OK some fan coil unit questions

[MikeSharp01]

There are many on here that extoll the virtues of fan coil units so I thought it would be fun to see if we could put a couple in - one for each bedroom. Looks straight forward enough but two questions come up. 

 

Firstly what pipe to use from the plant space to the units. I have found THIS stuff which looks like it should work. Will it?

 

Secondly do I need to provision for a condensate drain back to the plant room and away or can an evaporative approach be taken as in most fridges?

 

 

[G and J]

So, I’ll assume you are using the fancoils for cooling.

 

Sadly I can’t answer your pipe question, I guess the insulation is only there to stop the pipes dripping condensation if you run your heat pump cold enough to cause condensation.  If I’ve understood what’s been said it may be enough to run the pump just above the dew point - but that might mean bigger fancoils as the delta between room ambient and water temp will be lower.  
 

But if you do run it above dew point you won’t need insulation on the pipes at all.

 

I'm guessing if you do run it real cold then you’ll need insulated pipes and the drain will become very needed.  
 

 

But hey, sorry if we made you feel left out of the fan(coil) club lol

[MikeSharp01]

7 hours ago, G and J said:

But if you do run it above dew point you won’t need insulation on the pipes at all.

We will be cooling the slab at the same time so the dew point will be important. I guess then that some sort of dynamic dew point (DP) monitoring / calculating will be needed as the various environmental factors affecting DP will be moving about all the time. The good old DHT22 in combination with the Lawrence approximation equation [
Td = T - ((100 - RH)/5.) ] should do that?

 

I cannot see how I can do a drain back the plant area given the way the pipe would need to run - so perhaps that is us out of this one, so that's a few grand saved before breakfast.

[Dave Jones]

pipe is standard plastic. If the void your running the pipework through is insulated already then no need but for the price of pipe insulation cover everything. if its ground floor then drains can be put in the slab, 1st floor run through the joists - easy.

 

Defiantly connect up the condensate drains and lose in the studwork, wish I had done this knowing what I know now. being able to drive it at 12c for free really knocks out the heat fast compared to 16c.

 

Dont forget to insulate the UFH manifold as well.

[JohnMo]

For the pipes just make sure they are barrier pipes, so you don't get air entering the system over time. I am just using Hep2O.

 

The fan coils will have a condensate collection point and a connector to run to drain. I decided running a drain away was too much work, so thought I would suck it and see and if I needed, I could put a small container underneath to catch drips. Which so far I haven't needed to do.

[MikeSharp01]

Just now, JohnMo said:

Hep2O

Are you insulating them? 

 

What water temperature are you running the FCUs at?

[G and J]

34 minutes ago, MikeSharp01 said:

We will be cooling the slab at the same time so the dew point will be important. I guess then that some sort of dynamic dew point (DP) monitoring / calculating will be needed as the various environmental factors affecting DP will be moving about all the time. The good old DHT22 in combination with the Lawrence approximation equation [
Td = T - ((100 - RH)/5.) ] should do that?

 

I cannot see how I can do a drain back the plant area given the way the pipe would need to run - so perhaps that is us out of this one, so that's a few grand saved before breakfast.

If you have the capability of setting the flow temp automatically just above the dew point then no drain required.  Ok, you won’t be able to run super cold so it’ll take longer to cool each room, but it’ll still be better than not having any cooling in each bedroom.  
 

If you do go for supercool you’d have to isolate the slab anyway or risk condensation on flooring - not an attractive look - so the above dew point working sounds brill.

 

What kit do you need to implement Rough Lawrence?

[G and J]

40 minutes ago, MikeSharp01 said:

I cannot see how I can do a drain back the plant area

It could drain to any convenient waste pipe using a dry trap methinks, and if a continuous fall is the difficulty then some Fancoil instructions say you can run the pipe to make a deliberate gentle u bend so as long as it has a fall from the unit to start with it’ll work perhaps.

[JohnMo]

Just now, MikeSharp01 said:

Are you insulating them? 

 

What water temperature are you running the FCUs at?

My pipes run outside to summer house so are insulated. But not insulated within the summer house. The UFH manifold isn't insulated either. Neither seem to get wet, possibly due my short run times

 

The target flow temp is 14.5. but the way it's all setup the flow temp can go down to 11.7. But the heat pump compressor rarely runs more than 40 mins (mostly 12 mins) before shut off and then just circulates cool water. Once the return temp hits 19.4 the cycle starts again.

