How to rewire a high limit aquastat

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  #1  
Old 02-08-19, 11:00 AM
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How to rewire a high limit aquastat

Hello,
I would appreciate any help with rewiring a high limit aquastat, a honeywell L4006a, on a oil fired steam boiler with a hydronic loop. The high and low limit and the steam thermostat are all wired to the SAME terminals on an oil primary control, a honeywell r7184p. In the attached pic, I have disconnected the low limit wires from this control. The issue that Iím having is that the high limit is turning on the burner in order to maintain 180, essentially acting like a low limit aquastat. I believe this is because the high limit is wired to the r7184p, where the terminals are for a ďtemperature control for a tankless heater onlyĒ, as stated in the honeywell wiring diagram. Could you help me rewire this high limit? Thanks

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Last edited by PJmax; 02-08-19 at 07:27 PM. Reason: enlarged pics
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  #2  
Old 02-08-19, 07:22 PM
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Welcome to the forums.

L4006A breaks the circuit on a temperature rise to the control setting. It is used for high limit or low limit control. When used as a controller or as a low limit, a separate high limit must be used.

I can help you with wiring but running a steam boiler with a hydronic loop is a little different than a straight hydronic system. I'm going to let the members with more boiler experience help here.
 
  #3  
Old 02-08-19, 09:58 PM
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Hi Pete thanks for your response. If you don’t mind I would like to know how you would wire it?
I’d really like to cold start but my boiler, shut it off for the summer, but not sure how I can do that or if I can?

Thanks
 
  #4  
Old 02-08-19, 11:04 PM
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Things that need to be considered....
Since this is steam and hydronic there needs to be two thermostats.
I'm assuming a circulator for the hydronic.

Typically a steam boiler is setup for cold start.
I'd also assume that the hydronic thermostat controls the circulator and the boiler.
When the hydronic calls for heat what keeps the steam system from heating also ?
 
  #5  
Old 02-09-19, 03:44 AM
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On a steam boiler with a domestic hot water coil the winter/ summer switch when moved to the summer position, adds the domestic coil temperature control to the limit circuit not allowing the boiler water temperature to exceed 180 degrees or another selected temperature. This hot water limit control is usually wired in series with the other limits and is shunted out with the winter/summer switch for normal steam heating. This keeps the heating system from sending steam to the living space in the summer. So, the normal controls are, a controller for summer use, a steam limit and a manual reset high steam pressure limiter. Myself, I never liked using a coil in a boiler for summer domestic hot water production but preferred a separate electric hot water tank for summer use. (my preference). You could control the electric part of the domestic hot water tank manually or by any means you prefer.
 
  #6  
Old 02-09-19, 05:27 AM
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Hi Pete. There are two thermostats. In the pic, the steam one is wired to the oil control and the hydronic one is wired to a switching relay. On a call for heat, the relay turns on two circs, one on the supply and one on the return. I believe the hydronic tstat only controls the circs. High limit aquastat is wired to the boiler. It is set to 180 so it can’t turn to steam and heat the system.
 
  #7  
Old 02-09-19, 05:37 AM
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Hi Steamboy. Are you referring to the aquastats and the pressuretrol? I have a WM sgo5 and oil control and I can’t find this winter/summer switch. That would be great if I can turn a switch and PREVENT the high limit aquastat from maintaining temperature.
 
  #8  
Old 02-09-19, 05:45 AM
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My goal here is to really save on oil costs. For example, on a 40 degree day, with the way the high limit is wired, the boiler WILL maintain 180 even though it only calls for heat twice a day. That means the boiler is on every 45 minutes or so just in case there’s a call for heat. I think that’s a waste of oil and money. If I can find a way to rewire the high limit so that it ONLY fires up the boiler when there’s a call for heat, I think I would be saving a lot of money. And I could also turn it off in the summer.
 
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Old 02-09-19, 11:01 AM
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The winter/summer switch would only allow the hot water aquastat to function in the summertime so the boiler would shut off on a water temp of instead of on steam pressure when heating was not necessary. The aquastat would be wired in series with the rest of the controls and would be shunted (jumpered) with a switch when steam heating is required. The switch would render the aquastat inoperable so the boiler would operate on the thermostat and the safety controls.
 
  #10  
Old 02-09-19, 11:59 AM
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E,
As far as wiring goes, it just gets tied in to where your TT goes on the boiler. It acts just like a stat. On a drop in temp, the boiler comes on just like a call for heat.

At 180* your boiler will not make steam and you will not get unwanted heat. Unfortunately the downside of a tankless coil is the boiler must maintain temp all year round to deliver constant hot water when needed.

