Boiler acts like there is always a call for heat

mccaslin15

I have a Navien NCB-180 boiler which acts like there is always a call for heat, even if the thermostat is disconnected. If I take out the thermostat wire that goes into the T/T terminals on the boiler, it no longer acts like there is a call for heat. I have three zones which have Taco zone valves on them, and a single pump.

What diagnostic tests can I do to track down why this is happening? What voltage or resistance reading on the T/T terminals causes a call for heat? What should the reading be when it is not calling for heat?

Thanks in advance for any help.

HVACNUT

TT should connect to the end switches on the zone valves. Seems like one of them is stuck closed. You can start by disconnecting the end switches one by one until you find the culprit. Another possibility is a short in the wiring between the end switches and TT.

HomerJSmith

What model Taco zone valves?

mccaslin15

The valves are Taco 572 valves.

One important piece I left out - the pump is currently controlled by a Honeywell L8124A aquastat - so the end switches of the zone valves connect to both the TT on the aquastat and the TT terminals on the boiler. If I connect the TT directly to the zone valves, the boiler behaves like it should, but I don't ever run the system pump. I'm guessing something is not right in the aquastat since there appears to be intermittent continuity across the TT terminals.

What are my options for controlling the pump?

Thanks again for the help.

Jamie Hall

Ah. I was wondering about the aquastat. There are several control strategies which can be used -- but by far the simplest is to let the zone valves control the pump, and the aquastat control the boiler. That way your pump would run whenever a zone was calling, and the aquastat would fire the boiler if the circulating water weren't warm enough (assuming that it's not just a high limit) and turn it off again if the water were too hot.

If you connect the boiler directly to the zone valves, it will fire whenever the zone valves are open, which you really probably don't need.

Now the aquastat does have to be right -- it should close a contact when the temperature is too low, and open when it is too high.

HVACNUT

mccaslin15 said:

One important piece I left out - the pump is currently controlled by a Honeywell L8124A aquastat - so the end switches of the zone valves connect to both the TT on the aquastat and the TT terminals on the boiler. 

How and why do you have a triple acting aquastat on a gas mod con boiler? 
You don't ever run the system pump? What kind of Frankenstein's monster is rigged up over there? 
Can you draw out a complete wiring diagram? I can't wait to see. 

Jamie Hall

HVACNUT said:

mccaslin15 said:

One important piece I left out - the pump is currently controlled by a Honeywell L8124A aquastat - so the end switches of the zone valves connect to both the TT on the aquastat and the TT terminals on the boiler. 

How and why do you have a triple acting aquastat on a gas mod con boiler? 
You don't ever run the system pump? What kind of Frankenstein's monster is rigged up over there? 
Can you draw out a complete wiring diagram? I can't wait to see. 

Me too. Sounds like someone got very creative...

mattmia2

The functions that aquastat normally provide are almost always pat of the control in the mod-con.

neilc

"well, yeah, but we been doing it this way for 30 years"
#what_manual?

EBEBRATT-Ed

Taco zone valves are 3 wire zone valves and the end switches have to be connected correctly as they are also a part of the thermostat circuits they are not truly independent sets of contacts if wired incorrectly

Drock

Just had this same issue it was the tstat.  Try a pro 1000 Honeywell used two other stats before we switched.  It solved the issue.  Best of luck.