[G and J]

37 minutes ago, Dave Jones said:

pipe is standard plastic. If the void your running the pipework through is insulated already then no need but for the price of pipe insulation cover everything. if its ground floor then drains can be put in the slab, 1st floor run through the joists - easy.

 

Defiantly connect up the condensate drains and lose in the studwork, wish I had done this knowing what I know now. being able to drive it at 12c for free really knocks out the heat fast compared to 16c.

 

Dont forget to insulate the UFH manifold as well.

So, if one has both the slab and fancoils all run together in a single zone, then one cannot run supercool for too long before the floor gets condensy (today's invented word!).

 

But if the fancoil at 12c cools a bedroom v quick is it ok to boost the lot for say 10 to 15 mins without risk of wet floors?

[JohnMo]

4 minutes ago, G and J said:

What kit do you need to implement

None, just a heat pump, don't be too greedy with flow temps. My WC curve for cooling is 14 at 30 and 14.5 at 20. Compressor restart hysterisis is 4.9 and shut-off is 2.8. Been cool and not much solar gain for the last few days ASHP compressor hasn't started at all. 

 

Any condensation you make is a result of the internal temperature and humidity, keep the temperature steady you shouldn't have issues, if you let the temperature swing about with timers etc you will be chasing your tail to keep condensation in check.

[MikeSharp01]

17 minutes ago, G and J said:

implement Rough Lawrence

Just a temperature & relative humidty (RH) sensor, such as the DHT22.

[G and J]

Adding up all the various things said it feels to me like the answer is put in a drain in case but don’t expect it to actually drain much; keep things constant and the house will run well; simple weather compensation will mean the only thing I’ll have to do manually is close the bedroom door on really hot days.  
 

Leaving me more time to operate my coffee maker. Spot on!

[JohnMo]

You can run heating at 70 degs on timers and zone valves, it isn't efficient. Trying to similar with a heat pump ends up with huge running costs. You can run the heating at low flow temps slow and steady in all the time. It is efficient.

 

Apply the same logic to cooling. Well insulated you can set the system so it's self modulating. Responding to demand. Even in 300mm pipe centres in a 100mm thick slab, you can see the heat pump responding quickly to solar gain. We get the solar gain around 3 to 3.30pm. This is a snap shot of the heat pump flow temp.

 

The dips are the compressor starting and running. Prior to 2.15pm heat pump is off for two hours. Then an 11 min run and off again until 3.30pm. Then a 20 min run and off for 20, on for 20 etc, in direct response to solar gain.

 

[MikeSharp01]

So I clearly need to think this through. I cannot run drains as all the pipe runs are in the ceilings so above and supposed unit. Insulated pipe runs not difficult in the ceilings Electricity is no problem - what controls do these things have?

[MikeSharp01]

6 minutes ago, JohnMo said:

The dips are the compressor

Is this not the very definition of short cycling? Would it be better to run the cooling longer at a higher temperature?

 

[JohnMo]

3 minutes ago, MikeSharp01 said:

what controls do these things have

None. Currently just run the fan in the fan coil in manual at lowest speed.

 

1 minute ago, MikeSharp01 said:

Is this not the very definition of short cycling

No, short cycling is uncontrolled short running and off cycling. I had 6 starts over a 6 hour period. Typically you would have many (10 to 20) restarts an hour and very limited heat or cooling given to the house. Typical short cycle is a couple of mins running and a very short off time. What you are seeing is capacity control when running below min turndown.

 

4 minutes ago, MikeSharp01 said:

Would it be better to run the cooling longer at a higher temperature

Yes but then you run into condensation issues in the floor. So it's all a balancing act, long runs are great, but if you have condensation issues not so great.

[G and J]

21 minutes ago, MikeSharp01 said:

So I clearly need to think this through. I cannot run drains as all the pipe runs are in the ceilings so above and supposed unit. Insulated pipe runs not difficult in the ceilings Electricity is no problem - what controls do these things have?

Actually in the ceiling void or hung from the ceiling itself?  If the latter I do understand…..  except some of the units can be mounted next to a wall into which the drain could pass.  

