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. 

 

 

[NCXo82ike]

5 hours ago, Gus Potter said:

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. 

 

 

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.

[JohnMo]

8 hours ago, MikeSharp01 said:

 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!

It's all related to local temp and humidity, that's why I say a bit of trial and error. We ran ours at 14.5 target yesterday, it ran for around 7 hours, completely dry. Internal humidity stayed around 49% all day.

[MikeSharp01]

5 hours ago, Gus Potter said:

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. 

Interesting stuff Gus, 

 

5 minutes ago, NCXo82ike said:

This would be an argument for cooling at below the dew point surely? With fan coils +  condensation drainage.

 

The challenge is the pipework, we have 25mm insulation around nearly all our pipework - not sure how much condensation is forming on the pipework beneath that but our Underfloor Heating (UFH) manifold is not insulated and that is my tell tale. Humidity is hovering around 46%.

[NCXo82ike]

4 minutes ago, MikeSharp01 said:

The challenge is the pipework, we have 25mm insulation around nearly all our pipework - not sure how much condensation is forming on the pipework beneath that but our Underfloor Heating (UFH) manifold is not insulated and that is my tell tale. Humidity is hovering around 46%.

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?

[JohnMo]

7 minutes ago, NCXo82ike said:

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?

Aren't you going down a rabbit hole of complexity for complex sake. Heat pumps like simple open flow systems, if you involve UFH you are limited to 14 to 16 Deg flow temp (depends on location), no dew point management specifically needed. Size any fan coils to operate at that flow temp. Otherwise your into electronic mixers etc.

[NCXo82ike]

Just now, JohnMo said:

Aren't you going down a rabbit hole of complexity for complex sake. Heat pumps like simple open flow systems, if you involve UFH you are limited to 14 to 16 Deg flow temp (depends on location), no dew point management specifically needed. Size any fan coils to operate at that flow temp. Otherwise your into electronic mixers etc.

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. 

[JohnMo]

1 minute ago, NCXo82ike said:

running a dehumidifier

Why would you need that? This is my 3rd year of cooling, no dehumidification needed.

 

3 minutes ago, NCXo82ike said:

also don't know how much of a real world problem the humidity will actually be when cooling.

None if you don't try too hard to go to low flow temperature.

 

4 minutes ago, NCXo82ike said:

Panasonic aquarea heat pump natively supports two circuits running at different flow temperatures

Almost all heat pumps do that. But your CoP is governed by lowest cooling flow demand temp for cooling and highest for heating.

 

Higher cooling flow temperature give excellent running CoP. Over my 7 running hours yesterday I had an average CoP 7.8, using a massive 3kWh of energy, while removing 22kWh of heat from the house.

[saveasteading]

17 minutes ago, JohnMo said:

removing 22kWh of heat from the house.

If only we could store that  for next winter. 

Actually it us the one thing that is useful about slinky ground source. Warm the earth for later.

 

Where you live, above 22C is getting too hot for most people. 

I was travelling in 32C yesterday and ok with it.  

We get used to it. 

 

 

[NCXo82ike]

32 minutes ago, JohnMo said:

Higher cooling flow temperature give excellent running CoP. Over my 7 running hours yesterday I had an average CoP 7.8, using a massive 3kWh of energy, while removing 22kWh of heat from the house.

That's fantastic!

[JohnMo]

14 minutes ago, NCXo82ike said:

That's fantastic!

We are lucky, we have a lower ambient in summer than down south, but our big windows drive solar gain and a need to cool. So when cooling we do get a great CoP, in winter a reverse situation can be true.

 

29 minutes ago, saveasteading said:

only we could store that  for next winter

That's a gshp. But sand 150mm down below surface and loads of trees preclude that, tried but gave up. 

[joth]

2 hours ago, NCXo82ike said:

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. 

This is how I set mine up (6 years ago) on our ecodan, the problem is it's not just the FCU loop that needs condensation proofing but also the primary pipework that includes the filter, pump, sensors, divert valves etc. None of those devices are designed for being insulated, indeed there's a risk of pump burning out if you really did, and they gather condensation as easily as the pipework.

Adding a second, much larger, FCU and keeping it at dew point is how I've used it for past 3 years. 

 

[saveasteading]

1 hour ago, JohnMo said:

That's a gshp. 

Yes that would be, but slinky types are actually getting heat from summer warming and not the earth's warmth... so solar really.

 

Is your sand glacial esker?

The only ground I've known where soakaways had to be designed to run more slowly.

(Except chalk which can be a problem)

 

[MikeSharp01]

2 hours ago, JohnMo said:

Higher cooling flow temperature give excellent running CoP. Over my 7 running hours yesterday I had an average CoP 7.8, using a massive 3kWh of energy, while removing 22kWh of heat from the house.

The is very good- why is it so good, what insights can you give us into getting that good? Our COP was all over the place until I let it settle at 16oC today we are getting above 3.4 at the moment I will keep an eye on it and we where it goes 7 is miles off.