A2A sizing - am I on the right track?

[Gill]

I've been trying to work out A2A sizing as the one installer that's come back to me hasn't been able to come up with a workable solution with two outdoor units.  I think it's entirely possible but my calculations are giving me lower requirements.    I've had a read through this thread but would appreciate any input on my calculations so far. 

 

I've used a basic online calculator for BTU (it doesn't take window U value into account or underfloor insulation so it's possible our requirements are lower than the figures noted)

 

Requirements

• 4 indoor units / preferably 2 outdoor
• Living room.  Currently heated by 2 x 3.4 kWh storage heaters.   Room size 105 m3.  BTU 13586 / 3979w
• Bedroom.  Rarely heated but 2kWh panel was sufficient over the winter.  Room Size  40m3.  BTU 4794 / 1404w
• Bedroom 2.  Office room / spare bedroom.  North facing room  which gets very cold without heat.  Room Size  36m3.  BTU 4285 / 1255w
• SunRoom.  Would benefit from some cooling in the summer months.   Generally we do not use this room over winter but there will be the odd occasion (Christmas dinner) where we need to heat it.  Room size 68m3. BTU 11517 / 3373w.

 

The bedrooms I was thinking 1.5.  The sundroom, 3.5 unit I think would be fine based on our use.  The living room I'm unsure of as it does have an opening to the (unheated) kitchen but I thought 5kW unit would be sufficient.

We are not planning to rip out the storage heating this year as will provide fallback if I get this all wrong.  

 

The installer was looking at 7kW in the sunroom, both bedrooms at 2kW and 6kW in the lounge.  I did explain our use of the sunroom but I don't think that has been taken into account .  Their rationale for 3 outdoor units is that putting additional units on a single external increases the refrigerant quantity into apparently unsafe levels.  They've been back and forth with Daikin, Mitsubishi and Toshiba but doesn't seem to be getting clear answers so are recommending three outdoor unit as 9kW is going to be max capacity for one unit.  I have to say I'm in the dark on this - I thought you just looked at the max capacity on the external and took concurrent usage into account.  

 

With my calcs we could put the sun room and the living room on one outdoor unit and the two bedrooms on another.   With the house layout the living room would need to share with the sunroom otherwise we'd be looking at three outdoor units.   

 

I'm not sure how I should consider sizing the external units - do they perform better at particular loads?   It's unlikely we'd be using both the sunroom and lounge room on full power for either heating or cooling.  I was considering up sizing the external unit that will feed the two bedrooms in case we decide to put another unit in the hall.   Would that have a detrimental impact on performance?   

 

Appreciate any input.   Thanks

 

 

 

 

 

 

* correction - bedroom unit on back wall beside outdoor unit

Edited April 22, 2023 by Gill

[RichardL]

Looks good
To keep in mind - indoor units connected to a single outdoor have to be in the same mode;
You can have one on a mini split on heat while another is on cool -  I think you took this into account already - i.e. bedrooms split from living.

Power
Where is the fusebox - ideally if you can get a separate circuit for the outside units (in my opinion - I am not an installer) its nicer/cleaner than tapping into a local ring/socket.

Sizing
1.5 in bedrooms seems reasonable

3.5 in that living room would probably work - consider if you want a wall or floor unit - the floor units really pump out the heat - I've a 3.5kW indoor happily heating a large dining room/up the stairs and landing in my house.

Starting point for the outside units is add up the kW of the indoor's attached to them.
So very roughly (and I'm not an installer)  a 5kW outside the bedrooms and a 10kW outside the living room.

Depending on the manufacturer you can downsize by say 10-15% and the indoor units will get less heat/cool performance accordingly (search combination tables for rules on same)

Sunroom
The sunroom is the question mark for me - I read the cooling point - its going to push you from another 5kW to a 10kW - i.e. (again roughly) 1kW typical draw to double that. Depends entirely how you use it of course - but heating or cooling in an uninsulated (guess) space feels like it needs some thought.

If you open the bedroom door and the sitting room door will enough cold air filter into the sun room for those few days it needs cooling?


