The only place to look is the tankless heat and hot water combo. Its a Triangle Tube Prestige brand.. I left a message with my hvac guy.. I wonder if anyone ever experienced anything like that. --96.253.xx.xx

What you are using is low mass high efficiency gas boiler with a heat exchanger where the water from the boiler circulates through heat exchanger the on the other side hot water. This type of boiler is low mass where should have a Spirovent air and dirt separator installed which will filter black sludge and vent out air so boiler works at maximum efficiency. Low mass gas boilers hold around 3 to 4 litres of hot water where high flow is required. Also on the boiler there maybe a outside sensor where that regulates the maximum temperature of heating loop. Where outside temperatures are not that cold then boiler fires at low rate. It maybe necessary to back flush the heat exchanger as to remove any restrictions. --216.110.xxx.xxx

keep in mind if you tested it what you may be experiencing is a crazy tenant --208.115.xxx.xx

I am going to back flush the heat exchanger first and see how it goes.. Thanks Robert. --96.253.xx.xx

You will need shut off valves along with pump where first isolate heating loop then with pump back flush boiler. If using cast iron radiators than it recommended install a magnetic filter along with spirovent air and dirt separator then once in while open dirt separator then close once water is clean. I had a lot less problem once air and dirt separators were installed on heating loop and high efficiency gas boilers as the building had cast iron gas boilers. --216.110.xxx.xxx

I schedule a plumber visit the day after. The tenant is living there, black flush with pump will be next on my todo (little extra work to set it up with limited space (The whole system is in a small closet, love that setting, space saver) --96.253.xx.xx

Installing a Spirovent air and dirt separator where the air will go out then the black sludge will end dirt separator where open valve then wait until runs clears. For low mass boilers they solve a lot of problems. Cast iron boilers can take a lot of abuse where if water is a little low or black sludge as there is more mass. Not only is the boiler going to stay clean but last longer. To compare a low mass boiler is like comparing a basic car to a high end car where more complex electronics and have a screen to read codes. Once air and dirt separators were installed then had no problem with heat on upper floors or black sludge in system as took out three cast iron boilers. A low mass boiler has a primary and secondary loop where water must run really fast through primary loop. The boiler heat up really fast for domestic hot water and heat as goes to maximum then gradually goes down. View some videos on a Air and Dirt separators for boilers on You Tube then will see how they work. For service you need someone who went to seminars on how to service this type of low mass gas boiler. They have a book plus if unsure they call help line. You hear b boiler starting up then boiler will ramp up to maximum where if over limit is exceeded then boiler will shut down you hear the heat exchanger bang. --216.110.xxx.xxx

Taco and Spirotherm are known companies for air and dirt separators. Rust, black sludge is cleaned out where once system is flush then the system will only require opening valve to drain rust and black sludge. All that acts a insulator where you that damages heat exchanger. Taco is a family company where can contact them. A good plumber can easily install a air and dirt separator then heating system is working at maximum efficiency. --216.110.xxx.xxx