Hot water on Shabbat

Question

The halakhic discussions I have seen assume that the water coming into the boiler is a pesik resha de-nicha leh, since you want the water to be heated up (otherwise not only will you have no hot water later, but the hot water currently in the boiler will become cold). But these poskim also assume that if it is a pesik resha de-lo nicha leh, that there is room for permit it. If R. Abadi holds that it is a pesik resha de-lo nicha leh, what then is the issur? Which leads to my second question. Do we say that pesik resha de-lo nicha leh is mutar le-khatchilah?