If you know the model you are likely to fit - try googling <model install pdf> <model combination pdf> etc... some of the detail docs are public.
Ultimately you need to trust the installer but you can go in with some knowledge.

[Gill]

9 minutes ago, RichardL said:

To keep in mind - indoor units connected to a single outdoor have to be in the same mode;

 

Thanks @RichardL for your detailed response. 

 

 

The current plan isolates our cold front room from our warmer backrooms so should be good here. We will need to run that bedroom in warming mode (day) while lounge /sunroom could require cooling. Our main bedroom doesn't need to factor in as we'd only cool occasionally and late night /overnight when the second bedroom won't be needing input. 

 

12 minutes ago, RichardL said:

Where is the fusebox - ideally if you can get a separate circuit for the outside units (in my opinion - I am not an installer) its nicer/cleaner than tapping into a local ring/socket.

 

Fuse box is in the hall. Separate circuit sounds sensible. Will investigate that. 

13 minutes ago, RichardL said:

3.5 in that living room would probably work - consider if you want a wall or floor unit - the floor units really pump out the heat - I've a 3.5kW indoor happily heating a large dining room/up the stairs and landing in my house.

Good to know. Its a bit of a guessing game going from E7 to on demand. Floor units I had discounted due to positioning. I'll admit I like the aesthetic of high wall mounted and our "easy install" option puts a unit on the rear wall. If we went floor unit that would be replacing the fireplace which won't work. Possibly there are alternative duct routings through the suspended floor that open up different positioning ( might require a pump for extract if I understand correctly). More reading required on my part. 

 

21 minutes ago, RichardL said:

sunroom is the question mark for me - I read the cooling point - its going to push you from another 5kW to a 10kW - i.e. (again roughly) 1kW typical draw to double that. Depends entirely how you use it of course - but heating or cooling in an uninsulated (guess) space feels like it needs some thought.

If you open the bedroom door and the sitting room door will enough cold air filter into the sun room for those few days it needs cooling?

The sunroom (uninsulated) gets quite hot but you are correct in that the heat can dissipate somewhat through the house. At this time of year on cold sunny days it provides a welcome free top up to the living room temps. Summer (if /when it comes to Glasgow) it gets hot in there. Last year it hit 36 with the velux open but that is rare . AC is more a luxury than a necessity so I'm not overly focused on getting excellent cooling in that room. 

 

Heating wise, over winter we only used it for Christmas day dinner. 2 x 2.4 kwh heaters kept it warm. Given the occasional use I'd be fine leaving a storage heater in there or using a panel heater as backup. 

34 minutes ago, RichardL said:

If you know the model you are likely to fit - try googling <model install pdf> <model combination pdf> etc... some of the detail docs are public.
Ultimately you need to trust the installer but you can go in with some knowledge.

 

Been looking at the Mitsubishi MSZ-LN range. Have downloaded the docs - Excellent advertised SCOP (whether that is accurate is anyone's guess). That range doesn't offer smaller models so will need to dig around. I've yet to find anywhere that makes it easy to compare. Orion is good but I've found their specs to differ from manufacturer. 

 

I'm hoping the other two installers come back to me. A big part of the problem is that (what little I know) I seem to know more than the installers which does not fill me with confidence! 

[RichardL]

@Gill  I've got Mitsubishi Electric - the Zen models though, one under your LNs.
Can't complain at the kit and the outside units quiet most of the time - when working hard its just the fan blowing air creating any noise.

Re the sunroom - if you use it at different times to the living room you can always undersize the outside unit a little since neither is on at the sametime.

Honestly - I'd bow to your fitter experience at that point though - my fitter had a 'gut feel' 'professional experience' call it what you will re inside unit sizes vs. room sizes which pretty much matched my BTU calcs. 


The other part to be aware of - the inside units on multi split all come with remote controls - they work fine, but by default you end up with a time clock on each inside unit rather than central control. 

I didn't ask my fitter re wired controls - instead fancied the project myself - sourcing components directly for a fraction of the retail price.


