This is one day's condensation from a small through-the-wall MHRV, during the most condensation-prone time of year. It visibly drips during Spring and Autumn and dries up once the outside temperature is above the internal dewpoint (about 10-12⁰C) - as expected.

By rights it should condense a lot more than this (litres per day) but balanced MHRV systems struggle to do this, as the incoming dry air has about half the effective heat capacity of the outgoing wet air. So even a "90%" MHRV is losing a whole lot more heat as latent heat of uncondensed water vapour, than it loses as unrecovered air temperature, the manufacturers strangely don't talk about this in their performance data...

Re the summer overheating, we are able to turn electric ventilation off during the summer and leave windows tilted open, overnight if necc. Thanks all for the ideas on dust and exhaust routes! Thinking our next unit (of several planned small mhrvs) will exhaust through a slate vent, perhaps intake through another, as making big holes through thick stone/rubble gable walls seems like hard work.

The overflow from a DHW system is a vital safety feature in the event that the cylinder boils (eg due to failed stat) and the steam displaces equal volume of boiling water out of the cylinder. Careful with any modifications!

Different climate zone I think - it doesn't get that hot here.