Central Fan Coil Unit - run from ASHP

[wardie9025]

Hi all,

 

Need some external advice for heating upstairs.  

 

Background - we are installing UFH downstairs throughout and removing all rads.  Heating to be driven from ASHP at max 35 degC flow temp, open zone control with weather compensation.  The whole slab comes up and down in temperature together.  I will DIY install MVHR from the wet rooms to supply bedrooms only but the 'official' ventilation method as far as building control is concerned will be trickle vents that we'll just keep shut.

 

The challenge is upstairs.  I don't really want big rads (really want to rip them all out to be honest so I don't have to worry about their pressure drop form the ASHP pump) and also want to be able to cool with the ASHP so thinking fan coil units and whilst I could go one in each room, I had wondered about a central unit in the loft.  I'd also like to be able to purge the house with cool evening air to prevent overheating in summer so....I was thinking two inlets to the FCU with dampers that either pull air from the landing when in normal heating/cooling mode) or the outside when in purge mode.

 

So my questions:

1.  Am I insane?  What am I missing that could make this more complicated than it needs to be?

 

2.  Does anyone know of a good controller that could do this?  If not my thoughts would be either a) I have some basic Arduino knowledge that I'd quite enjoy putting to the test on something real so I could build an override function for the purge mode and manually activate it, looping out the standard FCU controller or b) buy a relatively inexpensive PLC and pay a freelancer a few quid to design me a simple control ladder logic that I could upload into it.

 

3.  Is there a good way of running the MVHR and the FCU supply through the same ductwork or will  the FCU fan just completely override the MVHR unit fan?  If the latter I was just going to run two lots of ducts

 

Cheers!

[LA3222]

@wardie9025 A timely thread. A ducted FCU keeps coming back into my mind, even with the ASHP pumping cold water around it still gets warm upstairs. When I started building I very nearly put air con in but I didn't because lots of folks on here talked about using the UFH pipes and how that was good enough - that was, until J Harris actually bit the bullet and installed air con and then the thinking changed.

 

Too late for that ship on my house so I am looking to do the same as you. I haven't spent a great deal of time looking but it looks like a hard setup to find information/suppliers. I have the means to tap into a condensate drain and power in the loft. I was thinking like yourself, suck air out of the hallway and add vents into the bedrooms - should be sufficient to drop the temperature to a comfortable level to sleep.

 

FCU rads seem abundant but I do not want stuff on the wall, not so much for a ducted unit that could sit in the loft so I'm hoping this thread will yield some useful pointers.

Edited September 10, 2023 by LA3222

[Dave Jones]

we doing the same albeit full MVHR with a cooling unit on the incoming side taking a feed from the ASHP. 

 

For the first floor fitting panasonic fancoil units in place of rads to provide both heating (not really needed) and cooling in the summer. They do not need any 'central unit' just a flow and return just like a normal radiator.

 

https://www.aircon.panasonic.eu/GB_en/product/fan-coils/

 

 

[wardie9025]

11 hours ago, Dave Jones said:

For the first floor fitting panasonic fancoil units in place of rads to provide both heating (not really needed) and cooling in the summer. They do not need any 'central unit' just a flow and return just like a normal radiator.

 

 

Thanks @Dave Jones - I have looked at these but they are expensive to put one in every room (though I haven't done the sums on a centralised system yet, including my time to install).  The other thing is the size on the walls and the fact that they don't allow for any kind of passive purge for fresh outside air, to essentially get free cooling.

 

That said, I've heard a couple of people say upstairs heating isn't really needed with UFH throughout which I'm willing to try because we could always install a central unit in the loft later if needed but we do want to put some insulation in the ceiling/first floor to deaden some of the noise from the kids stomping/jumping around so this will reduce heat transfer upstairs.

 

@LA3222- I was looking at something like product: PAW-FC-D40-1/-R (panasonic.eu)

[Dave Jones]

the MVHR is the air mover no the coils. 

[MikeSharp01]

23 hours ago, LA3222 said:

that was, until J Harris actually bit the bullet and installed air con and then the thinking changed

Did he, I thought he was happy cooling the slab, I don't recall air con anywhere? We have lived with our build, not finished but windows / insulation in, pretty air tight and we nave not had a problem this summer / past couple of weeks with it over heating if we just put the upstairs windows in Tilt at night - and we cannot use the roof lights yet. I have planned a fan coil unit in the MVHR supply line and appreciate that this won't have a great effect but might be a valuable addition to slab cooling (50T concrete) for down stairs and, to be fair, our upstairs is only 50% of the downstairs floor area so maybe less work to do than many on the upstairs. 