If you eliminated the 4006 and had the boiler only run for heat, then when you didn't call for heat and you needed hot water there would be nothing to turn the boiler back on to maintain boiler water temp to keep coil hot.

I feel there is more to the story though. You mentioned circs. You do not need circs for steam or domestic hot water from a tankless. Do you have and extra storage tank for your hot water. You also do not need a high & low limit or a separate relay just for your tankless.

You either have a hot water loop heating another zone or an aquabooster tank. At any rate in order for you to have hot water when you want it your boiler must maintain constant temp. You will not find any summer/winter switch as factory equipment and you do not need it anyway because at 180 no heat heat will be delivered.

That aquastat is adjustable though and you can try turning it down to see if you still get enough hot water to cut down on fuel a little.

Hope this helps a little.
 
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Old 02-09-19, 02:05 PM
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I donít want to interfere with the pro help here, but just as an aside, which manual did this

the terminals are for a ďtemperature control for a tankless heater onlyĒ, as stated in the honeywell wiring diagram
come from? I canít find it in an R7184p manual or an L4006a manual.
 
  #12  
Old 02-10-19, 08:16 AM
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Hi spott. I apologize if I didn’t make it clear that I have a hydronic loop off a steam boiler and an electric heat pump water heater for my dhw. Which explains why I disconnected the low limit because I don’t need it.
Hi zoesdad. I found that information on a wiring diagram for the r7184p that is on the boiler.
 
  #13  
Old 02-10-19, 10:50 AM
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Are you saying you are using your tankless coil for your heating loop. If you are not using it for domestic hot water then why are you maintaining constant boiler temp instead of going through a relay and controlling it with your stat on a call for heat.

One stat controls the steam and the other controls the water zone. Why do you need 2 pumps with only 1 hydronic zone.
 
  #14  
Old 02-10-19, 11:59 AM
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Hi spott. Iím not using the tankless coil for the hydronic loop. Thereís a separate tapping into the boiler for the loop. Iíve attached a pic showing this.


I donít want to maintain temp which is why Iím asking how to rewire the aquastat. I donít think I can do that with the honeywell ra89a relay thatís currently installed.

Unfortunately I bought the house with this setup and trying to make it more efficient and economical to run.

Is there a way to wire this aquastat to this relay, or another one, where it can do exactly as you suggested, which is to control it with the thermostat on a call for heat?

Thanks for for all the advice.
 
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  #15  
Old 02-10-19, 12:12 PM
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Did you move these L4006Aís around and re-wire? Are you sure you removed the L4006A that was being used for the Low Limit, and not the one that was being used for the High Limit?

Well since Iím teaching myself a little here, maybe what I found would be helpful here Ė and maybe not. But for what itís worth:

If you look at this diagram you can see how two 4006Aís are being used: one for the High Limit and one for the Low Limit. They both have the contacts normally closed and the contacts open on a temperature rise. You can see the one on the lower left acts as a Low Limit, since - because it is normally closed, T-T on the primary control is continuously energized until a high enough temperature is reached, and then the contacts on the L4006A open and T-T is de-energized. When the temp drops below the differential the contacts on the L4006A close again and T-T is energized again and the burner starts again and so on. So this L4006A is used as a Low Limit and boiler water temperature is maintained all the time.

The L4006A on the top right, however, is also normally closed and allows continuous power to the control. If the water temp reaches the High Limit, the L4006A contacts open and kill power to the control, and the burner stops. When the temp drops below the differential the L4006A contacts make again and power is restored to the control and the burner runs again if there is a call for heat. This L4006A is being used as a High Limit.

You can see that the High Limit L4006A itself is not wired in such a way as to turn on the burner if there is no call from the thermostat. It just supplies power to the control and a call from the thermostat is required to turn on the boiler, that is, you can see that X-X in the diagram is energized on a call from the thermostat and that turns on T-T at the control.

Anyway, it seems to me, if one of your L4006Aís is turning on your boiler when there is no call from the thermostat, then BY DEFINITION it is being used as a Low Limit and NOT a High Limit. Thatís what makes me think you removed the High Limit and not the Low Limit.
 
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  #16  
Old 02-10-19, 12:32 PM
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Do you have a pic of you relay without cover and hydronic zone with pump. There is a way to do what you want. Run everything through the relay and control that zone with your stat just as you do with the steam zone.

Where 5 & 6 go to the burner, you will cut in your 4006 in series in between those 2 points.

https://www.supplyhouse.com/Honeywel...3bf4fc8f690bc5

https://www.supplyhouse.com/Taco-SR5...witching-Relay

Scroll down to install instructions and see schematic.
 

