have you ever un frozen a hot water central heating tank there bad enough in lofts but its in a greenhouseThe feed and exp tank shouldn't need to be that high above the boiler. Where the cold feed goes is called the neutral point and probably best just before the boiler and the open vent placed just after it but no valves in between boiler and open vent. The water level in the tank will vary a little bit with the water expanding as it heats and also from the pump. You would just be looking at a domestic pump running at low speed I'd recon so its not going to cause much change in water level. You would be better putting your pump on the return and pump back towards the boiler as it will not get as hot. I would think 3 foot or so would do above boiler to water level in the tank. I would just be careful as I think its likely to boil over pretty quick and you would be better maybe heating a tank of water and pumping from the vessel to your pipe radiator.
remember the boilers stopping working due to the valve pipe being frozen on the outside
both are the same thing
even alotments have frozen protection for there pipes but greenhouses are really bad for it
ive had to deal with frozen pipes in houses and factorys even with protection theyve still frozen