Fan Coil Units for use with a (cooling) ASHP

[saveasteading]

23 minutes ago, JohnMo said:

soak aways are huge, compared to normal

I'm interested.

The regs show ever bigger areas where permeability is poor.

On sand they don't seem to acknowledge that the water won't reach the end. 

 

So in our case we used perforated pipes heading different directions to spread it.... phase 2 and 3 extensions to the zone, the drawings and calcs got us the approval, but are 'yet' to go in.

[joth]

23 minutes ago, JohnMo said:

Set up to run long run for periods, this helps a lot. Plus our ambient outside helps. 

And down-sizing the ASHP to allow these long runs at a low  deltaT without short cycling, isn't it?
Our ASHP is way oversized so will bordering short cycle (5 min runtime, 10min period) when just cooling the slab to ~16deg.

I'm still getting COP of 3.5-4. But it's powered from PV - if I could double the COP it would give me about 30p more SEG payments per day on these hottest days.  

 

[JohnMo]

5 minutes ago, joth said:

And down-sizing the ASHP to allow these long runs at a low  deltaT without short cycling, isn't it?
Our ASHP is way oversized so will bordering short cycle (5 min runtime, 10min period) when just cooling the slab to ~16deg.

I'm still getting COP of 3.5-4. But it's powered from PV - if I could double the COP it would give me about 30p more SEG payments per day on these hottest days.  

 

Last year on our 6kW heat pump it cycled, depending on load, we averaged around 4 to 5 CoP over the day depending on a few factors doing cooling. This year with a 4kW it can run low and slow, our dT was about 2.5 degs. Yesterday's whole day CoP was 7.3 including standby time/energy.

 

We only cool if there is solar gains, so we have plenty of PV also. So it free. It would almost always get clipped from PV also, if we weren't using it, so it really is free.

[Michael_S]

Not sure if we can calculate an EER, we have a heat meter but not sure if it counts backwards.....

 

Gonna add some more fans to the rads and overvolt them a bit to see if we can get some more airflow - I know you can get actual sit on rad units but the fans are less than a quid each compared to about £100 for a unit that will only do about 1/3 the length of a fad and is probably louder too.

 

Currently we are using our second hand heath robinson heat pump - hoping to move to a 'proper' set up but MCS sound rules mean our preferred placement is max 55db that seems to strictly limit the number of 8-10kw units available and the one that seems to be best fit (Grant) does not cool. 

 

Does anyone know of any other units that fit the 8-10kw and 55db or less sound power criteria and do cooling?  Thanks

[JohnMo]

1 hour ago, Michael_S said:

Not sure if we can calculate an EER

It's the inverse of CoP, so if calculating CoP you just get a negative figure, exactly the same calculation. Take the negative away you get EER.

[Michael_S]

8 hours ago, JohnMo said:

It's the inverse of CoP, so if calculating CoP you just get a negative figure, exactly the same calculation. Take the negative away you get EER.

The heat meter measures the flow rate and flow and return temps and calculates a kwh heat transferred number which normally just increases like an electricity meter.  Not sure if the return is warmer than the flow if it will decreases the total kwh transferred counter. 

Edited Wednesday at 23:16 by Michael_S

[JohnMo]

7 hours ago, Michael_S said:

calculates a kwh heat transferred number which normally just increases like an electricity meter.  Not sure if the return is warmer than the flow if it will decreases the total kwh transferred counter. 

It calculates kW as either a positive or negative dependant on heating or cooling. The kWh heat delivered scale is exactly the same as electric meter, it only adds it doesn't run backwards. So in cooling the kWh counter stays static 

 

7 hours ago, Michael_S said:

heat meter measures the flow rate and flow and return temps and calculates a kwh heat transferred number

Not exactly correct, it calculates kW then adds a time function to get kWh.

 

Heat pump monitor software has a tick box in the settings to allow it to understand if you also do cooling and it runs an internal timer to distinguish between cooling and defrosting. Once timer elapsed it knows you are in cooling mode and attributes the correct CoP, SCoP values 

[SteamyTea]

On 27/05/2026 at 01:02, Gus Potter said:

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

Why in the Southern States the VCL is on the inside.

Mind you, with climate change affecting the regional weather patterns, there may need to be some rethink about condensation risks.

