How does an Aquastat circuit work?

RichC901

My Aquastat 8148E 1257 will sometimes run the circulators but not the burner after receiving a call for temperature. Tapping on it or jiggling the wires gets it going again. This seems to be a common problem. I've pulled the board out and did not see any burned out solder connections, and made sure all contacts were clean and tight when put back in. Problem still happens.

I'd like to understand the circuit better so I can effectively troubleshoot it. I'm an electrical engineer who worked for many years as a chip designer. So far this is what I've established:

- Thermostat relay is shorting TV and T when calling for heat.
- C1 and C2 are receiving line voltage and running the circulators.
- B1 and B2 are not receiving voltage when problem occurs, but receive 24V when working.
- There is a vent damper connected via Molex plug. The damper is not open when the problem occurs.
- The system is Dunkirk natgas, hot water heat, 3 zone w/ Argo zone controller.

What I don't understand is what is happening between the thermostat and burner connections. Specifically:

- Is the big relay for switching the circulator only?
- What is switching the burner on and off? It seems like there is only one relay on the board.
- It looks like the high limit adjust is a variable capacitor. I'm guessing the temp probe is a thermistor. How do these components work together to shut off the burner at the high limit temp? Is it a voltage divider with comparator?
- Where is a good place to probe the input to the damper so I can eliminate it as the source of the problem?

STEVEusaPA

Toss it in the trash and get an electronic one...
-or-
Basically, it's temperature driven. Your burner can run independently of the circulator, based on temperature.
When a call is initiated by the thermostat, T-T closes and energizes both the circulator circuit, and the burner circuit.
You also have a high limit setting with a fixed 15 degree differential.
So, once the high limit is reached, the circulator will continue to run as long as there is a call for heat.
When the temperature drops the 15 degree (differential), the burner will kick back on.
This continues until the call for heat is satisfied.
I've never tried to repair one. I test it, if something doesn't work as specified, I replace it.
Specifically, in your case, when any thermostat calls for heat (3 zone valves or 3 circulators-I am assuming zone valves), there is a slight delay as the zone valve opens, and the end switch in the zone valve closes. Any end switch closed, closes T-T on the Argo (or X-X, cant remember off the top of my head), and closes T-T on the aquastat.
Just one or two zones calling usually allows the output of the boiler to exceed the btu requirement of the zone, thus short cycling (running up to high limit).
Here's a link, if you're an electrical engineer, it should be a cake walk:
https://customer.honeywell.com/resources/Techlit/TechLitDocuments/95-0000s/95-6940.pdf

RichC901

Thanks for your help Steve. I've been staring at Fig. 12 in the pdf you linked for many days now. It's not complete. For example, relay is not on there, nor is the temp probe. I'm not experienced with this type of electronics, so hoping someone here can fill in the blanks.

I'm likely going to replace the thing, but want to make sure it's not the damper or wiring to the damper first.

What electronic control would you recommend? I've heard people recommend Beckett or Hydrolevel.

Paul48

Look at figure 6.....The 1K relay is operated by the t-stat. It has 2 sets of contacts 1K1 and 1K2 . The 1K1 contact operates your circulator. The 1K2 contact sets up the boiler hi-limit, which then passes through contacts on your vent damper, that ensure the vent is open before the boiler fires. The problem is probably in the Molex plug to the vent damper.

Jamie Hall

I'd go with @Paul48 's comment. Those vent dampers and their connections are really troublesome. What should happen is that when 1K2 closes the vent damper should open, and when it reaches its own end switch it should send the signal on to the burner to fire. There are several places in there where gremlins can lurk, but that thrice-blessed molex is a great place to start.

RichC901

Thanks so much @Paul48! The schematic in Fig 6 makes sense to me now. I wrote Fig 12 previously, but was looking at a different pdf, but same schematic. With your help, this is my current understanding:

* The 1k resistor between T-W represents the input to the relay.
* Current between T-W will close two relay contacts: 1K1 and 1K2. These look like capacitors on the schematic so I was confused.
* The high limit controller block should break the circuit above high temp and close the circuit below high temp minus diff (hysteresis). The schematic makes it look like just a variable capacitor.

So here are my troubleshooting steps for when there is a call for heat and circs are running, but burner is not and water is below temp:
1) Confirm T-B has 24V. If not relay is not working.
2) Confirm T-R has 24V. If not temp limit circuit is not working.
3) Confirm T-R at damper (point 2>> in schematic) has 24V. If not molex is bad.
4) Confirm T-B1 at damper (point 3>> in schematic) has 24V. If not damper is bad.
5) Confirm B2-B1 has 24V. If not, molex is bad.

Note I removed the plastic cover on my damper so I could probe the PCB. With the power off, I measured resistance between the 4 solder joints where the wire connects to the PCB and T, Z, R, B1 on the Aquastat to determine which solder joint corresponded to which point.