[G and J]

29 minutes ago, JohnMo said:

You can run heating at 70 degs on timers and zone valves, it isn't efficient. Trying to similar with a heat pump ends up with huge running costs. You can run the heating at low flow temps slow and steady in all the time. It is efficient.

 

Apply the same logic to cooling. Well insulated you can set the system so it's self modulating. Responding to demand. Even in 300mm pipe centres in a 100mm thick slab, you can see the heat pump responding quickly to solar gain. We get the solar gain around 3 to 3.30pm. This is a snap shot of the heat pump flow temp.

 

The dips are the compressor starting and running. Prior to 2.15pm heat pump is off for two hours. Then an 11 min run and off again until 3.30pm. Then a 20 min run and off for 20, on for 20 etc, in direct response to solar gain.

 

Am I correct in thinking the pump is running all the time?  i.e. the water is constantly flowing round the system thus allowing the ASHP to monitor?

[JohnMo]

Just now, G and J said:

Am I correct in thinking the pump is running all the time?  i.e. the water is constantly flowing round the system thus allowing the ASHP to monitor?

Mine is, but you can also set it to sniffer mode, so the circulation pump run for a couple of minutes and off for 5 minutes.

[Dave Jones]

1 hour ago, G and J said:

So, if one has both the slab and fancoils all run together in a single zone, then one cannot run supercool for too long before the floor gets condensy (today's invented word!).

 

But if the fancoil at 12c cools a bedroom v quick is it ok to boost the lot for say 10 to 15 mins without risk of wet floors?

 

i ran a test at 12 for an hour and the floors were ok, we have karndean and it was still dry to touch. point is you dont need any lower than 16 anyway, just set and forget.

[Originaltwist]

@MikeSharp01 yon have an inquisitive and poineering approach to stuff. How about a go at one of these DIY jobs?

 

https://originaltwist.com/2024/06/26/diy-fan-coil-heater/

Re the condensation issue. I think the 'just in case' drip tray at the bottom is OK, then just try not to need it.

I have suggested a way for the unit to run automatically when hot water arrives but haven't sussed the cold water yet - any suggestions welcome.

Edited August 19 by Originaltwist
wrong link

[JohnMo]

26 minutes ago, Originaltwist said:

when hot water arrives but haven't sussed the cold water yet - any suggestions welcome.

Mine runs via two thermistors, one temp above for heating and the temperature below for cooling. A simple pipe stat x2 could do that. Normally open for one normally closed for the other.

[LnP]

I'm trying to get up to speed on this, so excuse me if these are daft questions.

 

We're planning an ASHP with UFH on the ground floor and upstairs bathrooms. We want to use the ASHP for cooling. We're got 3 phase electrical supply and plan ~11kW of PV. So following questions:

 

1. The ASHP will only need to run in cooling mode when the sun is shining and for a few days a year. Would it be true to say that designing the system to optimise the performance of the ASHP in cooling mode is not that important?

2. Upstairs I'm thinking about fan coils in the bedrooms for cooling, but with the possibility that they could also be used for heating in the unlikely event they are needed. Downstairs I'm thinking about fan coils for cooling, i.e. no cooling in the slab. With that configuration, why wouldn't you have the fan coils on their own circuit via a zone valve and plan for a flow temperature below the dew point, i.e. insulate the pipes and have condensate drains on the fan coils? If your flow temperature is limited by dew point considerations, doesn't it put more uncertainty around whether you're going to have enough cooling capacity?

 

Thanks for your understanding if these are silly questions or I've missed something!!

[JohnMo]

Zones means a buffer, so really would not go that direction. You need to look at system volume and flow rates. But if you have fan coil up and downstairs why bother with UFH? Your making life complex and expensive.

 

52 minutes ago, LnP said:

optimise the performance of the ASHP in cooling mode is not that important

Why isn't it important, if the heat pump is short cycling you may not get any cooling effect. Plus constant stop and starts will decrease the life of the unit.

 

I did timings last year based on PV being available, this year on and forget, runs as it needs 24/7 uses nearly no electricity. House much more steady temperature also, peaks in temperature when sun comes out are quicker brought down. Set a weather compensation curve for cooling and heating and have the system self modulating so it's either heating or cooling and never off. A simple high hysterisis thermostat to flick between the two, or simple switch. Keep it simple.