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Mark II HW control problem

I built this house, physically, in 1993. I had a hand in everything, including the installation of floor heat supplied by 2 Mark II HW units running at 100,000btu. The have a controller that has an outdoor sensor.

I'm an electrician, by trade, so I understand how everything works.

I'll tell the whole story. I noticed that the floors were getting cold in our bedroom. All of the thermostats were calling for heat. There are 7 zone valves. The first thing I checked were the positions of each valve and they were all open. I noticed the quiet in the boiler room. The system pump wasn't working. The floor had been drained of it's heat. I bypassed the pump relay and let it run full time. Well that created a monster flywheel because it took so long for the thermostats to get a reading the temperature rose to 85° turning on the a/c.

Now, this happens every fall at the beginning of the heating season, but we know about that, but we were unprepared for living in a sauna.

All it took was time, but in running some test cycles the unit shut down on the safety circuit, not the temperature control circuit. That was charging the system with 215° water.

We have a combination of ceramic and vinyl floors with floor heat and baseboard heat that looks like actual baseboard where we have wood floors. My wife noticed that the baseboard, and a modern radiator in the bathroom was extra hot.

I diagnosed that it was a problem with the controller sensor. It read continuity hot or cold. I've kept a full crib of spares since 1995. I bought the complete service box and have replaced the components each time I use one.

I had a new sensor, I cut pressure to the system, bled it and replaced the sensor, and got the same results. I cut it off at 200°, not knowing the high limit is 215°. I should probably test that again.

I turned off boilers for about 15 minutes and restarted them.

I watched while my wife took a shower, never taking my eyes off the gauges. Both units operated flawlessly. I get endless hot water because we pump cold water through the left boiler's heat exchanger and into the second boiler for further heating before it gets to the mixing valve. Endless hot water on a 3/4" feed to the shower.

The units have had few problems over the years. I still had the original temperature sensor and installed it, thinking it was fine. It too, has continuity. Shouldn't it open at some point before the limit switch opens?

I watched it happen, and then it didn't happen at all in 4 subsequent cycles.

I'm mystified.


  • mattmia2
    mattmia2 Member Posts: 9,840
    We need a lot more information. Do you have a system design drawing of this? We need more information about these sensors too, most temp sensors are a thermistor that varies resistance with temp rather than a thermostat that switches on and off so the setpoint is set in the control rather than by changing sensors.
    EBEBRATT-Ed Member Posts: 15,752
    If you have the manual it should tell you the resistance of the sensors at different temperatures. I would disconnect and check all the sensors measure the temp they are seeing and compare that to the resistance you read.

    If all the sensors are good and seated in their wells or in the water or air, they are sensing then it may be the controller

    I am assuming the boiler is full of water and vented of air and at 12-15 psi when cold