L8148A aquastat differential failure

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  #1  
Old 12-19-18, 03:39 PM
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L8148A aquastat differential failure

While calling for heat and after reaching the 180 high limit, the burner shuts down. Good, that's what it is supposed to do. But then the burner may start again after only dropping a couple of degrees. Nowhere close to the 8º fixed differential. Is it a failure of the aquastat and is that repairable or can the burner be getting a rogue signal to start from some other source?
I get a lot of short cycling on a random basis. Sometimes as short as a few seconds.

Riello burner, Crown Boiler, 4 zones, 1 circulator pump, TACO controller, L8148A aquastat. All connections checked and tight.
 
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  #2  
Old 12-20-18, 07:00 AM
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I'm not a pro & can't answer your question but will comment that these are crude temperature measuring devices & 8* isn't much of a change. Could be your gauge is slower responding than the aquastat probe.

A related question of my own concerns that differential. With modern higher-efficiency condensing boilers flowing cooler water through the pipes I'm curious if 8* differential is still a good setting? Maybe 15-20 would stop the short cycling and save energy *despite* the lower efficiency of cooler water (on average) flowing? If one is faced with replacing an aquastat should one with an adjustable differential--set wider--be installed?
 
  #3  
Old 12-20-18, 07:50 AM
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Had the same issue here. I invested in an IntelliCon HW+ piggyback controller. It's totally eliminated the short cycling and has saved me quite a bit on fuel consumption. It holds off the burner and gets me about a 15 - 20 degree water temp swing, depending on load conditions. Much longer burn times means much more efficiency. The short cycling, especially for those with over sized boilers just consumes tons of energy. This unit allows a lot of the heat in the boiler to be drawn out before it allows the burner to fire.
 
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Old 12-20-18, 02:27 PM
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S,
In the sight below will show you your options if you're not happy with your current aquastat. They do make different ones with different dif. settings.

There is one I'll list that is electronic and is a replacement for your and has many more features including an adjustable high & low DIF settings of which you will diable te low limit in your case and adjust the high to your preference.

Hope this helps a little.

https://www.supplyhouse.com/Honeywel...ronic-Aquastat

https://www.supplyhouse.com/Aquastats-Wells-350000
 
  #5  
Old 12-21-18, 06:16 AM
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For a great aquastat that can save up to 20% on fuel costs install a $157 Tekmar “out door reset”. It is aquastat with digital menus to set various boiler settings.

ODR's sense and adjusts boiler temp to most economical level based on outside temperture. User /home owner can set high and low temps, differential, etc.. Fuel cost savings provide quick payback.

https://www.supplyhouse.com/Tekmar-2...iler-4150000-p

Other brands are far more expensive. The $247 Tacor PC700 is a rebranded Tekmark 256. It has connector that can only be used on Taco controls. https://www.supplyhouse.com/sh/contr...ext=taco+pc700

A different Taco Fuel Mizer has very rudimenary settings that do not allow for most efficient operation. The Tekmar 256 is best buy.
 

Last edited by doughess; 12-21-18 at 07:02 AM.
  #6  
Old 12-23-18, 09:35 AM
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Sometimes it is clear that the boiler temp drops down by 8-10 degrees Other times it only drops 1-2 degrees before firing as long as the thermostat is still calling for heat.
 
  #7  
Old 12-23-18, 09:44 AM
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I've seen the info on this universal electronic control.
I need to make sure it will fire the boiler only on demand (i.e when one of the thermostats is calling for heat).
I don't want to maintain a minimum boiler temperature during the summer when only the domestic hot water zone is calling. The DHW zone goes to a separate storage tank aided by solar collectors.
 
  #8  
Old 12-23-18, 09:55 AM
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L8148A aquastat

My basic question remains: How is the fixed differential built into this device and do they fail? I don't know where to look for a failure point. Is it a circuit board component, a piece of metal in the relay which I might be able to bend, a spring, or condition of the contacts, etc.
I hate to just replace the aquastat without knowing if it is the cause of the random short cycling.
It was installed in 1994 so I suspect there was no thermal grease used in the well. Could that cause the errant readings?
 
  #9  
Old 12-23-18, 10:00 AM
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I had an Intellicon installed and just took it off. The burner was still short cycling when it was in place and I could see no difference in consumption. Service techs say they have seen most of them fail or fail to produce and no longer recommend their use.
 
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Old 12-23-18, 12:25 PM
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Maybe one of the experts here can chime in, but I don’t think it is normal for a boiler to be constantly running up against the high limit. Maybe there is some kind of flow problem. This doesn’t address the differential problem, but if the boiler wasn’t hitting the high limit so much maybe that differential problem really wouldn’t be a problem at all.

Could be wrong.
 
  #11  
Old 12-24-18, 08:03 PM
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The L8148 technolgy is old expansion bulb sensing that moves a diaphragm which activates a microswitch to call for heat. When they age settings change.

Newer electronic ODR/aquastats are easily adjustable, more accurate and save up to 20%.

Frequently see on DIY.com people fixated on the old when new ones (often for less money) would fix the problem, provide better and more economical performance.

On a $1,000 heating fuel bill 20% is $200. Stowepotato can mess around with the old L8184, buy a new one $167 or buy a Tekmar 256 ODR aquastat for $157 and pay for it with the savings, which over the years can add up. 5 years x $200 = $1,000 While the actual savings may vary it is real money.
 
  #12  
Old 12-26-18, 04:02 PM
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Thanks Doughness.
Replacing the L8148A with a newer L7224U is the direction that makes sense to me. I suspect the erratic performance is due to poor contact of the bulb in the well. I have thermal grease and could pull the old unit, add grease, reinstall and test for a week or so. If it was not the cause of the problem, I'd end up replacing with the electronic unit. So why not just jump to the final step and save work and time.
I'd be surprised if I save 20% on fuel, but the real savings will come from avoiding premature service calls because the burner clogs up due to the short cycling.
Techs from two different oil companies have had no clue about what's wrong or what to do.
Got to Do-it-Myself!
 
  #13  
Old 12-27-18, 07:56 AM
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The Honeywell L722U cost $147. To later add Honeywell ODR module is another $300 or total $447.

https://www.supplyhouse.com/Honeywel...Outdoor-Sensor

The $157 Tekmar ODR with annual fuel savings is a super bargain. Stowepotato's old R8184U can still be used for circulator control and 24 VAC to power Tekmar.

Various vendors use a low price starter unit then drastically mark up add-on's. This thread is classic example.
 

Last edited by doughess; 12-27-18 at 08:37 AM.
  #14  
Old 12-28-18, 03:45 PM
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Got the L7224U in 2 days from Supply House. I've bought other items from them before, always with super good service.
Pulled the old L8148A and found it already had thermal grease in the well, so that was not the problem. Took 30 min. to install and wire up. Very simple to program once one reads the manual. Works like a charm.
Instantaneous temperature readings are markedly different from the slow acting analog thermometer originally installed. Default hi limit is 180º and differential is 10º. Turned the low limit to off so it replicates the L8148A cold start mode.
Don't know if it's worth springing for the outside temp. control unit. I do know I saved about $200 by doing this job. myself.
Thanks again doughness for your input.
 
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