Four failed aquastats in 18 months


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Old 01-31-09, 05:38 AM
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Four failed aquastats in 18 months

Hi - I'm hoping to get some help with a somewhat sticky problem. A friend (locally well-regarded HVAC professional) installed our propane-fired boiler and 4 zone system.

One zone is our domestic hot water. In the past year and a half four Honeywell L4080B aquastats have failed. My friend has replaced all but the third one for free. We are currently using gator clips to complete the circuit and run our hot water as needed. There has never been a problem with any other part of the system.

Since the fourth aquastat stopped working I am trying to suggest that we delve a little deeper into the problem before just replacing another. He is understandably less than excited about my problem and I thought I might do a little leg work to see if anyone else has any ideas.

The set-up is: NTI Trinity boiler, Taco SR 504 switching relay, Super Store Ultra hot water tank, and Honeywell L4080B aquastat.

When the aquastat fails, it seems to do so gradually or intermittently rather than catastrophically. It also (to the best of our recollection) only seems to happen when we are using our very large, very inefficient wood stove in the basement. Ambient temperature in the basement gets up to approx. 110 degrees F at times.

Any advice is greatly appreciated!
 
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Old 01-31-09, 08:03 AM
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The 4080B is a high limit control.

How is it being used? Is it the operating control on the SuperStore?

Do you know if it's the actual switch that's failing and not making contact? or is the temperature part perhaps drifting out of range and not sensing correctly?

Not that I have any ideas really... just trying to get my head around the application and possible failure modes.

Well, that's not really true... I do have one idea, and I'll give a personal experience to illustrate.

A piece of equipment I work on uses what are called 'reed relays' on a printed circuit board. Soon after we started shipping product, customers began reporting failures, which we traced to these reed relays not making contact. Manufacturer was sent a sample of the relays for analysis and reported to us that it appeared they were not being applied properly because we were not switching ENOUGH current! There is apparently a MINIMUM current that is required in order to keep the contacts clean! We did a slight modification that involved adding a resistor to increase the switching current and they have worked flawlessly since then.

BUT: It appears from looking at the specs on that part that they are rated for use with millivolt gas valves... 0.25A @ 0.25 to 12 VDC ... note DC ... don't know if that matters here ... you are switching a relatively low AC current at 24VAC if it's connected to the SR panel ... I'm gonna look and see how much current is switched on those panels... it's just the internal relay coil that's being turned on and off, not much current.

It's _possible_ this is part of the problem... but on the other hand, how come they aren't failing by the millions? I'm sure you are not the only one with that setup!

P.S. The max ambient temp rating on that unit is 265 so I doubt your 110 is part of the problem.
 
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Old 01-31-09, 08:10 AM
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Taco SR spec:

All Switching Relays are relay type DPST, have a thermostat current of .18
0.18 Amps... hmmmmm... that isn't much. I wonder...

Maybe have your tech contact Honeywell and ask if this could be a possibility.

But still, why aren't they failing wholesale? by the millions? or maybe they are and we just haven't heard about it?
 
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Old 01-31-09, 09:31 AM
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If you have a multimeter could you test the resistance of one of the failed aquastats?
 
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Old 01-31-09, 03:11 PM
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Many thanks for the speedy and thoughtful replies.

I don't have access to the failed aquastats and I am sure that the only failed one I have is the one sitting dead in the water, so to speak.

I'll remove it and see if I can figure out what's failed.

I was unsure whether ambient temperature specified in the manual was temp in the tank or in the air. If it's air temp then I am sure that it's not an issue.

Thanks again for some ideas to start with.
 
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Old 01-31-09, 03:14 PM
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Oh, yes, in reply to your questions:

It is the control in the Super Store

I'm pretty sure that both Honeywell and the local distributor have been consulted and they're stumped

Thanks again
 
 

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