Air-to-air ASHP replacing warm air heating

[Gooman]

10 minutes ago, severnside said:

I can provide some update here. We discussed our situation with a commercial AC company. They have looked at ducting and it's insulation condition. They think a ducted AC unit can work in our situation. We are going to get it installed next month and see how well it performs. We are getting this unit to replace Johnson & Starley Aquair (with Grant Oil Boiler)

https://www.orionairsales.co.uk/mitsubishi-heavy-industries-air-conditioning-fdum140vh-ducted-ceiling-concealed-14kw48000btu-r32-a-15031-p.asp

 

AC Company will make some custom Plenum to connect to existing Warm Air ducting. This will also provide option to add some fresh air.

 

I've heard from other users and from AC companies themselves that there could be some gotchas here - but no-one has any hard data on real-world examples! There's been various musings about difference in cross-sectional area of ducting in a typical warm-air system being much, much less than in a typical AC installation and the effect this might have.

 

It would be great to get some results from an actual installation - both subjective and specific (airflow rates and temperature curves).

 

If that's not a problem, the other potential issue is if you run the AC in cooling mode - the original ducting wouldn't have been designed for that and might have problems with condensation. That's going to be much harder to identify on a test - other than maybe putting a humidity probe and/or camera down the ducting to see what happens.

[Gooman]

@Jeff V The other option is to go with something temporary until the Heat Pump/Duct tech is better proved ... second-hand J&S units do crop up on eBay from time-to-time, and might be an option if you have a local J&S-certified engineer who can install it.

[Ommm]

I am a novice at this, but if I understand it right the Aquair is just a big fan coil box: hot water goes into a heat exchanger, fan blows air over the fins, heat is transferred from the water to the air.  In an air2air system the emitters are roughly the same, only with refrigerant fluid rather than water.

 

As discussed on other threads on this forum recently, fan coils don't seem very complicated to make.  It's a case of having a suitable fan/heat exchanger combination to do the transfer, and sizing it for enough airflow/heat exchanger area to transfer the amount of heat at the flow temps that you want (I agree it's not obvious how to calculate this).  Fan coils for ducts are very common in commercial premises.

 

As you say, the diameter of your ducts is going to limit how much air you can deliver and the temperature of that air how much heat.  But you can still increase the airflow (until you hit limits like noise) and the flow in the fan coil.  Perhaps you might be able to divide your ducts and install multiple fan coils in strategic locations - that increases the total heat exchanger area and so each one has to work less hard.

 

For cooling, if you can get at the outside of your ducts you can (and should) insulate them.  If they are buried it would seem to me condensation is less of a problem - there's no free air hitting the outside of the duct.  You might eventually cool the concrete/etc to the point where it drops below the dew point, but that might take a lot longer and offer more chance for evaporation if you aren't running the system continuously.

[severnside]

1 hour ago, Gooman said:

I've heard from other users and from AC companies themselves that there could be some gotchas here - but no-one has any hard data on real-world examples! There's been various musings about difference in cross-sectional area of ducting in a typical warm-air system being much, much less than in a typical AC installation and the effect this might have.

I compared the air flow rates. My current Aquair S20's flow rate is 20m3/min. I never feel or hear the air noise. I have put my feet on ducts to feel it's working but it maintains the temperature. The Ducted AC units airflow rates are from 22m3/min to 48 m3/min. So I believe it should be OK.

I discussed condensation risk with the Engineer. He mentioned that all the ducting is metal ducting and wrapped in insulation so risk of condensation is low. Also we won't be running air temperature too low even for cooling.

Not many companies were interested to do retrofit so we are doing it on risk. If it works then it will be cheaper than UFH install cost itself.

[severnside]

I can share some details on our install. Air to air ducted unit was installed yesterday. There was very little disruption in the house as Air handling unit was in Garage it was replaced by Ducted unit and old oil tank replaced by external unit. 

It was 25C yesterday so we tested cooling mode which worked really well. It was modulating reasonably well and consuming about 1.4kw during cooling. 

At full speed the unit is quite than previous Johnson Starley unit it replaced. 

Internal controller uses 2 wires (used old thermostat cables) and give access to lot of functionality. 

There is option to add fresh air to system with one duct connected to outside fresh air and controlled via manual damper

 

 

Edited June 24, 2022 by severnside

[severnside]

[severnside]

[severnside]

I can see lot of activity in other threads about adding cooling options to ASHP. I will update my experience based on recent heatwave to provide reference for anyone visiting this thread

Positives

+ Cooling worked really well and system was working at half capacity most of the time. System maintained temperature around 23C

+ We only ran cooling during solar generation, so no additional cost of running. We averaged about 12-15kWh usage (guesstimate)

+ Not much noise from ducts or external units

Negatives

- Warm Air heating Ducts are designed for heating so downstairs gets more air than upstairs. Cooling needs opposit of that, so upstairs wasn't as cool as downstairs

- System was modulating to lower capacity with auto mode even though set temperature wasn't reached. I could override it by chaning fan speed

- Need to ask everyone to close windows in summer while AC is running

Edited July 20, 2022 by severnside

[Gooman]

@severnside - really interesting results!

