Aquastat Electrical Repair FAQs

Electrical wiring or failures at boiler aquastat controls

Boiler Aquastat Control Electrical Problems Q&A

Tip: See aquastat wiring guides and manuals at AQUASTAT CONTROLS 

Also see AQUASTAT TROUBLESHOOTING

Also see AQUASTAT DOESN'T TURN ON the CIRCULATOR PUMP FAQs

HydroTherm boiler with a Honeywell controller - I get no voltage at the zone valves

I have a HydroTherm boiler with a Honeywell controller. I have 120 Vac on the transformer of the Honeywell controller but no output (24 Vac) to the zone valves.

I can't figure out what's going on and I have no heat right now. Can anyone give me a clue what might be wrong?

I can attach a photo but can't figure out how to do that either! 2016/03/06 Paul

Reply:

Paul:

If the transformer is working and you see nothing at the zone valves I suspect an open or broken low voltage wire.

Check and confirm voltage at the thermostats as well. You can email me photos using the page bottom or top CONTACT link.

Tapping on a boiler aquastat control as a diagnostic step

Boiler will call for heat and boiler turns on but the burners do not ignite. After a couple of taps on the aquastat the burners will fire and heat as it should. Honeywell L8148E On 2021-04-22 by Neil

by (mod) - look for a loose electrical connection, bad relay, or bad temperature probe

@Neil,

When tapping on an electrical control causes it to operate I suspect burned contacts or a failed relay or a loose electrical wire connection.

What do I do if the circulator pump runs but boiler does not fire up

O turn up thermostat circulating pump goes on but the boiler does not fire up checked water psi bleed everything (Nov 21, 2015) tom

Reply:

Tom

If circulator runs and boiler temperature drops but the burner never fires check the boiler reset control. See AQUASTAT RESET BUTTON

Furnace repairman said aquastat is not necessary - just wastes oil. Is that true?

A furnace repairman suggested that having an aquastat on a furnace is a ploy by the fuel supplier that burns more oil than necessary. Is an aquastat necessary?

His comment made sense, since hot water is not necessary all the time.

Reply: If your "furnace" is a hydronic heat (hot water heat) system it's a boiler and it needs an aquastat or a limit control.

Baloney.

The aquastat is a critical control function on heating boilers and on some water heaters. It provides safety high limit temperature control as well as a low limit and differential control that assure heat in the boiler if a tankless coil is used on the heating system boiler.

See details about the HI, LO and DIFF optimal settings

at AQUASTAT HI LO DIFF SETTINGS

However it is true that IF your heating boiler does not include a tankless coil, that is if you make domestic hot water (for washing and bathing) using a separate water heater, you may not need to use the LO and DIFF functions of an aquastat. In that case there are two options:

1. You could replace the aquastat with a single function HIGH LIMIT controller, though the cost of doing on an existing heating so is in my opinion not at all justified given option 2
   
   See SINGLE FUNCTION LIMIT SWITCH
2. You can simply disable the LO / DIFF feature on the aquastat. To do that - which costs nothing -
   
   see AQUASTAT LO & DIFF DISABLED
   
   Watch out: there are reasons to keep some heat in a heating boiler even when it is not being called-on to make heat or hot water, to prevent condensation, corrosion, solidification of deposits making them hard to clean, and because some models of older cast iron heating boilers may leak when left cold.

When should the heating zone circulator shut off?

The circulator shuts off as soon and the burner shuts off. The installer says this is correct. It seem to me it should run for awhile to heat up the house. This is a new Aquastat. (Feb 19, 2014) Jim Kyle

Reply: when the thermostat is satisfied (if you're not in Canada)

Jim, what the circulator does may depend on how it was wired. And customs vary: e.g. in the U.S. vs Canada circulators are not hooked up the same.

If you are in the U.S. and your home heating system is wired as usual when the thermostat calls for heat that turn the circulator ON. The circulator will keep running until the thermostat is satisfied.

The temperature of the boiler water is what turns the boiler on and off. They are independent.

More: on many systems even if the thermostat stops calling for heat and thus stops the circulator, if the burner had turned on due to a drop in boiler temperature, the burner will typically keep on heating until the HI limit is reached

You don't say what controls are installed on your system, and I'm just nor sure what your installer did. You could take a look at the help telephone number for your boiler manufacturer or perhaps more directly, the brand and model of controls in use on your boiler to see if their specifications have been followed, or you could as the installer what the heck they did.\

Weil McLain Boiler won't run even when I jump the thermostat terminals but runs if I press the relay in the L8124A Aquastat

Hi I have a weil mclain boiler with 2 zone heating ( honeywell valves) . The zone valves work perfectly ( motors are working , microswitches good) . When i jump the thermostats , and the both valves energise, the boiler won't turn on.

I was able to reset the boiler by turning it off , then would run for a couple of days but on he third day as the thermostats would be calling ( levers pressing on microswitches) for heat , the furnace would not turn on ( only turns on to manitain water temp per hi lo settings).

Tried turning on/off boiler but would not work. Took cover off aquastat relay Honeywell L8124A, and manually started boiler by pressing the Burner circulator relay plate ( something the thermostats should be doing) .. It started

Then afterwards the thermostats returned to normal firing up the relay on their own. ANy thoughts would be greatly appreciated / Thank YOu ! (Jan 14, 2015) Anonymous

Reply:

Anon

If on thermostat call for heat the zone valve opens but the heater does not turn on the end switch could be defective, but it'd be a surprise to have the same failure at the same time on 2 valves.

A simple VOM or DMM test can indicate whether or not the end-switch on the zone valve is working or not.

If it is, the problem is at the boiler or its controls.