[joth]

On 10/09/2023 at 07:45, Dave Jones said:

They do not need any 'central unit' just a flow and return just like a normal radiator.

Presumably also a condensate drain? Or you're limited to apex 13°C min temperature . Which to be fair is sufficient for our house 

 

[joth]

On 09/09/2023 at 22:33, wardie9025 said:

 

So my questions:

1.  Am I insane?  What am I missing that could make this more complicated than it needs to be?

 

2.  Does anyone know of a good controller that could do this?  If not my thoughts would be either a) I have some basic Arduino knowledge that I'd quite enjoy putting to the test on something real so I could build an override function for the purge mode and manually activate it, looping out the standard FCU controller or b) buy a relatively inexpensive PLC and pay a freelancer a few quid to design me a simple control ladder logic that I could upload into it.

 

3.  Is there a good way of running the MVHR and the FCU supply through the same ductwork or will  the FCU fan just completely override the MVHR unit fan?  If the latter I was just going to run two lots of ducts

 

1. The ducts for effective cooling needs to be much larger than MVHR pipe that just provides fresh air , not energy transport.

I found 200mm ductwork the sweet spot, along with 1m 3 slot linear grills.

The space needed for central air is all in the duct work. Depending on the building shape it maybe trivial or maybe a major headache. You may need an oversize trunk and branch layout, best done with solid galvanised ductwork 

 

2. I use Loxone which is hugely flexible but bit of a learning curve. The miniserver looks expensive but with no annual subscription but ongoing software updates I reckon it's pretty good value over its lifetime really

 

3. No. Use separate ducts. The only compromise I share the linear slot diffuser plenum in one room for both FCU and MVHR supply purposes. This was by chance (the only place i could put the fcu outlet was where the MVHR outlet already was) but works well.

 

 

I have a hybrid: one room with dedicated FCU and two others share an FCU. An electric baffle controls air to one of the two rooms. (the other is the office and we just let that get air all the time. It has a much longer duct run so find it naturally gets less preference when the guest bedroom outlet is also open)

 

In winter when no active cooling is needed I draw air for one bedroom from the other, circulating air between both rooms and the landing between. This really helps balance the temp across  the building (else the empty room gets cold and our room to warm). Probably not a wise idea for covid safety though, LOL

We nearly put an extra skylight in the loft space, wish I had as then we could easily purge the loft overnight which would supply all the cold air needed for all the night time cooling.

Getting airtight extremal electric baffles installed is bit of a faff. I've been eyeing up main MVHR supply pipe,to share that existing penetration. if I put a branch in it and a duct mounted inline airtight baffle I could open this to steal cold air into the FCUs. (The balancing exit path would be out the skylight over our hallway, I think even when raining it'd be safe to crack it open half an inch)

 

 

[joth]

4 hours ago, MikeSharp01 said:

Did he, I thought he was happy cooling the slab, I don't recall air con anywhere? We have lived with our build, not finished but windows / insulation in, pretty air tight and we nave not had a problem this summer / past couple of weeks with it over heating if we just put the upstairs windows in Tilt at night - and we cannot use the roof lights yet. I have planned a fan coil unit in the MVHR supply line and appreciate that this won't have a great effect but might be a valuable addition to slab cooling (50T concrete) for down stairs and, to be fair, our upstairs is only 50% of the downstairs floor area so maybe less work to do than many on the upstairs. 

He certainly did install an a2a, you even commented on it 🙂

 

 

Edited September 11, 2023 by joth

[LA3222]

5 hours ago, MikeSharp01 said:

Did he, I thought he was happy cooling the slab, I don't recall air con anywhere? We have lived with our build, not finished but windows / insulation in, pretty air tight and we nave not had a problem this summer / past couple of weeks with it over heating if we just put the upstairs windows in Tilt at night - and we cannot use the roof lights yet. I have planned a fan coil unit in the MVHR supply line and appreciate that this won't have a great effect but might be a valuable addition to slab cooling (50T concrete) for down stairs and, to be fair, our upstairs is only 50% of the downstairs floor area so maybe less work to do than many on the upstairs. 

Yep, he did. Loads of threads on it as he put one of the pre gassed units in at the top of his hallway if I recall correctly.

 

I read a lot on this forum before I started building and learnt a lot. There is some bits on here which I think are pushed out with rose tinted glasses on.

