Hi @heat-pump-newbie, you might find the capital cost of a small MVHR room to recover heat from the bathroom extract isn't financially viable. Generally MVHR is for whole house implementation & running 24/7, where the energy saving potential is much higher. What's the duty cycle of your bathroom extract fan - on with the lights and then over-running on a timer, or on & off based on relative humidity? If it's a very infrequent duty cycle, fan running for perhaps than than 1hr a day, then MVHR isn't going to be cost effective. Energy to run the fan in the MVHR unit could well be in excess of any energy recovered! Also, over a day, you'll be losing far more heat through the bathroom walls and ceiling than what is intermittently extracted through the bathroom fan. The heat loss through the building fabric isn't quite so obvious as a fan sucking out indoor air, but it's happening all the while you're heating the house. You can consoled that the majority of heat your are losing is 'free' anyway thanks to your ASHP.

Thanks Allyfish - I don't have an extractor at all yet, I just have the bathroom window open till it feels dry in there. I'd also like to hang the washing there on rainy days (ie quite frequently) so it'll get a lot of use if it's humidity controlled.

Also there's no way I'll get whole house MVHR as that would be far too involved and the house is by no means airtight enough, or ever likely to be. 

Posted by: @heat-pump-newbie

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Thanks Allyfish - I don't have an extractor at all yet, I just have the bathroom window open till it feels dry in there. I'd also like to hang the washing there on rainy days (ie quite frequently) so it'll get a lot of use if it's humidity controlled.

Also there's no way I'll get whole house MVHR as that would be far too involved and the house is by no means airtight enough, or ever likely to be. 

Instead of an MVHR you could possibly consider a dehumidifier.

Ah. in that case, as Derek says, a dehumidifier is your best best. They are good for drying clothes and towels in a very cost effective way, as well as being very effective at preventing mould and damp in rooms that can experience high humidity levels. A bathroom ideally needs a controlled rate of air change via an extract fan, but dehumidification for relative humidity control would also be effective. An intermittent use extract fan removes odours, but some dehumidifiers on the market have active carbon filters, and could also do that.

Ah - sounds useful... Can you show me an example please ? I'm only familiar with a freestanding unit that collects condensate in some kind of bucket ! I presume there's something better suited to use in a bathroom. I'm more interested in comfort and simplicity than RoI here. I hoped something could just be installed in the wall and remove humidity without sucking out all the heat.

Doing a quick scan of dehumidifiers I've only found free standing units so not the kind of thing that woudl work automatically in a bathroom. 

Does anyone have experience of the Envirovent heatSava which is a single room MVHR ?

@heat-pump-newbie most dehumidifiers are free-standing, so you need to be a little careful with 240V in bathrooms and where you locate it. Most can be connected to a permanent drain, to save switching off once the internal condensate bucket fills up. That might require the unit to be off the ground i.e.: a shelf or similar, to gravity drain. Quite a few have carbon air filters as standard or as options. Ebac make very well rated models in a UK factory with control features like laundry mode and even a tent to go over your clothes horse! (No, seriously...

EBAC.COM "https://www.ebac.com/laundry)"

You'll get cheaper imported ones, but you do tend to get what you pay for, and Ebac's guarantees are up to 5 years and customer service are good. I've got a 30yr old Ebac CD30 industrial dehumidifier in my garage still going strong, keeping the humidity below 40% for a classic car and trailer tent, and a much newer Ebac washing machine in the utility. Ebac also make domestic heat pumps now.

Posted by: @heat-pump-newbie

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Now that I've got a warm bathroom heated by my ASHP I want to prevent losing all that lovely heat from being extracted with the steam from the shower ?

How do I choose what single room MVHR would be appropriate ? Everything I see online directs my to whole house systems or just technical stuff I don't understand. Help !

BLAUBERG.CO.UK "https://www.blauberg.co.uk/blauberg-mini-air-decentralised-heat-recovery-unit-single-wall-mounted-d-mvhr-ventilator-smart-wifi-control"

Vento Mini Single Room Heat Recovery Ventilation Unit | HRV

Eliminate condensation, mould & damp in small rooms with Vento Mini Stop wasting energy & heat and install Blauberg UK's decentralised single room heat recovery units.

Thanks Brendon, that's exactly the kind of thing I was looking at. Would you recommend the Vento Duo for a bathroom as their brochure suggests or would the Vento Mini be sufficient ?

@heacol this looks pretty awesome. Does/can it replace an existing extractor fan in a bathroom.

I've also found the Envirovent heatSava which I like. Does anyone have experience of this ?