Wouldn't touch glow worm with a barge pole, but only my opinion. They sell bucket loads of them but are designed for their particluar market.
I would go for the Vaillant Ecotec Plus 837 as a better option to the Glow worm. Better made, more reliable, longer warranty, no water scale inhibitor needed as has a 5 year anti scale warranty.
Ecotec Plus 831 will give just over 12l/m 837 will give around 14.5l/m.
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