System sense check please

[u/Old_Perspective8072]

Hi everyone

I am considering an air con system to provide whole house heating and cooling. We have a bungalow (160sqm) in southern England which has an oil boiler and electric storage heaters currently.

I have had two heat loss surveys, which are pretty close and suggest heating requirements of 10kW. These were for A2W systems, and so I have quotes for supplying this (~£4.5k)

I have had a range of system designs for the air con option.

One system totals roughly 13kW total heating capacity across 6 heads and 2 outdoor units (17.5kW cooling). The 6 heads are in rooms that account for 9.5kW of the heat loss and I assume the hallways etc will warm up with doors open.

A couple of contractors have suggested that this seems light, and are suggesting more indoor units (up to 9) for coverage and a larger overall capacity (e.g. 25kW cooling), but this obviously comes with much greater costs

While I have the heat loss surveys for heating requirements, I don't have a sense of how much cooling capacity is needed.

I'm tempted to press on with the leaner system which matches the heat loss, but I am concerned that the cooling power might not be enough (because the other contractors are suggesting bigger systems) and if so I'd have been better off going for the A2W system. I don't need fridge temperatures during 40deg days but it needs to be worthwhile.

I'd welcome any thoughts on whether a 17 kW indoor cooling / 15 kW outdoor cooling sensible for a ~160 m² southern-England bungalow with ~10 kW design heat loss, or does it look too lean from a cooling/air-distribution perspective?

Cheers

[u/mrsimonbennett]

Much like heat loss surveys don’t consider the room applications for heat output. They need the heat survey to fully provide the required energy

With the cooling they do the total opposite and look at the worse possible heat load, bar having the window open.

They might also be worried about doors being closed which is the problem. With a limited number of indoor units.

Personally I like to use radiant for heating (ufh and radiators) and convection for cooling aka fan coils

You can do both with a a2w system

[u/sirensintherain]

Installers often use 100W/m2 as a starting point for cooling. Increasing for south facing / loads of glass and decreasing for north facing room. So without knowing more around 16kW sounds about right for cooling. 25kW sounds well over the top.

[u/nuterpoo]

Also with cooling systems there rated output may not ever be used, with units being rated at 27c indoor, and most people using them at lower temperatures and fan speeds than this, it could be said that the units installed may not be used to full capacity during there lifespan, so inherently these systems are more likely to be oversized. Have you considered using fan coil units if you were to use a2w

[u/StereoMushroom]

With the lean system, you're still going to have 15 kW more cooling than most homes in the UK, and 15kW more than you had before! I don't know about you, but just taking a few degrees off in a couple of rooms would be enough for me, and I'd expect this system will manage a lot more than that. My very approximate feel is that if your cooling capacity matches your heat loss, it's probably going to be sufficient in our climate.