What are common boiler short cycling causes

What are the common problems to cause a hydronic boiler to short cycle (starts & stops)? Aquastat going bad? (Jan 13, 2015) Anonymous

Reply:

Loose electrical connections, shorted thermostat, poor thermal sensor contact, as well as defective aquastat.

Boiler short cycling is discussed in AQUASTAT TROUBLESHOOTING and in detail in our article BOILER SHORT CYCLING . Was that u clear or did you miss it? please take a look and let me know if questions remain.

New boiler aquastat keeps clicking so I turn it off

Just install a new aquastat to my gas boiler it continues to click. So I turned it off. On 2017-11-30 by wayne

by (mod) - How to Check first for a loose wire or bad electrical connection.

Wayne,

Check first for a loose wire or bad electrical connection.

If clicking continues let's look more closely at what is clicking - probably a relay switch in the aquastat

. If so, check the connections to the circulator or burner that the aquastat is trying to operate. If those were working before, and apologizing that I am shooting from the hip, let's do these basic aquastat tests. If none of those sort this out I'm left suspecting a bad relay and thus a bad control.

Tell me the control brand and model and I'll check further.

How to Conduct Basic Boiler Aquastat Controller Tests:

1. Turn the wall thermostat all the way down

so it is not calling for heat

2. Using a DMM or VOM

assuming you know how to do this safely and are aware of shock hazards, else STOP here

else with great care not to get killed, check for line voltage (120VAC) at the power terminals to the aquastat (i.e. power is on) by finding 120VAc at the two Line-IN terminals, L1 and L3 are typical markings.

3. Check the low voltage transformer output:

Check for 24VAC at the T and TV terminals - you will find 24VAC at one of these and 0 VAC at the other. If both terminals show 0 volts then the transformer is bad.

4. Now turn the Thermostat all the way UP

so that it is calling for heat (remember to set it back down later). You should see 24VAC on BOTH of the thermostat or TT terminals inside the aquastat. If you don't then the thermostat is not calling for heat and most-likely you've got an open thermostat wire or a bad thermostat itself.

5. Now while still calling for heat, check that the aquastat is sending 120VAC

voltage to the burner (the two B terminals B1 and B2) confirming that the aquastat is trying to turn on the burner (oil burner) or trying to open the gas valve if it's a gas boiler. This should be turning on the burner.

6. Now while still calling for heat, check that the aquastat is also sending voltage to the circulator pump -

look for 120VAC at the C1 and C2 terminals. This should be turning on the circulator.

If voltage is everywhere it should be and a relay is clicking constantly OR a transformer is buzzing, those component(s) are probably bad.

Wayne: continue reading about aquastat diagnosis at

AQUASTAT TROUBLESHOOTING 

It is possible for me to convert a line voltage thermostat wiring to low voltage?

Hi, I have a wiring/installation question regarding triple aquastats. I am interested in converting my Honeywell dual aquastat to a triple aquastat heat manger, perhaps the Hydrolevel Hydrostat or the like. I have a separate domestic hot water source.

The dual aquastat presently operates a two zone hydronic oil fired boiler running at 82% eff. Unfortunately both zones are controlled by simple line voltage thermostats.

After reading the schematics on triple aquastats, it seems apparent that low voltage wiring is necessary, especially for the heat manager to work properly.

Is this true? Is there no way to operate a triple aquastat with two zone line voltage thermostat wiring?

It is possible for me to convert the thermostat wiring to low voltage fairly easily in the house as well as wire in a switching relay to allow the second zone to communicate with the triple aquastat.

I'd appreciate any advice on whatever options will prevent the boiler from running to high limit 24hrs a day during the heating season.

I know that was a lot of questions. Thank you for your time! On 2017-01-15 by Joiry

Reply by (mod) -

Joiry,

With some care and engineering help it's likely that one could convert a 120V line voltage system such as line voltage thermostats to switch low voltage controls by using an appropriate relay switch.

But someone should call the Wise Lady from Philadelphia (Peterkin Papers, Lucretia P. Hale).

It sounds to me as if you are adding additional layers of cost and complexity (and thus unreliability) to an existing system that perhaps should be scrapped instead.

Watch out: usually where line voltage thermostats are installed they are controlling electric heat.

I don't understand how you've ended up with a mixed system that uses line voltage thermostats and an aquastat such as the R8182D with low voltage (24VAC) terminals (typically controlling a hydronic - hot water - heating boiler)

by Joiry I see line voltage t-stats controlling circulators on oil fired hydronic boilers all the time.

Hm really? I see line voltage t-stats controlling circulators on oil fired hydronic boilers all the time.

They have nothing to do with electric heat typically.

Maybe there are a lot of older houses in my area that may have new systems but still have clunky old 14 gauge control wires running through the walls attached to rotting, mercury t-stats barely hanging on to the plaster wall.

In any case, I'm more concerned with how to wire an additional zone to a triple aquastat. The triple aquastat schematics themselves seem simple enough. There are terminals for the burner, a circ and it's t-stat (low voltage), and a ZC ZR for additional zones.

That last part has me a bit puzzled. I assume I need a switching relay that has a ZC ZR on it in order to connect the second circ to the aquastats low limit functionality, but I have so far only found schematics online for 4 zone switching relays. One of those sounds a bit like overkill.

by Joiry Oops?

Nevermind! I looked a little more closely at a single zone switching relay schematic and it instructs, in the tankless coil version, which terminals to use in conjunction with ZC and ZR on the triple aquastat.

I figure where there is a tankless coil, there is a low limit functionality, which is ultimately the thing that I am looking for.

With low limit, I will be able to maintain a lower boiler temperature throughout the heating season except during the instances where there is a call for heat.