Boiler/Aquastat burning out 40 VA transformers

rmcbay

Hi everyone, I'm having a problem with my boiler for residential hot water/radiant heat, hoping someone has an idea what may be wrong. I have contacted the two Heating/Cooling companies in town and they say they are too busy and not interested in a service call.

I have a Hydrotherm boiler model 2HW5 manuf by Hook & Ackerman between 1968-1979 (yes, I know it's old). It is a simple single zone heating system. The electrical supply voltage comes from a 40 VA transformer, that steps voltage down from 120 VAC to 24 VAC. Demand is controlled by (an equally old but reliable) Honeywell T42A1X2 thermostat (the type with the Mercury bulb switch). The boiler over-temp is controlled by a Honeywell L4006A-1066 immersion probe aqua stat.

The 24 VAC comes from the transformer with two wires, a hot (red) wire, and a common (white) wire, to the aqua stat. The hot/red wire goes to the aqua stat switch where current is normally flowing unless the temp rises too high. The common/white wire bypasses the aqua stat. Both red and white wires then go to the natural gas valve in the boiler from the aqua stat.

I came home the other day to find the heat out. Upon inspection I discovered the old 40 VA transformer was burned out, hence, no power to the gas valve on the boiler, so no ignition. I replaced with a new 40 VA transformer, and it also burned out. A check of the new transformer with ohmmeter revealed the primary side burned out. A check of the 120v AC circuit supplying the primary side of the transformer reveals no problems, consistent 120v, breaker not tripped, able to run other items under load.

I obtained another new transformer but installed it with a 5 amp fuse wired on the secondary side of the transformer on the hot/red wire between the transformer and the aqua stat. Upon turning on the circuit, the fuse blew. I replaced all the wire from transformer to aqua stat, and aqua stat to the gas valve, but the same problem occurred/fuse blew. I then wired the transformer directly to the gas valve with a new fuse (bypassing the aqua stat) and the boiler fired right up.

Being somewhat in disbelief, but not knowing what else to do, I ordered a new aqua stat (now Honeywell L4006A-1678) essentially same model. I installed it and the fuse from my transformer again blew, as if there is an electrical short somewhere. I again wired directly from the transformer to the gas valve (bypassing the aqua stat) and boiler runs fine.

This does not seem to make sense because both the red & white 24 v wires in the aqua stat appear to be insulated or separated from any contact with the water pipes or any electrical ground. I have even powered the transformer from a completely separate 120 v circuit in an effort to diagnose but problem persists.

I am stumped. Anyone have any idea where to look next?

Jamie Hall

Check all the wires involved. May I suggest that an ohmmeter is cheaper than multiple fuses? What you are looking for is a short to common/ground in one of the wires. I think I would proceed by disconnecting all of the red wires at both ends (keep track of what goes where!) then test the wire from the aquastat to the gas valve. Also test the gas valve itself. Then test the wire from the transformer to the aquastat -- and the aquastat itself (both terminals). None of those should show a short to ground/common -- although the gas valve should show continuity but at more than more (considerably more) than 5 ohms. Then start reconnecting, starting at the gas valve (check the other end of that wire; should be the same as the gas valve). Connect the aquastat to gas valve wire and check again (should be the same as the gas valve on both terminals) then the gas valve to transformer wire and check at the transformer end -- again should be the same as the gas valve.

If none of that shows the low resistance/short, some back..

EBEBRATT-Ed

the secondary side 24v side of the transformer should be fused at 1.66 amps.

What else is hooked to the 24 volts? You only mention the gas valve and the aqustat. You have a thermostat and thermostat wire as a minimum. Is this a standing pilot?? Is this a millivolt or thermocouple system? What is controlling your circulating pumps is the 24 volt connected to them with a switching relay??

rmcbay

Thank you both for your responses. I have checked the wires from the transformer and from the aqua stat, and I am unable to find a short. The gas valve has 6 ohms across the terminals that are being used (TR-red) & (TH-common/white) when everything is disconnected, and I am unable to locate a short from either of these terminals to ground, with wires on or off.

There is nothing else hooked up to the 2-wire 24 volts out of the transformer. It goes directly from the transformer, with hot wire across the aqua stat switch & white bypassing it, and then directly to the gas valve.

This is an old system, with thermocouple on the pilot. Although I can't see (most of) the wires, what I believe I have figured out is: the thermostat actually controls the hot 120 v that goes to the primary side of the transformer as well as the 120 v to the circulator pump. In other words, the hot wire coming out of the circuit breaker in the box goes to the thermostat, where it is interrupted or connected according to heat demand, the hot then goes to a junction box in the boiler room, where the hot is pigtailed to the transformer and to the circulator pump, again, powering them both on/off simultaneously depending on heat demand. Not exactly the most efficient system I'm sure, but that's what I have before me.

Jamie Hall

6 ohms on 24 volts, my friend, will give you a 4 amp draw. Too much. Something wrong there (96 voltamperes -- twice the rating of the transformer. No wonder it blows...)

EBEBRATT-Ed

If you have an amp probe you could check the amp draw of the gas valve. Wrap ONE of your 24 volt wires around the amp probe jaw 10 times and whatever your reading is divide it by 10 to get the actual amps.

40 va transformer=40/120 volt = .3333 amps on the 120 volt side

the 24 volt side is 40/24=1.66 amps That's what the transformer is good for

Sounds like a possibility of a bad gas valve

rmcbay

Thanks guys, I figured it out. Good call Jamie on your original comment, I discovered a short in the wires from the aqua stat to the gas valve. The wire was brand new, but had an intermittent short in the center portion of the wire, if it was manipulated in one direction it would short out, in the other it was fine (while checking with a meter) but shorted when current was applied. I purchased new wire again, installed it, boiler fires, all is good. Thanks again guys.

ratio

An insulation meter, AKA megger, is the test instrument to find that sort of problem. I use a Fluke 1587, it's paid for itself already through a few speedy diagnostics.