In the pic below your thermostat of the ducting in the garage - is that the new ducted unit? What of the ducting shown there is original? 

Our current J&S unit is in a brick-built cupboard with ducting above (for 1st floor) and below (underfloor for ground floor). I doubt the internal unit could be fitted in the ceiling of that cupboard and connect to the existing 1st floor duct - there's just not enough room to work in. 

Do you know if the unit you have can be mounted flat on a wall, or does it have to be in the orientation shown?

[severnside]

@Gooman

Yes, the ducted unit shown suspended from ceiling is the new unit. Original ducting in this image is covered in black insulation.

You can see the second picture with original J&S Aquair unit to show original ducting. 

 

I believe the unit should be mounted horizontally for condensate tray to work properly. If used only for heating then that will not be an issue. The installer suspended it from ceiling to reduce

vibration noise, i did ask him to install on existing base used for J&S unit

On 24/06/2022 at 09:53, severnside said:

 

[RenewableNeil]

@severnside Your set up looks really interesting.  How is it performing in the colder months in terms of COP heat output and comfort levels?  

 

I have a 30 year old Lennox G10 Gas Warm Air heater that will need replacement soon, and I’d like to use the existing ductwork of the property with an air con unit - very similar to your set up.  The difference with ours is that the boiler is inside the house, so noise levels do become an important consideration.  Also I see from your photos that your unit is hanging off a concrete slab.  We would have joists there - I’m wondering if they would that be strong enough to hold this unit.

 

Thanks in advance!

[severnside]

@RenewableNeilUnfortunately we are in middle of renovation and we had to move out. So we couldn't test in winter season. We did test it in summer and air flow was better and quiter than previous system. The unit is not very heavy, check Mitsubishi specifications. It should be possible to mount it to joists. Do check noise specs from Mitsubishi if it will be indoor for you, ours in separate Garage

[earlsfield]

@severnside

 

We are in a similar situation to you, in that we have a 1970s building with a ducted heating system powered by oil and are wondering if an air to air heat pump would be a good replacement.

 

I was wondering what your experience has been over the winter and how your solution had held up? 

[severnside]

@earlsfieldwe are still out of house and build work is ongoing. We will be able to test it next winter. I have received lots of private messages around this topic, so I will do a follow up detailed post once I test it properly. 

[robin coles]

"Also a colleague of mine is having a new build done in France and he's having a Mitsubishi air to air heat pump distributed through ducting to wall outlets. Each outlet is motorized and has it's own thermostat. 

 

The gear my mate in France is using is here.

 

https://www.orionairsales.co.uk/mitsubishi-electric-air-conditioning-pead-m100ja-ducted-concealed-inverter-heat-pump-10kw36000btu-r32-a-240v41550hz-9921-p.asp

 

However, rather than the internal bit being a slimline ceiling void unit (as shown in that link) he's having a rectangular box type unit that sits in an internal cupboard and, I assume, receives the warm air from the external heat pump before sending it into the ducting."

 

I can now provide an update to my mates system in France. As above it used the Mitsubishi heat pump but the inside bit was a French company called Aldes, specifically the Aldes T One Air. This has as least two models; one for air only and one that does air and domestic hot water.

 

https://www.aldes.fr/products/t-one-air (right click - convert page to English in Google) 

 

You can clearly see that it's basically just a heat exchanger and fan assembly unit. Below is my mates installation

 

 

He told me the two white flexi hoses are two and from the heat pump. The two grey on the RHS are flow and return to his domestic hot water cylinder (I assume this may be standard immersion type but with two elements i.e. one heat pump and one electric top up). The other grey lead must be power. As I said previously his goes into a ceiling void ducted network and then down in the wall to wall vents, each one with a thermostatically controlled auto shutter. This is on a new build though. 

 

They also do the https://www.aldes.fr/products/t-one-r-aquaair as below which integrates the immersion cylinder

 

 

Unfortunately still not found such a product in the UK and just been rejected by another two installers. Simply won't touch air to air. 