 

My house is 280m2, air tightness is well into passive and same for insulation. The super insulted and airtight house works against you in the summer as once the heat is in its a pita to shift. I have ufh in the slab downstairs and in a 50mm pug screed upstairs.....running the ASHP in cooling mode is pretty pump and not enough to keep the house cool. It certainly helps but is insufficient by itself.

 

I would advise all new self builders to consider additional cooling mechanisms for the bedrooms, whether that's FCUs, aircon or whatever. The climate is supposedly only going to get hotter🤷‍♂️

[wardie9025]

@joth - thanks very much for the detailed response!  Very helpful.

 

Some follow on questions If I may....:

1. What are your flow and return temps for the water side to the FCU in heating/cooling mode?
2. Are you able to advise what make/model or FCU and dampers you used.  I know you can get a few different brands but I'm keen to understand what people have experience with.
3. Are the baffles (dampers?) binary position or modulating?  How do you control these/did you programme the Loxone yourself?

 

Thanks!

 

[MikeSharp01]

23 hours ago, joth said:

He certainly did install an a2a, you even commented on it 🙂

I didn't recall that - my brain is getting  less well connected every day - anyway I think it makes sense to provision for one in the main bedroom of our place as its only a couple of holes through  the fabric.

[joth]

16 hours ago, wardie9025 said:

@joth - thanks very much for the detailed response!  Very helpful.

 

Some follow on questions If I may....:

1. What are your flow and return temps for the water side to the FCU in heating/cooling mode?
2. Are you able to advise what make/model or FCU and dampers you used.  I know you can get a few different brands but I'm keen to understand what people have experience with.
3. Are the baffles (dampers?) binary position or modulating?  How do you control these/did you programme the Loxone yourself?

 

Thanks!

 

1. The buffer tank* currently runs at 17°C setpoint +/-2.2 hysteresis, so 14.8 - 19.2­°C

I would like to run it a tiny bit cooler than that, perhaps 12-17, but I need to finish some (a lot) more pipe insulation work before I can do that, and frankly this config got us through 8 people staying over on the hottest day of this year, so I think it's not bad

 

I've never needed to use them for heating, but in theory if they did it'd run at 24 +/- 2.2. It should probably be a bit higher than that to be effective, especially given no condensation risk on heating.

 

Loxone does have logic blocks to control flow temp based on load+weather compensation, but I'm not using them because wiring that into a mitsu FTC6 is non-trivial. (Either has to go through their gross Melcloud cloud API, or I need to rip out the melcloud wifi dongle and replace with modbus interface)

 

2. The smaller is a panasonic (Systemair) one, and the larger is something salvaged from the restaurant refit, details here and here.

 

3. Dampers are really basic on/off things from aliexpress

https://www.aliexpress.com/item/4000627671878.html

Loxone can modulate valves (and windows and shading) based on time of run but never attempted to apply it to these as the time of run seemed totally random. And for our use on/off seems fine

 

Now I've proven the whole lot works well, if doing it again I'd do it properly with galvanized damper and belimo actuator

 

Damper damper damper. I'm baffled why I keep calling them baffles.

 

 

* - Note that while my system was designed without buffer tank and does not need it for UFH, I found adding FCUs very much necessitates addition of a buffer tank, as the nature of blown air is it's mostly useful just when the room(s) are occuplied, which can often mean very low kW demand on the heatpump which led to egregious short-cycling prior to adding the buffer. So I now have an unusual but IMHO very sensible design where UFH runs without a buffer in winter, but the FCUs run with a buffer in summer.

 

 

Edited September 12, 2023 by joth

[Duncan62]

Couple of  FCUs:

No idea if they'll sell to consumers.

 

https://www.troxuk.co.uk/fancoil-units/pwx-ea11f88aee974359

 

https://www.advancedair.co.uk/products/fan-coil-units/epic-ecm-fan-coil-unit

[wardie9025]

On 10/09/2023 at 19:31, wardie9025 said:

That said, I've heard a couple of people say upstairs heating isn't really needed with UFH throughout which I'm willing to try because we could always install a central unit in the loft later if needed but we do want to put some insulation in the ceiling/first floor to deaden some of the noise from the kids stomping/jumping around so this will reduce heat transfer upstairs.

 

To update on this one I can confirm that following the cold snap, we do actually need heating upstairs! 🥶  To be fair no loft insulation in a good portion of the house yet though as contractor is being slow and I'm running duct work but gonna get it in for sure after the last week or two!  Looking at 150mm duct work for heating as primary use. 

