Vaillant Ecofit Pure 30kw Error F20/F23 from underfloor heating

30 Jul 2013
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United Kingdom
Hello all

Need some assistance and hoping someone can help me with this daily nightmare.

I had an kitchen/diner extension in the summer and the builder installed underfloor heating on a rack across 3 zones for this area. It kept causing the the old vaillant ecotec pro 18kw heat only boiler to fail as the builders plumber said it wasn’t powerful enough to power the underfloor heating and also the rads and water tank.

In Oct, I got my normal plumber to install a new vaillant ecofit pure 30kw heat only boiler. We did not upgrade the pump etc.

For some info, the rads and hot water tanks is controlled via Nest and the underfloor is controlled by a separate heat miser setup.

On a daily basis in the morning I keep waking up to error F20 or F23 causing no hot water or heat in the house. Sometimes I get this error during the day as well.

The plumber changed the 2 port Honeywell valve in case there was an issue with that, but this did not solve the problem.

We concluded that it was definitely something to do with the underfloor as when the underfloor was turned right off for a whole week we had no issues whatsoever and ran smoothly just on the radiators and hot water tank.

We then turned off the radiators and hot water via my Nest, and just switched on the underfloor it seemed to be ok with the few occasions of it pushing out the same error.

Not sure what else to do here, any advice is much much appreciated…

Many thanks
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I may wondering if there’s a restriction in your underfloor heating?
No, like a kink in the pipe, or something similar, maybe a pipe insert has come loose?
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My guess is that you either don't have an automatic bypass valve or it's set too high. shouldn't get a f20 with heat only as it doesn't apply. F23 is caused due to heat rate going up too quickly.

The other probable cause is that the boiler is too big. We have had similar problems with the older 428 / 438 ecotecs. dropping the max output down to what your property needs along with pump being on max ( highest fixed) speed solves it. These boilers do not like modulating pumps,
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