The Mitsubishi electric install and combination files are out there - sometimes on European or even Australian installer sites though.
Google:
MXZ-2F33-42_3F54-68_4F72VF_Combination_Tables.pdf   (3.3-5.4kW outside)
MXZ-3F54VF_Comination_Tables.pdf   (5.4kW outside)
MXZ-5F102VF Combination table.pdf   (10.2kW outside)

 

Edited April 22, 2023 by RichardL

[Gill]

10 minutes ago, RichardL said:

The other part to be aware of - the inside units on multi split all come with remote controls - they work fine, but by default you end up with a time clock on each inside unit rather than central control. 

I didn't ask my fitter re wired controls - instead fancied the project myself - sourcing components directly for a fraction of the retail price.

Id be happy enough rigging something up for myself. Was one of the negatives I keep seeing mentioned but it will still be a massive improvement on our E7 storage heating so I'm good with that. 

 

12 minutes ago, RichardL said:

Honestly - I'd bow to your fitter experience at that point though - my fitter had a 'gut feel' 'professional experience' call it what you will re inside unit sizes vs. room sizes which pretty much matched my BTU calcs. 

I'm hoping the other two companies I approached will come back to me as I'm feeling a bit like the non expert advising the experts. Hopefully that will change with an on premise assessment and further refinements. 

[Radian]

1 hour ago, RichardL said:

The other part to be aware of - the inside units on multi split all come with remote controls - they work fine, but by default you end up with a time clock on each inside unit rather than central control. 

If you have WiFi connected devices, like My Daikin A/C, then there's a scheduler that can be shared by multiple units. Investigate the smart options if available for the equipment you intend to buy.

[RichardL]

2 minutes ago, Radian said:

If you have WiFi connected devices, like My Daikin A/C, then there's a scheduler that can be shared by multiple units. Investigate the smart options if available for the equipment you intend to buy.

Yes -  personal decision only - but last thing I want is WIFI integration, apps, cloud etc etc.
(Worked in IT too long to have that hooked into my central heating - certainly not turning my house into an IT support zone). 

There is a lot to be said for an overall house level timeclock and a few zones with basic on/off/temp.

Mitsubishi Electric has some really simple 2 wire setups for wall mount buttons - just needs some digging.
A bit of phone cable and jobs a goodun.

Agree with the point of research up front!

[merlyn]

We have a 40+ year old warm air system already, a gas one. We are considering blanking off the upstairs ducting from the rest of the house and heating a plenum chamber with an A2A heat pump like the Daikin Ducted – Mini Sky Air R32 for instance. https://aircon-online.co.uk/product/ducted-mini-sky-air/ The current chamber has ducts off it for 2 bedrooms and study and bathroom already. The bedrooms are unheated throughout the winter (electric blankets to the rescue) The study and the bathroom are used at the different times so a modest unit can be fitted. The ducts room inlets have physical valves to open and close when they are needed. Very '70s tech. In summer the bedrooms get very warm and sticky so units will be operating overnight in air con mode.

 

The lounge  will have its own indoor unit and the gas heating will provide the hot water for now, otherwise it will be turned off. I couldn't help noticing that I could make your life really awkward by suggesting you could heat at least all the house from one indoor unit in the attic and another in your sun house.

 

Fascinating Thread so far. Thank you

[markocosic]

Careful cooling through ducts if they're not known to be insulated well. Condensation and mould/rot may ensue.

 

A ducted unit will need a return air plenum to operate - I assume that you've though this through but can we sanity check?

 

I assume that heating a single hall and leaving doors open is out of the question?

[merlyn]

The engineers putting in the ducts 40+ years ago insulated them. Warming the attic not a good plan much better it goes to bathroom. I'd renew the old insulation as I am of sound mind. 

The air is exhausted into the room whether hot or cold. Unlikely to be cold air into the bathroom though. 

Absolutely,  heating the hall and leaving the doors open is a very strange. 

Edited April 24, 2023 by merlyn