Fixing neglected multi-zone radiant heat system with Q90 75 boiler

ShawnC715

First of all, I would like to all that provide valuable information to us DIY homeowners here. I have been reading multiple articles here to help figure out some problems that I am having with a 21 year old Carrier / Dunkirk Q90 75 that has been working intermittently. Like many, mine has been short cycling. I just moved into a three house property that has been neglected and the bare minimum has been done to keep things working. The HVAC company recommends replacing the system with tankless system for heat and and hot water. I am hoping to get by with current system for a few more years since there are many other issues that require repair. I have ordered a replacement igniter to see if this helps with my issue. I also plan to follow the manual for annual cleaning. The Dunkirk Tech said that the cast aluminum heat exchangers get pretty dirty and cause this issue, but I have not seen any good references dealing with the cleaning of this boiler.

The system itself is a mess. There are three zones. Two downstairs in floor zones and one heat coil for the upstairs. There are two thermostats and there is wire in the wall for a third. The HVAC place has no clue how this thing is wired and working and said it would probably take them a day to figure it out. I am hoping fix this with a 3 zone controller from Taco and thermal actuators on the manifold, but am not sure that is an option. So any information or thoughts are welcomed . Currently,there is a Johnson Controls A419 installed with the probe zip-tied to the hot feed line, which I am not really sure what it is doing. There are some other issues with the layout as well there is ball valve that is open that connects the hot side to the cold side which seems to me should be closed. There is a large component on the the top which looks like an air separator but it is in between two elbows. So, I believe it is too close to the elbow to be effective. There is another component that comes out the back of the boiler that looks like a second air separator, but it has a schrader valve on it and the expansion tank is connected to that. Based on the videos that I have seen on other setups, this all seems pretty messed up to me. I also plan on get some test strips to see what kind of condition the the water in the system is in and if is has any antifreeze in it. I am in Northern Wisconsin, so I would assume it would be a good idea to have antifreeze in the system.

I know there is a lot here, but any information is greatly appreciated.






would assume it would be a good idea to have antifreeze in the system.

I know there is a lot here, but any information is greatly appreciated.

Jamie Hall

First things first. A tankless system -- presumably a tankless water heater is meant -- is the wrong device to power a heating system, though in some situations it might be OK for hot water. Not, however, in a three family, unless each apartment has its own hot water supply. For the heating system you need a boiler. Which you have.

What problems are you actually having with the system? Is there adequate heat in the various apartments? You say something about short cycling, but what is cycling? The boiler (I can well see it might, from what little I can see of the piping)? Circulators?

Some pictures of the boiler and the piping from farther back, so we can try and trace out the piping and the connections, might be helpful.

ShawnC715

My apologies, there are three separate houses. One of the houses has a boiler with radiant heat (radiators), another has forced air and the system pictured is in an unoccupied house that will be used be used for short term rental. The pictured system is in a 1500sq ft two story house. The boiler fuel source is propane.

The short cycling that I was referring to is that the system would go through its start up and through the sight window you could see the igniter glow then the flame would appear momentarily and the system would shut down. After a minute or so it would restart the process. It seems to be working now, but obviously there is an issue. The condensation drain does not seem clogged and the air intake and venting is through the top of the house and they are right next to each other. One thermostat it upstairs and the other is downs stair. There is thermostat wiring in the wall down stairs for a third zone that is not hooked up.

For clarification, the system they want to put in its place is a wall mounted condensing boiler that is made to heat the house and and provide domestic hot water.

Additional pictures of the system.

Jamie Hall

The short cycling to which you refer is a problem with the safety or ignition or gas train in the boiler burner. Not to mince words, but any reasonably competent tech. should be able to diagnose the problem and fix it in a few hours -- there aren't that many possibilities. It certainly doesn't warrant a whole new boiler.