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protecting transformer

Constantin Member Posts: 3,796
Install a 120VAC outlet, then pair it with a <a href="http://www.elkproducts.com/products/elk-trg1640_2440.htm">24VAC ELK power supply</a> (that is, if 40VA is enough). Not only does the ELK have a LED to tell you if the secondary is providing power, but it also incorporates a polyfuse to protect the power supply from shorts on the secondary side. The polyfuse auto-resets, which is also nice...


  • chuck172
    chuck172 Member Posts: 90
    protecting transformer

    Is it a good practice to install an in-line fuse, say 3 amps or so on the 24 volt side of a transformer?
    The transformer powers zone valves.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    I do not know today's prices.

    In the old days, a transformer cost a few dollars, a fusepost cost about $0.50, and a fuse (Littlefuse type) cost $0.03. And labor was essentially free. That was 50 years ago. I do not know the prices now, but labor is surely not free.

    I think the question you need to address is what failure modes do you wish protection against? I.e., how are the components likely to fail, what is the cost of failure, and what would you like to happen when that failure occurs.

    So, is your purpose of the fuse to protect the zone valves or the transformer. My guess is it is not worth it to protect the transformer. If the valves, you need to think about the failure mode of the valves. If a valve burns out (goes open circuit), no need for a fuse. If it shorts out, it could harm the transformer, and keep the other zone valves from operating until the shorted one is removed. If that is what you are worried about, you need a separate fuse for each zone valve, because if you use one fuse for all of them, when the fuse blows, they all stop working.

    Not being a working heating professonal, I do not know the failure modes of the devices used here. I used to design electronic systems and my impression here is that the failure rates of these things is very low, at least after they have been working a month or so. My guess, in the case of zone valves with a motor, it is possible the gears might get jammed and the motor would draw excessive current. A fuse could be helpful there. But otherwise, it is probably not worth the trouble.
  • Techman
    Techman Member Posts: 2,144

    I find it much easier to install an inline 24v circuit breaker rated at 3.2amp.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    Unless Code requires it for some reason............

    Of course. And even if there is no reason, or one you disagree with. Where I used to work, people sometimes remarked: "There is no reason for it; it is just our policy."
  • Rollie Peck_3
    Rollie Peck_3 Member Posts: 24
    24 Volt fuse

    A standard 24 volt Class 2 transformer is designed in a special way so that in event of a short in the secondary circuit, the output drops to a level that will not overheat the number 18 wire used in most heating setups. I'm asssuming that it won't overheat itself, as well. This is also a very good reason not to use non Class 2 transformers in heating systems. Use multiple transformers if one transformer does not have sufficient capacity. That being the case, a fuse wouldn't be necesary in the secondary circuit.

    Rollie Peck,

  • Why not use

    a transformer with a built-in circuit breaker?


    My hydronic and controls suppliers stock them.
  • Honeywell

    AT150F-1022 circuit breaker transformer is a great way to go. There are also similar transformers from Robertshaw and White Rodgers.
This discussion has been closed.