 

@severnside in your pic of the new distribution unit are each of the four outputs going to individual rooms i.e. 4 rooms or do they sub divide again? It's just that size ducting (obviously I'm guessing at the size ~100-150mm diameter maybe?) seems very similar to what we have from 1965! One of my concerns though is that our current night storage heater supplies warm air to 10 outlets, though one is now blocked off. And even that leaves 4 rooms that were never part of the main heating system so currently have stand alone Dimplex Quantums. But if we hoped to include them on the ducted network with an ASHP that's 13 outlets that look very much like your 4.

 

 

 

 

 

[severnside]

On 24/06/2022 at 09:53, severnside said:

Hi Robin, If you see the image above, this is picture of old warm air system. Top part is return duct and bottom part is supply duct. There is no individual room level control other than vent louveres. The 4 pipes you see are connected to return (3 return and 1 fresh air if needed) to provide volume. There are 3 further pipes not visible in photo connected to supply ducts. These old warm air ducts start as one duct and then branch off into individual rooms.

So it should be possible to use exisitng ducts in your case. We also built an extention and it wasn't possible to extend ducts there. We put 3 Air to Air split units in new part connected to a separate outdoor unit. Hot water is direct electric immersion connected to solar panel diverter.

[robin coles]

Hi Severnside, 

 

Thanks for the answers, I get it now. 

 

As you say ours is the same idea; a big rectangular main trunk line is fed warm air from the plenum chamber and this then branches off to the rooms. And our water is identical to you so haven't got that additional load for any potential ASHPs. 

 

Can I ask "We put 3 Air to Air split units in new part connected to a separate outdoor unit"  So is that something like the ceiling mounted AC units in each room or do you mean it's one of the ones that get hidden away but with 3 ducted outputs from the heat exchanger? And heated by an separate outdoor unit? So you've got two ASHPs have you? 

 

In our case the heating demand from those three additional rooms we've got is not always needed so we could go for some of these electric panel heaters that so often get advertised these days. But for now we have to stick to the Dimplex Quantums as due to the main house heating we have to be on an Economy7 tariff. 

 

Thanks

 

Robin

[severnside]

On 06/07/2023 at 20:22, robin coles said:

Can I ask "We put 3 Air to Air split units in new part connected to a separate outdoor unit"  So is that something like the ceiling mounted AC units in each room or do you mean it's one of the ones that get hidden away but with 3 ducted outputs from the heat exchanger? And heated by an separate outdoor unit? So you've got two ASHPs have you? 

We put 3 wall mounted split AC units connected to an additional outdoor units. So we have 2 outdoor units, one for existing Warm Air ducting and other for 3 additional split ACs. We did look into ducted for new rooms but these will be used occasionally so having 3 separate units provide more flexible control.

[Archer]

Just to add onto this thread for some further advice and to share the latest with our plans. We are also in the position of updating a 70's gas Warm Air system (J&S). 

 

We've got two quotes for two different solutions that we are mulling over;

 

Option A from one Contractor is all electric, using a ducted air to air system; to install 2x Outdoor Air to Air HP units and 2x indoor units - one serving the ground floor using our existing steel ducts and the second serving the first floor, with new ducts and ceiling supply and return vents in each room (they are currently in the floors). The kit for both would be 12kw  Mitsubishi Heavy Industries HP's. 

 

Option B from a different contractor is a hybrid system - a gas Multi-Calor boiler and air handling unit on the existing ducts (ie. both floors), with a separate Daiken 12kw HP which provides backup/ mild weather heat if needed but is optimized as an A/C cooling unit for the whole house. 

 

We're trying to decide between them now - the gas hybrid solution is around 15% cheaper but the warranties are just for a single year against 7 years for the twin A2A option. Obviously there are other pro's and cons as well. 

Edited July 27, 2023 by Archer

[Archer]

@severnside with your system, are the controls quite intuitive can I ask? On the Multi-Calor there is some smart tech which modulates the fan speed right down as rooms near their target temperature to try and avoid some of the on/off hot/cold effect that you can get with blown air systems... Does your Mitsubishi ducted A2A do something similar or is it a bit less intuitive? Also, when weather is really cold in the winter do you tend to notice the house getting cold during defrost cycles? 

 

My other main worry is around the sizing. It just seems odd that one guy reckons a 12kw A/C will work for the whole house while the other has specified double that essentially... I trust him but neither engineer seemed very interested in looking at room heat loss calcs, it was all "rule of thumb" stuff and both advised over sizing rather than risking the units being undersized. I wondered if for the all electric option part of the thinking was to find a unit with a big enough fan to push enough air round the ducts for a whole 60m2 floor of a house. 

 

What do you think, should I be asking more questions about the sizing...?

[sharpener]

50 minutes ago, Archer said:

What do you think, should I be asking more questions about the sizing...?

 

With a 2:1 disparity yes I think you should. Either one of them is going to cost you much too much or the other one will not heat your house.

