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Modine HW unit heater

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limejuice
limejuice Member Posts: 3
in Plumbing
Before I had my boiler replaced, the old hot water unit heater in my basement would only come on when heat was called for. I switched to natural gas in 2017 and the idiots that installed my new boiler (among other egregious mistakes) just wired the fan on the unit heater to the incoming power to the boiler, so it just runs whenever the fan switch is on, regardless of whether the thermostat calls for heat. I know that the correct way to do this is to install an aquastat, however I eventually want to put radiators in the basement to replace the old unit heater, so I’d like to avoid laying out the cash for the aquastat if possible. Could I simply just wire the unit heater to where the circulator is tied in, or would that overload the relay? The boiler is a Crown natural gas atmospheric draft from 2017.

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  • mattmia2
    mattmia2 Member Posts: 9,672
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    Is the basement a separate zone with a separate pump or valve?
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,859
    edited December 2020
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    What "relay" brings on the "Heat"?

    So let me guess for this example. The zone thermostat turns on a Switching relay that operates the burner on one set of contacts and a circulator pump on the second set of contacts. Look at the relay specifications and see what the contacts are rated for.

    Now, look at the motor on the fan and the motor on the circ pump. What is the starting load rating? If the addition of both the motors starting loads added together is greater than the relay contact ratings... the answer is No.

    If they are just a little over, then you may get away with it for a short time. If they are well under the rating of the contacts, then your answer is YES.

    But you already knew that ... you just needed a reminder.

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

    mattmia2
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,859
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    If you have a Honeywell the R89A or an R845 to bring on the circulator then the rating is about 12 Amp @ 120V. that is good for about a 1/2 horsepower motor.

    Here are the ratings for the Taco SR503 multi-zone relay


    As you can see the 6 FLA rating is 1/2 that of the Honeywell and this has a 36 LRA so a 1/3 HP motor is all you can put on those contacts. Your circulator might be 1/25 HP so look at the fan motor on the Modine and see the HP or LRA ratings is on that motor.

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

    mattmia2
  • mattmia2
    mattmia2 Member Posts: 9,672
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    In addition to just using the ciruclator relay, I am thinking if it has its own thermostat you could use a ~$15 RIB relay at the unit heater and just turn the fan on with the thermostat.
  • jumper
    jumper Member Posts: 2,262
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    mattmia2 said:

    In addition to just using the ciruclator relay, I am thinking if it has its own thermostat you could use a ~$15 RIB relay at the unit heater and just turn the fan on with the thermostat.

    General method is for thermostat to control fan. Any heating when fan is off is not truly wasted.

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