VAT -install and supply ASHP

I'm a subcontractor supplying and installing a heat pump in a residential property. I'm invoicing the main contractor rather than the homeowner. Do I zero-rate the VAT on my invoice to the main contractor, or should they apply domestic reverse charge and CIS?

Hi 
The domestic reverse charge only applies to standard rated (20%) and reduced rated (5%) supplies not zero rated supplies.
Guidance on the domestic reverse charge can be read here:
VAT domestic reverse charge technical guide and I would advise you to look at the sellers flowchart here:
VAT domestic reverse charge technical guide Flowcharts
The conditions that need to be met in order for a heat pump installation to be a zero rated supply can be found in notice 708/6 here:
Energy-saving materials and heating equipment (VAT Notice 708/6)
Thank you.