 

Remind us of its age, construction and floor area and we might be able to suggest something.

 

For reference I have had several estimates now and am reasonably confident a 12-15kW HP will be enough for a 200 m2 barn conversion in the SW.

[Archer]

6 hours ago, sharpener said:

 

With a 2:1 disparity yes I think you should. Either one of them is going to cost you much too much or the other one will not heat your house.

 

Remind us of its age, construction and floor area and we might be able to suggest something.

 

For reference I have had several estimates now and am reasonably confident a 12-15kW HP will be enough for a 200 m2 barn conversion in the SW.

 

It's not exactly like for like as one quote assumes that a 15kw gas burner (the Multi-Calor Udara) will do the heavy lifting for winter heating, whilst the 12kw heat pump is used primarily for cooling but can also provide backup/ shoulder season heat. He was suggesting that because of the refrigerant gas it doesn't run that efficiently at the higher temperatures needed when the weather gets cold. If we have the gas option we are really limited on which HP can be used because it needs to be compatible with the Udara and also the Lennox cooling coil that goes on our warm air. 

 

Our property is a 1970's build, detached with around 130m2 GIA. Insulation varies from 75mm to 100mm mineral wool in the walls and 250mm in the roof. It's mostly timber frame - actually quite well constructed, but the air tightness won't be great. 

 

My instinct was that a 12-15kw unit would be correct but unclear whether there's something about retrofitting 2x ducted A2A units into the ducts that is giving some constraints. The engineer seemed to really struggle to find appropriate indoor units that would be large enough to push air around 10 or so outlets per floor. He hasn't said it explicitly but I wondered if that's because of a fan issue, not necessarily the overall capacity of the heat pump... But obviously we don't want a system that's hideous to run because it's greatly oversized. 

 

Just wondered if anyone on the forum has come across something similar?

Edited July 28, 2023 by Archer

[Gooman]

Apparently, J&S are rumoured to be working on an air-to-water ASHP unit + heat exchanger that would fit on the same ducting footprint as our current J&S unit.

 

As it's air-to-water it would be eligible for the government grant (air-to-air isn't).

When it might see the light of day is anyone's guess though!

[Archer]

On 28/07/2023 at 12:10, Gooman said:

Apparently, J&S are rumoured to be working on an air-to-water ASHP unit + heat exchanger that would fit on the same ducting footprint as our current J&S unit.

 

As it's air-to-water it would be eligible for the government grant (air-to-air isn't).

When it might see the light of day is anyone's guess though!

Just my luck that this will be launched right after we've finished our bespoke, custom job! 

[severnside]

On 27/07/2023 at 23:25, Archer said:

@severnside with your system, are the controls quite intuitive can I ask? On the Multi-Calor there is some smart tech which modulates the fan speed right down as rooms near their target temperature to try and avoid some of the on/off hot/cold effect that you can get with blown air systems... Does your Mitsubishi ducted A2A do something similar or is it a bit less intuitive? Also, when weather is really cold in the winter do you tend to notice the house getting cold during defrost cycles? 

@Archer The controls are basic but sufficient. These are similar to commercial units you might see in offices. You can set the temperature and turn on / off. It feels different as it's always running on low setting once temperature is achieved instead of on/off like radiators.

 

On 28/07/2023 at 06:39, Archer said:

My instinct was that a 12-15kw unit would be correct but unclear whether there's something about retrofitting 2x ducted A2A units into the ducts that is giving some constraints. The engineer seemed to really struggle to find appropriate indoor units that would be large enough to push air around 10 or so outlets per floor. He hasn't said it explicitly but I wondered if that's because of a fan issue, not necessarily the overall capacity of the heat pump... But obviously we don't want a system that's hideous to run because it's greatly oversized. 

The way I analysed it if it was sufficient was to compare it with existing system. Our exisiting unit was Johnson Aquair S20 which can push 1224m3/h. Mitsubishi FDUM 140 can push 2880m3/h (Max) and 1320m3/h (Min). We found that it's better than previous unit in pushing air to furthest outlets and much quiter. 

For heating capacity analysis our previous boiler attached to Aquair S20 was 20kW, which was not running all the time. I did a basic heat calculation using the spreadsheet available on the forum and asked the installer to install one size bigger than calculations. AC installers always over spec and use rule of thumb. Also they calculate based on cooling load. AC unit heating capacity is higher than cooling, so if you spec for cooling it would be over spec for heating. These units do modulate a lot so overspec is not bad, e.g. our 14kW Mitsubishi can modulate 5.0 to 14.5kw

 

We haven't used heating much on the unit since install. We have just moved back in the house after renovation, we turned the heating on yesterday to heat the empty house and it was working well. We are still to test in full winter