The boiler has to be able to command the pump on demand, there is no RF technology that can do that period, even wired isn't 100% but no wireless solution will ever be acceptable for a 'mission critical' application such as this. This is put into a house if someone is so lazy as to not want to wire it they'll pay for it in safemode trips when the boiler goes down for unknown transmission issues.
I'm not sure if this is an engineering or practical question, the technology you're trying to use is not able to reliably replace a wired solution, which even after installation of the control wire would cost LESS than the wireless solution, in materials and time. Wire pulls are not that troublesome for something that needs to be able to function if there's a solar flare, or someone using a microwave oven that has broken shielding, or any of the other unbounded failure modes for an RF signal.
As a basic example, a simple low power short range RF system could be completely blocked if someone set a cookie sheet at the wrong angle in relation to the RF signal.