Last edited by spott; 02-10-19 at 01:48 PM.
  #17  
Old 02-10-19, 02:56 PM
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Hi zoesdad. I did remove the wires for the low limit as that aquastat is attached to the tankless coil. This is in the pic from my first post.
Both sets of wires, and the steam thermostat, are all connected to the SAME terminals on the oil primary control, the r7184p. As a result, both aquastats act the same which is as a low limit.
 
  #18  
Old 02-10-19, 03:04 PM
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Hi spott. I have attached the pics that you requested.
The green Taco circ is on the supply and the red b&g circ is on the return and both flow through a heat exchanger.
Iíll take a look at your links shortly and will let you know if I have any questions.
 
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  #19  
Old 02-10-19, 04:39 PM
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That 89 relay is only being used to operate your pump on a call for heat and has no way to turn the boiler on. The aquastat is being used for that purpose just as if you were using your coil. Every time the temp drops below the set point it turns on the boiler if you need heat or not.

That aquastat has to be tied into the relay so it operates only on a call for heat.
You only have 1 pump on that relay. What runs the other pump and what heat exchanger and why 2 pumps.

If everything runs fine except the way the aquastat is wired, look to see if you can wire that 4006 in series between the stat and burner from that relay instead of the current wiring directly to the burner.

Just a thought.
 
  #20  
Old 02-11-19, 02:47 PM
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Spott,
I suspect the heat exchanger is either a plate or shell & tube type. The reason for having a heat exchanger would be the low operating pressure of a steam boiler. What I don't understand is if both circs are on the boiler side. One should be on the boiler side of the HX & the other on the hydronic loop side.
 
  #21  
Old 02-11-19, 04:48 PM
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Grady,
I was thinking about a plate heat exchanger also but that would mean 4 pipes being involved. Two to the boiler and 2 for the plate for hydronics. Check out post #6. Has 1 pump on supply and 1 on return and 4006 directly wired to primary like a tankless operation, maintaining boiler temp.

I asked if he could post pics of the setup to see exactly what he has.
 
  #22  
Old 02-11-19, 09:15 PM
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Hi spott. I think using the sr501 would solve a number of my issues. Just would like to clarify some things.
Would the 24vac/Com on the sr501allow me to run a common wire to the Nest? This would solve the power stealing issue of the nest.
Can I rewire the aquastat from the oil primary, the r7184p, to the 5 and 6 on the sr501 and would this allow the aquastat to control the burner through the relay? This would make my boiler a ďcold startĒ boiler.
If this setup works I believe I can turn off the boiler for the summer, but only after itís been thoroughly cleaned correct? Thanks for all the great advice.
 
  #23  
Old 02-12-19, 12:19 PM
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E,
Yes, your 4006 would be connected to 5 & 6 then to your primary TT so the boiler would only operate on a call for heat instead of maintaining temp constantly.

That 24V terminal can be used for your nest. I have never installed a nest so I don't know what the specs are but according to Taco if you fully charge your nest battery first that terminal will maintain it.

At the bottom of the page I'm posting there is a number for TACO to call if you want to verify.

https://s3.amazonaws.com/s3.supplyho..._PROD_FILE.pdf

Hope this helps a little.
 
  #24  
Old 02-12-19, 02:03 PM
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Here's a fairly long Q & A on a steam boiler with a hydronic loop. About 3/4 down the page is a question & answer about control wiring. https://heatinghelp.com/systems-help...-steam-boiler/
 
  #25  
Old 02-13-19, 08:40 AM
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Hi spott. This maybe a dumb question but how would I wire the 4006 to the 5 and 6 on the sr501 AND the tt on the boiler? Pigtail and cap each wire? Thanks
 
  #26  
Old 02-13-19, 12:49 PM
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I think this is the way. I think the second way would also work. Maybe the second way would be safer. I think they both would satisfy the link Grady pointed to. ButÖ need the pros to check this. I could be wrong.
 
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Old 02-13-19, 01:24 PM
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Thank you Z. I have still not figured out how to show schematics.

The first schematic is how I would do it.

The second using the L1 I wouldn't recommend simply because by cutting in between L1 & L1 on the primary, if the 4006 was satisfied it would interrupt power to the primary which is also needed for the steam zone.

In the first it would take the stat and the 4006 to start the burner. The if the 4006 reached temp but the zone is still not satisfied then the burner shuts off but the pump stays running until the zone is satisfied. If the temp drops below the aquastat setting the burner would come back on.

Hope this is more clear due to Z's schematic.
 
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