Position of ABV

At the moment the bypass for my boiler is a 15mm gate valve.

The boiler is in the garage. 22mm pipe goes from it to the ceiling, runs along inside, then splits. 22mm heads off downstairs to heat the ground floor, the other half of the split reduces to 15mm to heat upstairs. The heating feeds a couple of rads before it reached the gate valve under the floor. Pipework carries on heat the bathroom and the other bedrooms.

All the rads bar the bathroom have TRVs fitted and I'm going to fit a TRV in the bathroom later in the year.

My question is, can I swap out the gate valve with an ABV at it's current location?

as long as the gate valve is before any 2 port zone valves then yes you can

As said, as long as theres no zone valves before it then yea its fine to replace it where it is. Assuming its a combi boiler?
Just pay attention to the direction of the flow of water round the system, the auto bypass will have an arrow on it and must be fitted so that the water is flowing in the direction of the arrow or else it wont do anything at all

No zone valves and it's a modulating combi boiler so straight swap.

Looking up the boiler manual, it says the pump head min flow rate is 28.5l/min.

Now in my naive world, I'd set the ABV to 28.5 but I did read somewhere a complex method of looking at pump head and calculating pipe resistance and wind direction to come up with a figure. And a counter method of boiler on full, open the ABV until there was flow then turn it back one turn.

And 15mm ABVs are extortionate so it it looks like it'll be a 22mm with a couple of reducer