Different effectiveness between extractor and dehumidifier

I don't know whether single-room MVHR is effective (cost-wise or humidity-wise)?

Yes, but not in the Summer.

I'm looking at that based on your suggestion. At first glance it seems like a more elaborate and expensive solution than I was seeking.

oh yes! I'd quite forgotten about Summer. All doors and windows thrown open most days does make a big difference.

They claim to recover about 75% of the heat. That means you can run them in trickle mode without losing too much, which is important in a bathroom where it might get uncomfortably cold if a regular fan were venting to the wintry outside.

Extractor fans that blow out need the air replacing from somewhere. If you don't open a window it'll come from inside the house somehow, which will likely mean cold air will come in via air leaks. You might not feel the bathroom being cold unless the bathroom itself leaks, but some other spot will get colder because of it.

Dehumidifiers will reduce moisture, but they won't do anything for air quality - cooking, 'toilet smells', pet/human odours, gas hob. The advantage of MVHR is you're simultaneously pulling in fresh outside air instead of recirculating stale inside air.

Single room MVHR are a bit expensive for what they are. I have a used one of these to play with:

The trouble is the fan is loud (perhaps because it's used, but I don't think so) and I need to work out how to quieten it down, which is awkward. There are some hacks here:

But even at lowest PWM it's not quiet enough for me. The other option is just to DIY an equivalent, perhaps with a matrix of copper tubes like a steam loco boiler. But I need to work out how to do a tangential/cross-flow fan (pulls air across the fan blades like a water wheel) as that doesn't seem to be a thing you can easily buy in a flat form factor, only a long cylinder.

Theo

Thanks for the very useful information.

We have a heat pump tumbler drier which lives in my workshop - no decent space in the house.

Obviously when the weather is nice and dry and sunny it doesn't get used. A washing line does the job.

It doesn't condense 100% of the water out of the exhaust. The room gets warm and perceptibly damp - which I don't think my tools (or the car in the attached garage!) would like very much.

In summer we put on an extractor fan. It sucks out the warm damp air, and warm dry air comes in at the opposite end.

In winter we put on a dehumidifier. It dries the air, and the power consumed by both devices make the workshop nice and warm.

Andy

Maybe in the bathroom (except for smells), but only specialist dehumidifiers meet the requirements for UK bathroom installation and from what I recall they are rather expensive.

In the kitchen, you definitely want air (including grease, fumes from burning gas, etc., as well as water vapour) sucked out.

If only Water Vapour was the actual problem in the bathroom.....

How do straws fail?

Maybe stacked layers of coroplast board, with alternating hot/out going left to right and cold/in going back to front, then some sort of 3D printed 45° manifold?

No idea, but they appear to have spares for everything. Perhaps they get clogged with muck?

For decent heat transfer you need the walls to be as thin as possible, or made of heat conducting material. I'd guess coroplast is thicker plastic as it's designed to be sturdy?

The Tempra has outlet (from the room) on the sides and inlet on the bottom to prevent airflows mixing, with a flap to close the bottom so you can stop it letting in hot air in the summer. I'd guess you might need to do something similar - inlet in one plane and outlet in another. A nice big ~120mm outlet fan on the face of the unit is easy, I suppose you could side mount a smaller ~60-80mm fan for the inlet if you didn't mind a deep unit.

Theo

Well, I did say "except for smells"! They are unlikely to be a hazard (just an inconvenience), whereas condensation causes mould (which can get dangerous) and damage to grout, sealant, etc.