i mean if you look at it this way you dont even need to modifie the boiler connect the hose to the boiler , and the boiler does the job itself. But i dont know how the water reacts if you heat it up or etc. and if or there will be problem with the plants . I got another idea more manual intesive but worth a try.
Where you live right now is it cold ?
if yes. You probably have a radiator heating ur rooms .
Get a few smaller or some big buckets the plastic normal ones.
Fill them up and let them warm up near the radiator. Mix the water here and there .
Yeah its a hassel but its not expensive and before you get a solution its still better than nothing.