 

@joth - note you said 200mm as sweet spot but this was more for cooling?    Would be really keen to understand your set up and experience?

 

Cheers!

[joth]

15 hours ago, wardie9025 said:

heating as primary use. 

 

@joth - note you said 200mm as sweet spot but this was more for cooling?    Would be really keen to understand your set up and experience?

Correct, as mentioned it was just installed for cooling.

This winter the logic has cut in a couple times to try and heat the office. So far they don't actually seem to do much compared to heating. Not sure if it's because the outlet vent is up at ceiling height, or the flow rate is not high enough to transfer much energy. It's really not necessary so I've not investigated much.

Bigger issue is the buffer tank (volumizer, in line on the zone return leg) does not seem so effective for cooling, the ASHP runs for a very short burst. Imagine this is because I plumbed it for cooling with the inlet on the bottom, draw off at the top. With hot water it raise straight to the top so the ASHP quickly sees a warm return temp. 

[JohnMo]

One thing I have noticed with our fan coil for heating, is if the heat pump switches off for a period of time, or is doing hot water, the room temps drop, as the fan keeps going and start active cooling.

 

You need a buffer really, which is in effect kept hot by the heat pump, so the fan coil can have a steady stream of similar temperature water and is not affected by any off periods of the HP.

[LaChab]

I was wondering if a few of these would work if they were fitted into the ceiling........ https://www.myson.co.uk/products/kickspace.htm

Even with water at 40c, just one could probably contribute 1kw, enough for a whole well insulated house! And they're cheap enough to fit in each (downstairs) room.

Run all day off nightime off-peak heat.

There's a sophisticated variant, but it's a bit pricey. https://smithsep.co.uk/catalogue/caspian-universal-concealed-fan-convector/

[joth]

Yeah not bad shout on plinth heaters especially if you could install them at floor level.

If paying more for the fancy unit I'd want something that could cool too (ie wih condensate tray).

And why on earth did they name it a UVC 🙄

[wardie9025]

2 hours ago, JohnMo said:

One thing I have noticed with our fan coil for heating, is if the heat pump switches off for a period of time, or is doing hot water, the room temps drop, as the fan keeps going and start active cooling.

 

You need a buffer really, which is in effect kept hot by the heat pump, so the fan coil can have a steady stream of similar temperature water and is not affected by any off periods of the HP

 

Thanks @JohnMo - I was thinking about this the other day and worrying a bit imagining the ASHP COP dropping significantly because of all the potential short cycling or just the rooms getting cold.  On reflection though we have a thick (85mm, a bit more in some places) screed that seems to be doing a good job as a storage heater so this will help hold the thermal inertia in the system I think.

 

 

1 hour ago, LaChab said:

I was wondering if a few of these would work if they were fitted

 

Like the look of those and probably less hassle than a central unit - it wont do my external free cooling though and this is quite important to me in our project.

[JohnMo]

16 hours ago, wardie9025 said:

On reflection though we have a thick (85mm, a bit more in some places) screed that seems to be doing a good job as a storage heater so this will help hold the thermal inertia in the system I think.

We also have UFH in the main part of the house, the fan coil is in a summer house we always keep hot. UFH used as a big buffer.

 

I would normally batch charge the floor in the house on cheap rate, but doing that we had issues with the fan coil and heat pump short cycling during the day, fan coil stopped heating effectively.

 

So have reset back to weather compensation running 24/7. For this to work we have set the WC to match the fan coil (10 degs OAT 31, -6 OAT 36) and use an electronic ESBE mixer for the UFH in the house. 

 

ESBE mixer can run two different temperatures (T and T2) via a volt free contact.  Our house thermostat moves the control point between T and T2, depending if the house is above or below 20 degs.

 

T is set for a fixed flow temperature of 28 deg and T2 set to 36 so it takes whatever flow temp is available from the heat pump. So above 20 in house, fixed flow temp, below 20 variable flow temp (based WC curve)

 

Today it's 7 degs OAT, flow temp is about 31.5 from ASHP, house mixer about 80 to 90% open flowing 28 degs, heat pump is cycling 30 mins on, 30 mins off. During an off cycle the flow temperature drops to about 23 degs before the heat pump restarts, so mean temperature is just below 30.

[JohnMo]

Forgot to mention at the moment we are doing hot water via immersion during cheap rate as a trial for a few days, while I get the weather compensation curve setup. Found there is quite a delay restarting CH mode after DHW mode, while everything cools down from 50+ to low 20s to enable restarting CH mode.