[Michael_S]

On 28/05/2026 at 08:13, JohnMo said:

It calculates kW as either a positive or negative dependant on heating or cooling. The kWh heat delivered scale is exactly the same as electric meter, it only adds it doesn't run backwards. So in cooling the kWh counter stays static 

 

Not exactly correct, it calculates kW then adds a time function to get kWh.

 

Heat pump monitor software has a tick box in the settings to allow it to understand if you also do cooling and it runs an internal timer to distinguish between cooling and defrosting. Once timer elapsed it knows you are in cooling mode and attributes the correct CoP, SCoP values 

Not a lot of software on our heat meter, it is a Danfoss Sonometer 1100, it has a screen you cycle through that gives you live readings for flow and return temp and volume and a calculated heat transferred in kwh measured since the unit was new, nothing else.

[JohnMo]

13 minutes ago, Michael_S said:

Not a lot of software on our heat meter, it is a Danfoss Sonometer 1100, it has a screen you cycle through that gives you live readings for flow and return temp and volume and a calculated heat transferred in kwh measured since the unit was new, nothing else.

Couple that to heat pump monitor, that will add an electric meter, a few bits and bobs, and you get full monitoring.

 

We have the same heat meter. 4th or 5th push of the button also gives you kW as an instantaneous reading. You have dT available to read. Will give stuff like this, it's ace for understanding what really happening.

 

 

[Michael_S]

2 hours ago, JohnMo said:

Couple that to heat pump monitor, that will add an electric meter, a few bits and bobs, and you get full monitoring.

 

We have the same heat meter. 4th or 5th push of the button also gives you kW as an instantaneous reading. You have dT available to read. Will give stuff like this, it's ace for understanding what really happening.

 

 

Yeah - it definitely doesn't run backwards or have the heat/cold kwh registers that some models have so agree it can give instantaneous output and possibly if I understood modbus or whatever could give telemetry but I don't and suspect it would need extra modules to do so?

[Gus Potter]

@MikeSharp01 Interesting stuff..

Yes it can be, if you have the will to live. I think pragmatism should prevail here and, at times I'll just need to contradict myself! for the common good. 

 

What I was referring to was what happens in the walls of the house. To explain this in lay terms and to use a qualitative example. If you have good double glazing and say overnight the house is sitting inside at 18 deg C and the temperature outside drops to 10 deg C. The outside pane of glass may end up at 6.00 am in the morning at about 11- 12 deg. Now suddenly a warm moist weather front of air blows in. You can often see condensation on the outside of the glass. This happens quite a lot in Scotland for example. If you have rubbish glazing then the outer pane will be much warmer as it gets heated from the 18 deg C air inside the house and thus you are less likely to observe this. 

 

Now if you can see that then you might ask.. well are the walls that we can't see inside doing the same. That is in summary what I referred to as reverse condensation. 

 

But this thread is not really about that. It's to do with condensation inside the house when you cool it on hot days. The fundamental questions are: 

 

1/ How cool do you want your house on a hot day and what am I prepared to pay for that? 

 

2/ Am I going to do something that could compromise the design of the house. This could be by causing condensation to occur in a place where it can't be easily vented out and thus promote things to rot / corrode. We spend loads of time on BH detailing air tightness and so on.. but we are always thinking (so far until say this thread) from the inside out. 

 

3/ Am I able to accept some cooling say from my Air source heat Pump (ASHP) and treat this as a bit of free lunch. It won't be perfect but it's a good pragmatic trade off.

 

Now for me, take any underfloor heating / ASHP or similar combination. My immediate approach is to ask, can we keep it simple, cheep to maintain and by default retain value in your house. 

@JohnMo sums it up for me here with practical elegance. 

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.

In summary there is no perfect answer. The systems are generally set up to heat the house, but can cool to some extent.

But for a bit of pragmatism lets look at what could go wrong if you get condensation on the heating pipes in cooling mode.

1/ If you lag the pipes then water gas will still get in and condense. If running for a long time with adverse humidity then water will collect and run out the gaps in the insulation. Now if that happens over a ceiling it will cause staining. 

2/ Much depends on how you use and respect the house. If you are boiling up pasta big time or drying washing inside then that is an issue. The building regs are set up for this, but.. 

3/ We need to consider the frequency of occurrence. It's ok if from time to time you get a bit of condensation, so long as you get rid of it quickly enough then the risk of harm is low. We will quite happily take a couple of buckets of water and wash floors! That will change the humidity rapidly.