Ask Caleffi:  Relay Controls
January 15, 2015

Caleffi Z-one Relay Controls have "resettable" fuses

One of the really nice features of the Caleffi ZSR and ZVR Zone Relay Controls is the resettable fuses for the 24 V circuits. When a resettable fuse trips, it is not "blown" and does not need to be replaced, it just opens. If any of the 24 V circuits (thermostat wiring or zone valve wiring) experience a short or exceed the 40 VA rating of the transformer, the resettable fuse will open the 24 V circuit to protect the transformer. When this happens, the board will still have line voltage, but no 24 V (just like blowing a glass fuse). The relay board is fine, but there is a field wiring problem that needs to be addressed and fixed. Once the resettable fuse has cooled down (line voltage removed for a short time), it will close again.

If this happens, disconnect the line voltage from the relay board. Check out all the 24 V wiring to make sure there are no shorts. Make sure the sum of all the loads does not exceed the VA rating of the transformer(s). If all that looks OK, disconnect all the field wiring and then re-apply line voltage to the board. Check to see that you have 24 V at the R-C thermostat terminals, making sure the transformer and resettable fuses are OK. Power down again. One-by-one, reconnect the 24 V circuits and power-up the line voltage. This process should help to isolate the problem circuit.

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Blog post comments
Patrick Nyberg
February 09, 2016

The ZSR 103 relay is having a problem with the priority DHW. The aquastat closes but the DHW does not turn on. I kill power to the relay and turns it back on then the DHW engages. This is very intermittent and frustrating. What could be the problem. It worked fine for the first 4 months and these last 2 months have been a bug bear to trouble shoot.

Cody Mack
February 10, 2016

Sorry to hear about your troubles, Patrick.

I would need a bit more information before trying to accurately diagnose. Though if the lights are shutting off on the ZSR and come back on when you reset the power it would be likely that you have an intermittent short / ground somewhere in your low voltage wiring. The fuse is designed to protect the transformer and board against this kind of problem.

I would suggest contacting us directly with any further questions.

Cody Mack
Caleffi North America

jonathan valdez

In reply to by Cody Mack

November 18, 2020

hello, i have a ?. i have the zsr103, it sounds like your saying the "resettable" fuses are low voltage. is this true? because my is reading 120v on both of them. the top "resettable" fuse is burnt to a


In reply to by jonathan valdez

November 19, 2020

Jonathan, The resettable link protects the low voltage side of the system buy shutting down the line voltage to the transformer. It is on the 120v side of the transformer.

Stephen Smith

In reply to by dan_firkus

November 19, 2020

Hi, our ZVR103 unit has just failed. Upon inspection we have found scorched "re-settable" fuses. Is there any explanation for this except a power surge of some sort?
The unit has been operating without issues since installation 3 years ago.


In reply to by Stephen Smith

January 07, 2021

Stephen,  This is an indication of power surge that has taken out the safety device.  This could have been an external power surge.

Allan Lillies
April 13, 2017

I want to change the controls on my existing system. I have 3 - 2 wire thermostats with 8 - 2-wire thermal zone valves on a single zone pump. How many of my 3 w 24v zone valves can I connect to a single zone control on your SVR106 Zone controller? Alternately can I use one 2 - wire thermostat to
input to more than one T-STAT?

Cody Mack
September 12, 2017


Sorry for the late reply!

Our 3 and 4 zone controls have a single 40 VA transformer while our 6 zone control offers two 40 VA transformers. If you have 3 thermostats controlling 8 thermo-electric actuators that will work just fine with our ZVR103 (3 zone control). You can wire multiple actuators in parallel to the zone valve output on the board. The ZVR will also be able to control your pump for these zones as well!

The ZVR103 with it's single 40 VA transformer is more than capable of handling a dozen or more 3 watt thermo-electric actuators along with the thermostats to control them (if they're power consuming).

Best Regards,

Cody Mack
Applications Engineer
Caleffi North America

A Schuerger
September 24, 2021

Hi. My transformer looks blown. Can that be replaced without replacing the whole unit? Any idea why that may have happened?


In reply to by A Schuerger

September 27, 2021

The Caleffi ZSR pump control that you have has the transformer soldered into the control board. You are not able to replace just the transformer, you will need to replace the entire unit.

Bernie and Bourgeois
April 17, 2022

My ZSR106 does come on when I turn on the power switch than all light shut off again, probably a short?



In reply to by Bernie and Bourgeois

April 26, 2022

Bernie, It sounds like you may have a short in the low voltage wiring on the thermostat side. You would want to disconnect the thermostat wires and power the board up to see if it remains energized. If it does you will need to locate and repair the shorted wiring.

alex ricard
May 27, 2022

Hello I would like to know the part number of the blue fuse ( the burned one) on the attachment photo.

Thank you


In reply to by alex ricard

May 31, 2022

This component is soldered into the board. It is not a replaceable item it is designed to protect the board. You will need to replace the entire board and locate the short before replacing.

October 05, 2022

I have a zsr103. None of the zones are circulating. The thermostats open the valves on each zone when calling for heat, The pumps don't turn on. The output pump fuses are good. I measure 120v at the input, the transformer that's soldered on measures 24v. However, there's no power on LED. also, none of the zone LEDs light up. What do I check next?


In reply to by Tom

October 05, 2022

Tom, You should have a "green" power light lit at all times.  The "red" zone lights will illuminate whenever a specific zone thermostat call for heat.    If you don't have the green power light the board may be shut down by the internal resettable breaker.   I would disconnect the thermostat wires to all zones and any other low voltage wiring and power the board back on.  If the power light comes on you have a short in the low voltage wiring.  You can also, using your voltmeter look for 24v between the R & C terminals at any of the thermostat locations.   I would recommend calling Caleffi Tech Support for other troubleshooting tips 414-238-2360. 

November 21, 2022

Hello, I have the ZSR103 multi zone relay. I've been having issues with the relays turning on and off extremely fast regardless whether there is a call for heat. It's worked fine for a couple years and I have not changed any wiring, any ideas thanks!


In reply to by Evan

November 29, 2022

Evan,  This could be thermostat, wiring or relay issue. I would start by disconnecting the thermostat and valve wiring at the relay board and see if the problem goes away.   If it doesn't it is a relay issue.   If it goes away then it more likely is a wiring or thermostat issue.  You can jump the board out to troubleshoot.   If you have questions or need assistance please call Caleffi Tech Support 414-238-2360.