Navien Combi Boiler/water Heater

[dustel]

I had a plumber replaced my boiler and water heater for a Navien combi. I have nothing but problems. the house is a multi family house, currently 4 apartments. The heat is not hot enough to heat each apartments. the plumber is telling me that due to a high demand of hot water, the boiler can't heat enough. my thermostat I set it to high 70's but it can't get pass 71. when it is warmer, the boiler seems to better, but when it drops say like 20 or below, then it is a problem to heat the apartments enough. please help.