Boiler Transformer Wattage

Jakek

Today I did an electrical audit and found the 24v boiler transformer is using 30 watts when idle and no call for heat. (Small residential system.) The transformer is not powering the T-stat or anything else, just the natural gas regulator.

I found this surprising high. Is this normal or should I invest in another transformer?

mattmia2

How old is the boiler? What kind of combustion controls does it have? If it has solid state controls, the controls could be using roughly that much power (or if there are some contactors or safety valves that the transformer is powering that are always energized). Transformers have some losses in the energy used to magnetize the core, but 30 w is more like the output of a typical transformer. How are you measuring, are you sure it is correct?

Jamie Hall

That does sound a bit high. Simple test disconnect the transformer secondary (only need to undo one wire, not both) and measure the power draw of the transformer. That will give you the transformer loss.

EBEBRATT-Ed

Most transformers are rated at 40va (watts) maximum although smaller and larger transformers are out there.

How did you measure this wattage??

Jakek

@Jamie Hall - I did disconnect the load from the transformer entirely and wattage was the same when no call for heat.

I'm thinking the transformer is on its way out.

How I measured: I recently got a "Efergy" system which attaches to the house mains and computes the wattage. I've found it pretty accurate once configured. To check the boiler transformer, I turned off every other breaker so I was only monitoring that circuit. Since the transformer is connected to a switch I could be sure nothing else was on the circuit.

As an aside, I verified the Efergy system with a "kill-a-watt" plug-in meter in circuits where that + known load are the only things connected and made sure the two meters matched. I have more accurate meters at work but this is good enough to find problems.

mattmia2

If it is really drawing 30 w with nothing connected it is going to be really hot, that energy has to go somewhere, not just warm but so hot you can't touch it. Think about how hot a 25w lamp gets.

How accurate and precise is this device with very small loads. That is a really wide range if it can measure both a couple hundred amps and a couple hundred milliamps. It is not gong o be able to do that all without having some sort of multi range circuitry. If you really want to know, put an ammeter in series with it. The power factor of the transformer could be causing issues with measurement as well.

mikeg2015

> @mattmia2 said:
> If it is really drawing 30 w with nothing connected it is going to be really hot, that energy has to go somewhere, not just warm but so hot you can't touch it. Think about how hot a 25w lamp gets.
>
> How accurate and precise is this device with very small loads. That is a really wide range if it can measure both a couple hundred amps and a couple hundred milliamps. It is not gong o be able to do that all without having some sort of multi range circuitry. If you really want to know, put an ammeter in series with it. The power factor of the transformer could be causing issues with measurement as well.

Really good point. An inductive load like that has terrible power factor. Cold skew by nearly 50% if I’m not mistaken.

Jamie Hall

Something is fishy. As @mattmia2 said, if that poor little thing is really drawing a quarter amp (to give you the 30 watts) or more, it's going to be very very hot.

Jakek

Yeah, that's why I think it's shot. It's very warm to the touch -- 25 watts warm at least.

Easy part to replace. Glad I found it now and not when the boiler stopped firing.

Thanks everyone.