Honeywell L8148A Aquastat problem


Old 08-20-16, 01:29 PM
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Unhappy Honeywell L8148A Aquastat problem

I'm baffled. Hopefully someone will have an idea.

Have a Honeywell L8148A connected to my FHW boiler. The aquastat is not firing up the boiler. The boiler is dead cold.

I have 24v across the T screws. I have 120vac on L1, ground on L2, L3 jumpered to L1 (factory installed).

Temperature dial is set to 160. High limit is unchanged from the factory setting (190 I think)

Boiler does not fire up to heat to low limit.

I set my thermostat to 90. Its only 75 in the room, I hear the thermostat click on to call for heat.

The boiler still doesn't fire.

I do not get 120vac across C1/C2, nor do I get it across B1/B2.

Also do not have 120vac on either B or R

I ran down to the supply house yesterday afternoon to pick up another aquastat, figuring this one went bad.

Just finished swapping it out. Same problem. Doesn't turn on the burner to get the boiler up to operating temperature. Voltages check out on T1/T2 and L1/L2/L3.

Lost. Not sure what else can be wrong? Help?

Last edited by vtwin; 08-20-16 at 01:46 PM.
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Old 08-20-16, 04:07 PM
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You should not have 24 volts across the T screws when the thermostat is calling for heat. Try jumping them.
Old 08-20-16, 07:26 PM
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Thanks. Disconnected the T wires, and powered up the aquastat. If I jumper the T leads, it will fire the boiler.

So, this would suggest I have a bad zone valve somewhere?
Old 08-21-16, 06:24 AM
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Depends how your system is configured. If you have zone valves with individual thermostats and end switches, then yes, may have a failed switch (or the actuator) or could be a break in the wiring.

If none of the zones fire the boiler I'd suspect a wiring issue. Could also be a limit (hi temp or whatever) depending on how they are wired in.
Old 08-22-16, 05:53 AM
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Thanks. I don't mess with the zone valve wiring. The house has three zone valves, two feed new construction and the third feeds a hydro/central-air air handler which services the "old" section of the house.

If it were simply zone valves I could handle the wiring, but the air-handler has a half-dozen transformers and switches in it which also connect to the thermostat and zone valve. The wiring to handle cooling, heating, turning the fan on and off, etc. is beyond me.

My guess is the wiring problem is contained in the hydro-air unit, I'll have to trace out the circuit with my multimeter when I get home tonight and see if I can determine where the 24vac current is originating from.

I've been looking for a reason to ditch the hydro-air unit (we do not use central A/C -- way too expensive to even contemplate given electricity rates) and instead subdivide the rooms the hydro-air unit services into several zones with baseboard heat.

This may be a good opportunity to do just that, for the cost of some baseboard, a few zone valves and thermostats, and a zone valve controller, I could do the conversion over the long upcoming weekend.
Old 08-22-16, 07:03 AM
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The Honeywell L8148A has an internal 24 volt transformer. It is wired so that when the T terminals are shorted together either by the thermostat or by the end switch on a zone valve it will fire the boiler (assuming water temp is below setpoint). You will measure 24 volts on the T terminals with nothing connected to them. Something external (thermostat, end switch, relay contact in air handler, etc) has to short the T terminals together to fire the boiler.

Good luck!
Old 08-22-16, 02:58 PM
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Is this something that just happened,will any of the other zones fire the boiler?
Old 08-22-16, 03:04 PM
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Give us some pictures of the wiring. The more the better.

The wiring may appear to be difficult but it's not.
Old 08-22-16, 03:53 PM
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In the past, before I've concluded that my Aquastat was responsible for a Boiler malfunction, I have independently hot wired 110 directly to the Burner, and then to the Circulator, just to verify that they will function when supplied with power.
Old 08-23-16, 05:21 AM
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The L818A is 50 year old technology. Rather than buying and installing another L818A aquastat, consider spending $30 more for a modern out-door-reset. ODR's provide better control, have LCD display of temperatures and settings while lowering fuel costs.

A modern technology ODR is the Tekmar 256 Boiler Control. A microprocessor-based control designed to regulate the supply water temperature from a single boiler based on the outdoor temperature. The Taco PC700 is a rebranded, for a $100 more a high priced 256. It has Taco brand green housing with a short 4 wire cable and plug rather than terminals used for connections on the 256.

You can still use the L818A for 24 VAC to power the 256 and circulator control .

Last edited by doughess; 08-23-16 at 06:40 AM.

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