference between flow & return temperatures is too great. The heat isn't getting away from the boiler fast enough, the boiler interprets this as a blocked pump and shuts down. Given a bit of a rest, the temperatures equalise a bit but the starting point is higher so it works OK.

Pump might be undersized. Boiler is almost certainly too powerful for both the system and the house. As a very rough rule of thumb, 1kw per radiator plus 0.1kw per square metre of UFH is sufficient.

My late 1970s 4-bed semi needs 9.5kW at full load in the middle of Winter, just to give you an idea. 9 rads and 40sqm of UFH (so rough rule of thumb makes that 13kw, but I've done a proper heat loss calculation on the building to come to the 9.5).

I suspect the minimum output of your boiler exceeds the maximum requirements of the house, and as soon as it tries to ramp up from cold it overheats
Hmm. I’m not sure it is oversized. The house is detached 3 storey with 6 bedrooms. The underfloor heating is heating a room which has a really huge ceiling (5.5 metres high). Pretty large heat loss because the roof insulation isn’t great. Lots of plumbers advised us to go 35kw. But worth thinking about - thanks.

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