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Adding Ecobee to Honeywell L8148E 2 zone hydronic system

Rack Member Posts: 2
edited May 14 in Thermostats and Controls
I would like to install 2 Ecobee thermostats to my current 2 zone hydronic heat system. I have a Honeywell L8148E Aquastat with 2 Honeywell V8043F1036 valves for my zones. I currently only have 2 wire thermostats on each zone.

First, I would like to know if the internal transformer is capable of handling the additional power requirements of the new thermostats. The only transformer in the system is the one integrated in the L8148E. I can add a new transformer, but would like to avoid if not necessary. I went to purchase one, but the local HVAC tech I talked to told me that he would just run the thermostats off the integrated transformer. It seems there are varying opinions on the forum about this. I know the Aquastat is rated to power 2 zones, but not sure if I will overwhelm it with the ecobees. I'm guessing the worst case scenario is that i may have to replace the integrated transformer if I overload it, which doesn't seem that difficult, or expensive... so I'm inclined to try it.

Second, I would like some help with how to wire up the new thermostats correctly. I will attempt to create a drawing like others I've seen on the forum to show what I currently have, but it will have to wait until tomorrow. From the manuals I've been looking at, I'm not convinced my system is wired correctly. It has been working since I purchased the home 12 years ago, but it seems strange that all the manuals seem to show different wiring schemes than what I have. I had A/C installed immediately upon purchasing the home, and it appears that the installers cut the old wiring from my thermostats, and ran their own. The old wire is still present, just cut off at the valves. I have no idea if they changed anything.

My current wiring is as follows. From my V8043F1036 valves, one end switch goes to Tv on the 8148. The other end switch is jumpered to TR, which is connected to T on the 8148. TH is connected to W on the thermostat, and R on the thermostat is connected to Z on the 8148. Z is jumpered to W.

Thank you in advance for any advice you can provide.
Mad Dog_2


  • 109A_5
    109A_5 Member Posts: 1,001
    edited May 14
    Hello @Rack,

    You did not mention if there is any other load on the Aquastat's transformer, like a damper and/or an ignition module.

    It may be possible. You add up all the VA (Volt Amp) rating of all the loads.
    EDIT for clarity.
    The VA rating of the transformer in the L8148E is 50 VA.
    It appears that about 33.6 VA Max. is usable for external loads (1.4 Amps).
    V8043F1036 at about 8 VA each.
    3.5 VA per EcoBee (this should be further verified).

    If you are unsure, I would venture to say it is less expensive to burn up an external transformer than the transformer in the Aquastat from additional loads and/or wiring errors. For the cost of an additional transformer why stress the transformer in the Aquastat ? You could add fuses too, to protect the transformer(s). Although fuses may not protect from a mild overload.

    National - U.S. Gas Boiler 45+ Years Old
    Steam 300 SQ. FT. - EDR 347
    One Pipe System
    HVACNUT Member Posts: 5,464
    Is the air conditioning thermostat separate from the heating thermostats?

    If one or both thermostats are heat/cool, then take the power from the AC transformer. 

    The L8148E has a 50 va transformer so I think it would be OK but personally I would still install a separate fused 40 va transformer and isolation relay for the thermostats. 
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 6,695

    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 6,695
    edited May 14
    I looked up some typical control current usage numbers. If you have an automatic vent damper on your L8148 control the total VA usage of all the working parts is over the transformer rating. It may work for some time however the transformer will operate at an elevated temperature, eventually causing insulation failure on the windings, resulting in transformer failure. I recommend that you use an auxiliary transformer for at least one of the zone valves.

    If you do not have an automatic vent damper, then you will be just fine using the existing L8148 transformer with the new thermostats.

    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • Rack
    Rack Member Posts: 2
    edited May 15
    Thank you for all the comments. I do not have a powered vent damper on the boiler. The aquastat does power a honeywell v800a1021 ignitor module.

    The A/C system I had installed has since been removed and replaced with mini splits, which each have their own sensors and remotes. The thermostats are solely for the heat system.

    It looks like I can probably get away with not having a separate transformer, so I'll probably just wire it as Ed described for now. Heat season is almost over, and I'll have time to clean up the rats nest of wiring over the summer. I may opt to add an additional transformer for peace of mind, but I'd like to get everything wired up right now to see how it all works together.

    I'll have to look into the fuse and isolation relay as well. Right now, the only protection for the system is the master on/off switch. (edit: the switch is fused) I realize you just said fuse and isolate the external transformer, but I will see if I should add anything else.

    I'm a very handy DIY guy, but never messed with heating and A/C much. I enjoy this process of learning, but don't want to make expensive mistakes along the way.

    Thank you all for your help.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 6,695
    No Vent damper = use existing L8148E transformer. As far as fuse on a transformer that was referenced above. Whenever I install an auxiliary transformer for zone valves or other apparatus, I like to start with this device on the R terminal of the low voltage side of the transformer.

    This comes in handy when you make a mistake. The factory installed smoke stays inside the transformer.

    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • colinjwarren
    colinjwarren Member Posts: 2
    I am also installing two Ecobee thermostats in a Honeywell L8148E two zone heating system, with V8043F1036 and V8043F1051 zone valves. I have determined that the total load on the transformer will be 45 VA, so I should be ok with the L8148E’s 50 VA built-in transformer. However, my current wiring differs from that in EdTheHeaterMan’s diagram. From the V8043F valves one end of both End Switches goes to T on the L8148E, the other ends are jumpered to TR, which goes to TV on the L8148E. These connections are the reverse of EdTheHeaterMan’s. Is it safe for me to just re-wire my system to accord with EdTheHeaterMan’s diagram? Also, on my valves the TR terminal is at the outer edge and the TH is in the middle, the opposite of what is shown in the diagram above. Is this significant in terms of which end of the End Switch is jumpered?
  • HomerJSmith
    HomerJSmith Member Posts: 2,309
    edited November 20
    What I would do. I would verify that the current thermostats cable has only 2 conductors. Sometimes there are other conductors in the cable that have been cut at the exit out of the sheathing because one only needs 2 conductors. You can check at the zone valve where the cable goes to the thermostats.
    There are work arounds for 2 conductor cable. Replacing 2 conductor with 3 conductor cable would be ideal, if possible.

    The aquastat transformer is capable of running 3 zone valves, current wise. That transformer would drive the aquastat relay and the zone valve motors and other equipment. I would use one auxiliary 40VA transformer and two relays, one relay for each thermostat. The relays could be SPST normally open. Wired up, the auxiliary transformer would drive the Ecobees and not the aquastat transformer providing power to the thermostats.
  • colinjwarren
    colinjwarren Member Posts: 2
    Thank you HomerJSmith. Confusingly, one of the two thermostats had 3 wires, but inside the wall this had been spliced to the old 2 wire cable. I managed to pull a new 5-wire cable through the basement crawl space and up the wall using this old cable. The total planned load on the L8148E relay transformer with the two new ecobees was 45VA, within the 50VA rated capacity of the L8148E. However, the wiring of the L8148E and two V8043F valves differed from that given by EdTheHeaterMan, and I wasn’t sure what would happen if I tried to rewire them according to his plan. So, in the end called a boiler engineer to come and do it. It took him 5 hours and several dead ends, but the ecobees are now working. He installed an auxiliary transformer, as you suggested. Now I hope the 20 year-old boiler (a Lennox) will last a few more years!