Any gas boiler's HEX can stand a static head of 30M, 3.0bar, because the PRV (pressure relief valve) is set to this, the question is, what is the maximum differential head they can tolerate, most will have 2.5M to 3.5m dP at their rated output at a dT of 20C. The boiler that one of these will be replacing is a (Vaillant) 438 ( heat only) which has run for years with a 8M pump, but this doesn't mean that the boiler HEX was operating at a dP of 8bar or anywhere near it as the dP is spread across both the CH system and the boiler HEX, if the pump is on the boiler flow then even less of a problem as the remaing head after a external ABV should be even less)
Even if the system (Vaillant) boiler is capable of withstanding a very high dP its a bit academic because its circ pump head (of 6/7M) is only capable of giving its max flow rate (of 17.92LPM for a 25kw boiler) at a dP of ~ 4.0/4.05M so depending on the boiler HEX dP will result in very little remaining head to drive it through the system., hence the requirement for LLH or the like if high flowrates are required. Vaillant system boilers as well have a internal bypass (ABV) that can only be adjusted to a max setting of 4M so again, (if I am interpreting correctly how it works) if a high system dP is required at even modest flowrates then the pump head will increase but not much use since it will just recirculate as the ABV is at its max setting of 4M.
A heat only boiler would do the task above very simply if it can tolerate a high dP.
