Honeywell L8148 Aquastat

Discussion in 'HVAC Heating & Cooling' started by Southern Man, Aug 2, 2008.

  1. Southern Man

    Southern Man DIY Hillbilly

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    I have one installed on a 3 year old oil fired residential boiler. I have the thermostat set to the lowest setting, 180F. When the burner cycles the temperature goes up to 240F, and the burner shuts off. Sometimes the T&P vents around the same time. The cycle repeats when the temperature drops to 170F.

    This doesn't seem like the proper behavior for the system. Do I need to replace the aquastat?
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    The more typical operation would be to shut off at about 210-220 or so. 240 may be the overtemp cutout shutting it off.
  3. Southern Man

    Southern Man DIY Hillbilly

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    530
    Location:
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    If I set it at 180 why would it cut out at 220?
  4. nhmaster

    nhmaster Master Plumber

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    S. Maine
    1 - the capillary tube is either not fully seated in the well.
    2 - The aquastat well is not mounted in the high part of the boiler or it,s
    located too close to the return.
    3 - The L8148 is defective.
    4 - The thermometer on the boiler is defective
    This is pretty much all it can be, one of the four things.
  5. Southern Man

    Southern Man DIY Hillbilly

    Messages:
    530
    Location:
    North Carolina
    I replace the entire unit and now the system works fine. I took apart the old one, and one of the lines on the circuit board fried itself.

    I have had more problems with electrical equipment at this place.
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    A whole-house surge suppressor wired into the main panel might help. Placing separate, higher capacity units next to your expensive electronics (tv, stereo, computer) are worth the effort as well.

    I put a power monitor module on my air handler that monitors the incoming power for over and under voltage. It prevents the system from turning on if the power is outside of the set range and it induces a delay on restart if the power if fluctuating (as often happens when it comes back on after an outage, however short). While wiring it up was more of a pain than expected, it does work. It's been awhile, but I think it was from IDC and I bought it at Grainger. Haven't looked for awhile to see if it is still a product.
  7. nhmaster

    nhmaster Master Plumber

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    That would be # 3 on my list. If it makes you feel any better, that's kind or rare. Happens, but not too often.
  8. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Many of today's HVAC units now have some extensive electronics in them. Older ones tended to be more robust relays and switches; now, most of that is electronic and controlled by a computer. My air handler has a 16-speed, electronically controlled blower motor and you adjust the max output with DIP switches. The things often only come with protection fuses, little surge protection. Repeated hits on electronics can be as bad as 'the big one'. Each surge can chip a little more away from the diode/transitor junction until eventually, it fails. Preventing that from happening on critical equipment is a minor insurance expense. The boiler has a computer, there's an external one that integrates the outside reset functions, and there's one for the radiant zone valves. So, lots of electronics. The mechanicals can often handle noisey or peaky power...the electronics less so. It's fairly cheap to protect them and much better than having to replace them after a storm and the power jitters when they turn it back on after a tree or lightening takes it out. Almost nothing will stop a direct hit, but there's lots of near misses to the power system that you can protect from fairly easily.
  9. Southern Man

    Southern Man DIY Hillbilly

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    530
    Location:
    North Carolina
  10. Southern Man

    Southern Man DIY Hillbilly

    Messages:
    530
    Location:
    North Carolina
    Fortunately mine's the old style electro-mechanical stuff- all Honeywell. In fact I just rewired the whole boiler last weekend with two new zone relays. I also put in a Beckett HeatManager which is obviously electronic so I do need to consider protecting that along with all the other stuff in the house that keeps breaking with a whole-house surge protector.
  11. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    The two things to consider on surge suppression are: response rate and amount of energy they can dissipate. Look for response rate in the nanosecond timeframe, and the most joules you can afford. A brand that has been in business for awhile doesn't hurt, either...build quality is important, too.

    Those I've looked at require them to be installed on two separate circuit breakers, one on each leg. Nothing else can be on that circuit, so you need at least two open slots in your panel. Installation is pretty simple, though.
  12. Southern Man

    Southern Man DIY Hillbilly

    Messages:
    530
    Location:
    North Carolina

    So they plug into a 240 amp breaker space? I figured that they would be wired before the main breaker.
  13. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,933
    Location:
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    I've only looked at a few, and yes, they plug into the power panel bus protected by a CB. You could use a 240vac one, or two individual ones, just make sure they are on opposite sides of the bus. This also protects some that may be internally generated, like say a compressor that could produce a surge.
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