Fan Coil Units for use with a (cooling) ASHP

[Michael_S]

On 03/05/2026 at 15:54, JohnMo said:

Had my cooling on this year already, one fan coil and UFH, flow rate 14.5 to 15 degs, zero condensation from fan coil

To be fair, last week with the easterly flow DPs were very low across the board despite the solar warmth.

 

I have just bought a bunch of computer case fans and and going to see how they work with our double rads and flow temp controlled to be above dew point; we will probably run the dehumidifiers in the summer when we have excess PV so minimal cost which should also help with this issue - at the expense of some additonal heat input from the dehumidifiers.

[LiamJones]

On 19/09/2024 at 11:42, Wil said:

Just for completeness, I email Megawave about their FCUs and got the following data sheets if anyone is still considering them:

 

I haven’t gone as far as an order.

FCU Data.pdf 659.83 kB · 11 downloads MWES Concealed FCU data sheet.pdf 7.16 MB · 9 downloads MW Free Standing- FCU（DC motor）manaul 23.6 wifi 1.pdf 1.03 MB · 6 downloads

@Wil did you order these in the end? 

[Dave Jones]

On 03/05/2026 at 14:55, LiamJones said:

@Dave Jones have you ran condensation drains to your FCUs if you’re running above dew point? If you have, do you think you needed to? I ran flow/return to upstairs bedrooms in case we needed heating upstairs. turns out it’s more of a case of needing cooling up there but I didn’t run drains to them. What do you think? 

 

no, i dont want the loop to be less than 15c to avoid dewpoint on the slab.

[Michael_S]

WE are running our flow at 16 (checking dew point to avoid condensation) and have some cooling.  We have mounted 8 PC fans under one rad as a proof of concept and it helps creating a small local cool zone, they are silent but we could actually do with considerably more airflow, may try a higher voltage power supply. Currently they are fixed using mini magnets but not that secure so may try mounting them using plastic trim strips.  Heat pump is drawing about 1/3 minimum input on an hourly basis and cycling frequently showing we are basically failing to extract nearly as much heat via the rads as the unit is capable of even at minimum modulation.  Inside air temp is 26C compared to 35C outside but no idea what it would be without the cooling.

[JohnMo]

I would tweek it down 0.5 degs at a time, leave for a while and check for condensation. You should be comfortable taking it down a couple of degrees. The lower running temp to will promote more heat uptake and a higher demand, so less cycling. It's a balancing act, so some trial error I am afraid.

[Michael_S]

22 minutes ago, JohnMo said:

I would tweek it down 0.5 degs at a time, leave for a while and check for condensation. You should be comfortable taking it down a couple of degrees. The lower running temp to will promote more heat uptake and a higher demand, so less cycling. It's a balancing act, so some trial error I am afraid.

All the rads are ok except the bathroom that is sweating a little, I guess we could just turn that one off?

[MikeSharp01]

5 hours ago, JohnMo said:

I would tweek it down 0.5 degs at a time, leave for a while and check for condensation. You should be comfortable taking it down a couple of degrees. The lower running temp to will promote more heat uptake and a higher demand, so less cycling. It's a balancing act, so some trial error I am afraid.

 I have had ours at 16^oC for a couple of days. It have brought the slab temp down, at 15 we get the faintest of condensation on the manifold at 14 it drips off!

[Gus Potter]

I posted some stuff a few years back about how you get, call it " reverse condensation". My post was driven as I live in Scotland. Here we get days, particularly on the west coast where the house is cooler on the inside and the Atlantic Ocean moist winds blows in. Most BH folk wrote me off as a mad Jock!

 

It totally reverses the dew point calculation and yes you do get condensation forming on the cooler house surfaces. 

 

Now in the south of the UK you are not accustomed to this.. but if you introduce air con into a very well insulated house then this kind of reverse dew point and internal condensation is worth a thought. 

 

In structural terms the odd bit of water gas condensation is ok.. but too much and you have a problem.