NCXo82ike

So yes the dehumidifier would increase heat by its power consumption: ~200w. So counter-productive. Quite possibly the decrease in internal RH would allow the floor temp to drop lower and the increase in system performance overall. Looking at Messana who still hydronic systems to rich Californians, the dehumidification is a well advertised component.

I guess it also makes sense to run minimal MVHR flow to bring in the least humidity possible?

So was Copilot lying to me saying the enthalpy core would drop the internal RH at outdoor temps needing cooling? Have you considered using a standard dehumidifier?

For those chilling, how do you feel with the increased relative humidity? Copilot's calculation gives me 85-90% RH internally if I cool my house to 21 degrees on mid 30 degree days, using only sensible cooling i.e. above dew point. Dehumidifiers? Guess that's a big win for standard air conditioning systems which have dehumidification modes.

That's fantastic!

Quite possibly. Still a little way of installation. But our planned Panasonic aquarea heat pump natively supports two circuits running at different flow temperatures, so it shouldn't be too much of a DIY hack. Admittedly cooling using a loop above dew point and then running a dehumidifier would be easier. I also don't know how much of a real world problem the humidity will actually be when cooling.

Yes, I'm thinking this would should not be on the same loop as UFH. But maybe using pre-insulated MLCP pipe runs, so they effectively have a continuous vapour barrier around?

This would be an argument for cooling at below the dew point surely? With fan coils + condensation drainage. If you can control flow temperature and fan speed you could balance cooling and dehumidification as needed.

yes, I think we should be able to do a reasonably comprehensive job over the beams and between the joists.

That is amazing, thank you. Getting great value from this forum! We will be doing some internal insulation on the brick wall above and indeed all the old solid walls. Insulating lime plaster where there's any chance of moisture in the walls and MVHR to extract humidity from the internal side. The plan is to bring those to U value of ~0.60. Then new walls and ceilings are just to modern building regs, not passive house standards. Ground floor is liquid screed on 150mm PIR on ground bearing slab. UFH in the screed and low profile overlay on new chipboard flooring in upper floors.

Thanks Nick. Here's the SE drawings, relevant section of regs drawings, and photo of the key parts . The steels B06 and B10 form cold bridges with the ground below. They support a solid wall above. You can see blockwork internally but we had to rebuild a section and couldn't make this a new cavity wall. The webbed joists hang off those steels and pocket into the old solid brick walls. So it's an all an inherent compromise- but I want to take sensible precautions to stop the kitchen vapour condensing on cold spots.

Thank Nick, does that mean we shouldn't insulate the beams then? They have external solid walls bearing on them so I (as a total amateur) consider them a cold bridge/condensation risk? Perhaps all academic given that we're getting crittall-style doors as a massive thermal bridge

It's a shame this level of detailed knowledge isn't common

First floor. We've got a new insulated slab over the whole ground floor

Some joists come off the steels, so these will have cool ends- the spray foam will hopefully stop vapour getting to condensation spots. We have the same situation with old joists in the solid walls. I've seen some recommendations to avoid internal insulation at the joist pockets to keep these warm, and others to insulate and tape around the joists for a continuous VCL.