My present aquastat, L4006A, controls just the burner (warm-start, hot-water boiler). I want an aquastat that will do the same, except shut off the pump when the boiler temp drops below, say, 140 deg during a heat call, and restart the pump when the temp gets back up to 180 deg. What is the model I need?

 

Why do you want to do that for Gilmore???? Are you having an issue?

Your aquastat already hold the circ off from the Lo setting/diff

 

Din't ya already ask this a while back? I pretty sure ya did, and I answered that I think the L4081B may be the one you need... if I recall correctly.

 

https://customer.honeywell.com/resou...0s/60-2105.pdf

See figure 6

 

No, the aquastat, and only the aquastat, will turn the burner on and off. It's a warm-start boiler, so the temp is maintained continously, even when there is no heat call.

The pump is controlled only by the thermostat - when there is a call for heat, the pump will run continuously, even if the boiler temp droops to 125 deg during a long heat call. (Radiation rating higher than the boiler output, so the boiler temp droops during a lengthy call.)

 

Hardwired... thermostat doesn't control burner.

[I read your mind... I knew what you were going to ask before you asked it!]

 

Boiler Bypass??????????

Wait the 4006 is only a single limit. How is it warm start?

 

When the boiler temp gets below the aquastat's sepoint, the burner will fire, come hell or high water - irrespective of the thermostat or pump. That's a warm-start boiler. The thermostat is not involved - the boiler temp is maintained continuously, day or night, or at least it is supposed to be. Except when the boiler temp droops down during a long call - the burner will keep firing and the pump will keep running - but the burner can't keep up with the radiation loss, so the boiler temp droops.

 

HUH? You have a 24 V pump?

Why can't you use this with 24 V ?

[this post is out of time sequence due to the time shift wormhole that has opened preceding the events of 12/21/12]

Gil, you can use the 4081 ...

Burner circuit on high limit side at 24VAC and circulator circuit on low side at 120VAC

 

Wow... this time shift portal that has opened up is really messing with my head!

 

Gil, in 10 minutes you will see my replies!

 

Trooper, at first I thought that aquasat, L4081B was the answer to the maiden's prayer. But that is for 120V. I need something for 24V.

 

Why not? 24VAC side replaces 4006, 120VAC side controls circ... no?

 

I hope it's not a sign that the Borg are coming....

 

OK, yes, that will work, thanks. I guess I was thinking of interrupting the 24-V circuit to the existing circulator relay when the boiler temp dropped below, say, 140 deg. That would keep 120V out of the aquastat but would make my digital thermostat go black when the circ shuts off on low temp. I'll ditch that idea and go with your plan.

The jumper in the 4081 will be removed, of course. Under what circumstances would the jumper not be removed?

 

Maybe not a bad idea to keep it all at 24V ... why would the thermostat lose power if you did that?

Not using battery power at t'stat?, OK, but don't you have three wires to the t'stat then? 24V Hot, 24V Comm, and the 'switched' lead? Or perhaps it's a 'power robbing' design?

If you wired the low into the switched lead from the t'stat I don't see why the t'stat would be powered off?

Tell me how the thermostat is currently wired and I might be able to come up with something easy to do.

Several examples in the install manual...

https://customer.honeywell.com/resou...0s/60-2105.pdf

 

T'stat is digital, non-programmable Honeywell T8775A. It's round, heating only, no batteries, with only two wires connected to it. I assume that when the 24V is interrupted (no heat call), the 'stat gets power by sneaking it through the circulator relay coil.

New info: the t'stat wire is 3-conductor, but only two are being used. So, maybe I can rewire the 'stat or replace the 'stat to accomplish what I want?

 

Yes, possibly. I believe there is a way to install a resistor that will allow the unit to sneak the current it needs through that.

Gimmee a few mins to check that out for ya.

Here's the manual for your thermostat, I don't see a 24VAC COMMON terminal so this has to be a 'power robbing' design. The manual says nothing about adding a resistor... but I'm sure it's possible. The trick will be finding the correct resistor valve to use.

I would use a 'decade resistor box', and just dial in the value that keeps the t'stat running with the limit switch open, but that's because I happen to have one... most ppl do not. I'm thinking I might try 250 ohm / 0.5 W resistor and see what happens... You only need enough current to power the thermostat, and not enough to operate the relay.

Maybe a call to Honeywell support? (good luck!)

http://cache.air-n-water.com/manuals...t8775c1005.pdf

 

The two wires are connected to the R and W terminals of the 'stat. The other 'stat terminals aren't used.

 

Or, just go with the schematic plan...

Something that has always puzzled me about these power robbing designs...

OK, I understand that they are powered by leakage through the relay coil under normal circumstances.

But, what happens when the thermostat calls for heat? For all intents, when the contact is closed between the R and W terminals, there is a short there... closed contact... how can any power be supplied to the t'stat in that condition?

Perhaps they have a large capacitor inside that can store enough energy to run the t'stat for the times that the t'stat calls for heat?

That's the only way I can see that these could work.

The document DOES SAY that the temperature setting is saved in non-volatile memory in the event of power failure.

Out of curiosity, I would be interested to know how long the t'stat will remain powered after disconnected from the circuit. If you are so inclined, and curious as I am...

Maybe you won't need the resistor? Maybe it will stay powered long enough to wait until the low limit is reached ?

 

I know that if I turn off the power to the boiler (which shuts off the power to the 24-V xfmr), the 'stat's screen immediately goes black, probably in a power-saving mode. Even if the setting is retained, the 'stat will have a blank screen while the circ is overridden. I would find it a bit unhelpful not to be able to glance at the 'stat to see the room temp and the little blinking icon that tells me there is a heat call in progress. Also, I might want to change the 'stat's setpoint during a heat call while the pump is overridden.

I will think about the resistor idea. I would first measure the resistance of the relay coil - then maybe shunt a resistor of maybe that value, or a little less, across the 4081's circ contacts - and experiment to achieve the largest resistance that will keep the 'stat's screen lighted. I will calculate the resistor's power dissipation, and double it for good measure.

I have the 4081 on order and I should receive it sometime next week. If the resistor idea doesn't work, I can always go back to the 120-V idea.

Thanks, Trooper.

 

That's what is puzzling me...

How is it that when power is cut the t'stat goes off, yet when the t'stat calls for heat and the relay closes between R and W, the t'stat does NOT go off?

Very puzzling...

There must be some 'logic' inside that tells the micro-P :

IF R/W is OPEN and power goes away, THEN shut down, ELSE IF R/W is CLOSED, power from internal source (charged capacitor?, internal Lithium cell?)

I sure wish I could find schematic diagram...

When in doubt, search for a patent! I'll read this later:

Power Stealing for a Thermostat Using a Triac With Fet Control

 

Here is a theory. First, the switch contacts inside the 'stat is solid state. When the switch breaks, there is 24V across R and W, and the 'stat is fully powered.

When the switch contacts makes, what is the volage across R and W? Not zero - it will be the forward drop across diodes, transistors, etc., making up the switch. That voltage drop must be high enough to run the microprocessor and display.


When the power is completely turned off, the voltage across R and W will be zero, and everything is off except the volitile setpoint memory, which is retained by a transistor.

 

I received my L4081B and installed it today. I decided to go with running 120V through the pump switch rather than try to haywire if for 24V and worry about the power-robbing thermostat. All the 120-V connections are push-in, and the guts of the aquastat innards are completely isolated. It is operating correctly, but I need to do some tweaking of the dials to optimize things.

This site excels at doping out Honeywell aquastats. Thanks, Trooper.

 

Great! glad it worked out for ya. You're quite welcome!