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Updating 35 year old heating system

LimeLight
LimeLight Member Posts: 5
I am upgrading my heating system in steps and need some advice on how to proceed.

Last year I installed a Weil-Mclain GV90+2 that heats two zones (upstairs/down stairs). The system has two Honeywell R845A1030 Switching Relays that provide power to twoTaco 007-F5 circulators. There are 2 wire connections between each thermostats and the R845A's, which are connected to the 'T' terminals and the 'W' and 'R' terminals on the battery powered thermostats. Terminal '5' and '6' of the R845A's are hooked together and then one has a wire to the GV90, attached to the 'BK' and 'BK' wires of the TSTAT plug.

I have bought two Ecobee 4 thermostats and am planing to pull new 5 conduit wire from the boiler to each thermostat location.

First question is can I use the 'C' wire from the TSTAT plug on the boiler as the 'C' wire for both Ecobee's keeping the 'W' and 'R' wires as they are currently wired?

Second question is that I want to replace the R845A's with a unit that has solid state relays. Would a Taco SR502-4 work out of the box to replace both with a single unit? Providing I have the 5 wire conduit run.

Thank you for any and all advice

Comments

  • Steamhead
    Steamhead Member Posts: 14,863
    The Common wire of a thermostat cable must connect to the same transformer as the other wires. This would be the transformer in the relay unit. Unfortunately, there is no common terminal in an R845, or any similar relay that I know of. And trying to connect two common wires together when they're fed from separate transformers is asking for trouble.

    When we need "common wire" capability, we generally use a "fan center relay" such as this one:

    https://customer.honeywell.com/resources/Techlit/TechLitDocuments/69-0000s/69-0482.pdf

    This type of relay mounts on a standard "1900" (4" square) electrical box, the common terminal on the transformer is accessible, and the relay itself can be replaced without replacing the entire unit.
    All Steamed Up, Inc.
    Steam, Vapor & Hot-Water Heating Specialists
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  • LimeLight
    LimeLight Member Posts: 5
    edited September 2018
    Thank you for the quick response. I will be pulling new 5 strand wire this weekend and only hooking up two of the wires to each of my battery powered thermostats.

    If I pick up a Taco SR502-4 I should be able to replace the R845s. The SR502 has 2 24v COM along with R and W for two zone thermostats and 120v N and H for two zone circulators.

    The only question I have is will the Isolated end switch X contacts kick on the boiler using the two BK wires that are currently hooked up to one of the R845s?

    Looks real simple to hook up using the current wires I have as long as I pull new wire to the thermostats.
  • Steamhead
    Steamhead Member Posts: 14,863
    Sure about the common terminals? I looked at the instruction sheet and they are not shown:

    http://www.taco-hvac.com/uploads/FileLibrary/SR502(102-082).pdf
    All Steamed Up, Inc.
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    Consulting
  • LimeLight
    LimeLight Member Posts: 5
    edited September 2018
    http://www.taco-hvac.com/uploads/FileLibrary/102-377.pdf

    Looks like it will do what I need. Not sure about the 100K resistor.
  • Steamhead
    Steamhead Member Posts: 14,863
    Looks like there are different versions.

    The resistor between W and C may be for a "power-stealing" thermostat that doesn't have a C terminal. These were common for a while but I haven't seen many lately. With a setup like this, everything is fed from the transformer in the SR502-4, so all the C leads from the thermostats would go to that C terminal on the board.
    All Steamed Up, Inc.
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    Consulting
  • HVACNUT
    HVACNUT Member Posts: 4,209
    > @Steamhead said:
    > Looks like there are different versions.
    >
    > The resistor between W and C may be for a "power-stealing" thermostat that doesn't have a C terminal. These were common for a while but I haven't seen many lately. With a setup like this, everything is fed from the transformer in the SR502-4, so all the C leads from the thermostats would go to that C terminal on the board.

    The Taco relays now have an R-C terminal block on the left side of the board.

    @LimeLight
    What model aquastat do you have? The XX dry contacts on the 502 wire up to TT on the aquastat.
    If the aquastat is a L8148A, it will be a cold start.
    If it's an L8124, it will maintain the low setting and limit at the high setting on a heat demand.
    Using the XX dry contacts will not hold the circs off when the low limit temp is met.
    In order to help prevent condensing of the flue gasses, you're better off using ZR/ZC if it's a triple aquastat.
  • Robert O'Brien
    Robert O'Brien Member Posts: 3,390
    > @LimeLight said:
    > I am upgrading my heating system in steps and need some advice on how to proceed.
    >
    > Last year I installed a Weil-Mclain GV90+2 that heats two zones (upstairs/down stairs). The system has two Honeywell R845A1030 Switching Relays that provide power to twoTaco 007-F5 circulators. There are 2 wire connections between each thermostats and the R845A's, which are connected to the 'T' terminals and the 'W' and 'R' terminals on the battery powered thermostats. Terminal '5' and '6' of the R845A's are hooked together and then one has a wire to the GV90, attached to the 'BK' and 'BK' wires of the TSTAT plug.
    >
    > I have bought two Ecobee 4 thermostats and am planing to pull new 5 conduit wire from the boiler to each thermostat location.
    >
    > First question is can I use the 'C' wire from the TSTAT plug on the boiler as the 'C' wire for both Ecobee's keeping the 'W' and 'R' wires as they are currently wired?
    >
    > Second question is that I want to replace the R845A's with a unit that has solid state relays. Would a Taco SR502-4 work out of the box to replace both with a single unit? Providing I have the 5 wire conduit run.
    >
    > Thank you for any and all advice

    502 will do it
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  • LimeLight
    LimeLight Member Posts: 5
    edited September 2018
    @HVACNUT

    The GV90+ does not have an aquastat hooked up too it. So will the XX on the 502 work to trigger the boiler if I hook them up to the BK wires that the R845A is currently hooked up too?

    @Robert O'Brien
    Thanks for the reply I just ordered a TSR502 be here on Monday. I have ran the new 5 strand wire to the downstairs thermostat and hooked up the old thermostat to the R845A, every thing works great. Plan to pull the upstairs wire tomorrow, it will not be as easy. Praying for it not to be stapled to the studs in the wall, fingers crossed.
  • HVACNUT
    HVACNUT Member Posts: 4,209
    > @LimeLight said:
    > @HVACNUT
    >
    > The GV90+ does not have an aquastat hooked up too it. So will the XX on the 502 work to trigger the boiler if I hook them up to the BK wires that the R845A is currently hooked up too?
    >
    Sorry. I was stuck in oil heat mode.
    Can you post a pic of what the black wires are connected to at the boiler?
  • LimeLight
    LimeLight Member Posts: 5
    @HVACNUT
    Page 58 of the attached. Terminal 5 and 6 of one of the R845As is connected to the two BK wires on the TSTST plug of the GV90+. My intent is to hook up the ISOLATED END SWITCH (X X) of the 502 to the same two wires on the GV90+.
  • HVACNUT
    HVACNUT Member Posts: 4,209
    Yup. Looks like the blacks are connected to R and G which is the thermostat or end switch connection.
    Done yet?