Aquastat L8148A


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Old 04-18-10, 05:56 AM
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Aquastat L8148A

Hello All,
I have a L8148A aquastat and have been having some trouble, I believe, with it. This aquastat has only a high limit setting in it that I can see and has a factory preset 15 deg differentail. What's happening is that I get some very short, like seconds, of burner run time. What I have noticed is that as soon as the zone is satisfied the burner shuts down with out reaching the high limit. I'm thinking that the burner should continue to fire eventhough there is no call for heat until the high limit is reached, am I correct? I have also noticed that the burner fires as soon as a zone calls for heat even though the 15 deg differential is not met, in other words the high limit is 180 degs, zone calls for heat and temperature is at 175 deg an burner fires as soon as zone calls for heat. Shouldn't the burner not fire until the temperature is at 165 deg? I havn't changed anything in the system and have just noticed this issue in the last few weeks. Any ideas would be great
 
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Old 04-18-10, 09:33 AM
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I'm thinking that the burner should continue to fire eventhough there is no call for heat until the high limit is reached, am I correct?
No, as soon as the heat call is removed, the system will and should shut down.

Shouldn't the burner not fire until the temperature is at 165 deg?
It depends... remember that this is a 'differential', with HYSTERISIS. What this means is that the burner will shut down on a temperature RISE to high limit. And only AFTER it hits high limit will it not fire again until the temp FALLS to 165.

For example: If there's a heat call, and the boiler temp rises to say 170, and the heat call ENDS BEFORE the boiler reaches high limit, the differential will not come into play.

So, if another heat call comes along, the burner will fire because the previous heat call did not drive the control to high limit.

What you are experiencing is pretty common during these 'shoulder seasons', where the heat demand is nearly negligent. You might get a heat call from the t'stat, and there's hot water in the boiler, so the burner and pump start, sends a shot of that already hot water into the radiation, which very quickly satisfies the demand and the system shuts down.

Bottom line is that if the heat call satisfies before the high limit is hit, there will be no differential action.

What is the minimum temp setting on your aquastat? Some only go down to 180, but some are lower... if yours is lower, you could try turning it down to say 160 or so during the spring and fall... or install an ODR control and let it happen automatically.
 
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Old 04-18-10, 09:39 AM
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Some thermostats have a setting that can help with this also... I think they call it 'dead band' or something like that. This would be on a newer digital 'stat... older stats have 'anticipator' settings, that need to be properly set up.

It widens the 'gap' ... sorta the same thing as the differential. It would let the house temp drop a bit, maybe 1 or 2 degrees before it calls for heat. Then, same as the boiler diff, it would call for heat until it hit the setpoint. If your thermostat has this feature, you might try experimenting with that. Problem might be that the temp in the home would swing more, might overshoot, and would become uncomfortable... but worth looking into.

What thermostat do you have?
 
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Old 04-18-10, 01:57 PM
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Thank you for the replies. I'll have to look into the thermostates although my gut is that isn't set able because I have digital programable t-states. I also don't believe their is any low limit setting on this aquastat the only setting that I can find is the high limit. So I guess it's going to be an outdoor reset!!!! Since I have a Taco 506 exp I guess the logicall choice would be the PC700, any thoughts?
 
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Old 04-18-10, 04:26 PM
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I didn't mean the low limit... I meant to take a look and see if you can turn the high limit down... some have an adjustment range of like 180 to 240... some have a lower range, maybe like 140-200 or so... so take a look and see what the range is on your aquastat and if you can, back it down to 160 or so and see what happens. Don't go too much lower than that though, because you run the risk of running the boiler too cool and inviting flue gas condensation.

I guess the logicall choice would be the PC700...
I dunno... if plug and play is worth something like $125 to you, then yes, it would be the best choice I suppose. If you aren't afraid to do a little wiring, the Tekmar 256 can be had for a lot cheaper and will do the same thing. You would only have to do a teeny bit of extra wiring.

What thermostats do you have? i.e. make/model
 
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Old 04-19-10, 03:18 PM
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The thermostats are Honeywell's model CT3200. I will do some research on the Tekmar 256 also since $125 could go towards beer for the project!!!!! Ok, I've read through the Tekmar 256 installation and it leaves me with a question: How would I heat me inderect hot water tank in the summer? It is now set up as priority on the Taco 506-exp.
 

Last edited by happyhomeworker; 04-19-10 at 05:12 PM.
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Old 04-19-10, 05:49 PM
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Take a look at the Tek 260, that one has indirect control on it... I didn't notice you had an indirect. But... we'd still need to look at the 'interface' to see how that will all work together. You might have to drink the cheap beer! But, at this point, I think the 260 is gettin' close to the PC-700 in price, and if it's close enough, might as well go plug and play with the 700.
 

Last edited by NJT; 04-19-10 at 06:13 PM.
 

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