Upon arrival unit was on and running. Customer stated the last couple of weeks the thermostat was turned up but the heat never satisfied to what it was calling for. Turned both thermostats up and the unit and zone valves worked properly. Put pipe clamps on the return and supply heating lines. Coming out of the unit when running was 180* F and water coming back to the unit was 150-170* F showing the unit is heating up the water and hot water is circulating throughout the heating pipes. Unit turned back on each time aquastat called for heat when return water hit 160*F. The unit is having a hard time overcoming the temps outside due to the extreme cold. Customer stated they noticed the issue when temps began to regularly were below 32*. Unit is working properly.