External heat sources like monobloc ashp, or pipes that might freeze up, could be protected from frost damage with an anti freeze valve. Its just a valve at the lowest point in the plumbing, that opens a bit when the temperature falls below 3C or so, to prevent freeze damage. Once it’s opened, slightly warmer water is present, and it will close again for a while. Link to one below.
https://www.seconrenewables.com/heat...lve-7537-p.asp