You cannot hook even CPVC to really really hot things. There should be an iron or brass bridge from the boiler to the PVC to sink some of the heat. The outlets on the hot water heater are iron and they are not really a PVC friendly union, nor are the unions to the HydraTherm, so you have to buy these cheap metal nipples and unions to bridge to the CPVC. The nipples are sealed with Teflon Compound (LIQUID!) and turned down really damn tight. I tightened them gingerly, and they leaked. So I threw caution to the wind, and cranked down on the unions to the HydraTherm like mad. Problem solved.