Boiler Installation-Hot Water Coil

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JohnC0310

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Figured I would try another thought on my mind today. I am in the middle of a restoration of a brownstone that is a design build. The heating has become another piece of the pie. The installation is converting an old 3 family apartment to a single family home. There is an AHU on each of the second and third floors that will provide cooling in the summer and heating in the winter by way of a hot water coil installed in the AHU.


The new boiler will have a zone for each of the AHU units (2) and also provide baseboard heat to the first floor. My question is this for the second and third floor- the thermostat is controlling the AHU on the respective floor so the 24 volt feed is derived from the AHU circuit board. My thought is to have the heating side of the thermostat control a general purpose relay coil that I install in the AHU that will close a set of contacts that I connect the thermostat line originating in the TACO (SR_506) boiler control switching relay in the basement for the respective zone/circulator pump.

My concern doing it any other way is connecting the 2 different power supplies together and possibly shorting a board. The plumber tells me that he does it all the time, I say have at it but let the homeowner know that the plumber is the one to call when the heat does not work or there is a new circuit board needed for the AHU.


Any thoughts on alternate methods???

[ October 11, 2005, 04:24 PM: Message edited by: JohnC0310 ]
 
Re: Boiler Installation-Hot Water Coil

Is this an engineered control system? If so, what do the instructions say? Heck, what do the instructions say anyway?

I would want to know if the boiler is looking for a switch closure or a 24-volt input. I would also like to know if the AHU is just closing a switch on the control board or if it sending out 24-volts.

If the boiler controller is looking for a 24-volt signal and that signal just closes a switch on the boiler controller that isolates its power supply, the work may already be done for you.

I think before you can make a decision, you have to know (in some detail) how the control boards in the boiler and the AHU are working. I'd push hard for manufacturers instructions and if you have a question, call them. Don't trust the plumber! lol
 
Re: Boiler Installation-Hot Water Coil

I'm not sure I understand your description of how you intend to connect the coil of the relay you are adding. You are correct in your concern about paralleling the two 24 volt supplies from the two AH's.

First I would check the AH control board to see if there is a set of dry contacts available for this purpose. If so just parallel them (each AH) and run an 18/2 down to the circulator control.

If no contacts are available then you will have to provide an "isolation" relay at each AH. The thermostat will operate that relay which will have two sets of contacts, one to operate the AH and the other to operate the circulator. Each relay will also have it's own 24 volt power source. Best way to do this is with something like a Honeywell RA-89A that has a transformer built in rather than building your own. That's what I used to use, check Honeywell to see what is new.

-Hal
 
Re: Boiler Installation-Hot Water Coil

Thanks for the replies guys. Each AH has its own circulator pump on the new boiler, the 120 volt being supplied by the TACO switch relay. The AH has control terminals for a flow valve or circulator that would be installed either at the AHU or remotely at the boiler. My concern with doing this is to have the boiler shut down for some reason and the pump would start for the AH and burn up. I would also have to run the 120v from each floor to the boiler.

The isolation relay is how I am going to wire it so the terminals for the AH board and TACO are separate. I wish that this job was engineered but it is a design build that has each trade coming in and hashing their own work out and the next guy comes in and performs miracles.

Of course the plumber installed the boiler in a corner and the pumps were facing the wall with the expansion tank above them. I found a mirror and taped it to the wall for help.....
 
Re: Boiler Installation-Hot Water Coil

The AH has control terminals for a flow valve or circulator that would be installed either at the AHU or remotely at the boiler.

Right, that's what I was talking about. They should be dry contacts that close when the thermostat calls for heat.

I would also have to run the 120v from each floor to the boiler.

Run an 18/2 from each of the AH control terminals to the thermostat terminals for each circulator on the TACO panel.
Low voltage.

My concern... is to have the boiler shut down for some reason and the pump would start for the AH and burn up.

The 120 volt feed for TACO panel should be wired with the boiler so that when it is shut down so will it and the circulators. Certainly shutting down the boiler safety switch should kill the circulators. The low water cutoff if used as well as the high limit aquastat just kills the "fire" whether it be gas or oil. At any rate I wouldn't be concerned about the circulators burning up, they can run continuously and they are thermally protected besides.

-Hal
 
Re: Boiler Installation-Hot Water Coil

Hal's advice is right on the money - I've wired many like this. One question that I have is how are you energizing the AH fan in the heating mode? Is the AH control circuit already set up for this with a "white" terminal? If the AH is set up from manufacturer for cooling only just be careful not to "back feed" condensor 24V. I've seen it happen in this application where a relay is used in heating mode to energize green fan terminal. This in turn connects to Y terminal in t-stat, consequently calling for cooling. No big deal to isolate it as long as you are aware of it.

John
 
Re: Boiler Installation-Hot Water Coil

The unit controls was wired by the HVAC installer. It was installed knowing that there would be a hot water coil so the HVAC guy ran the appropriate control wiring. The unit is also built for heating so I assume that there is a multispeed fan. Thanks for checking in though
 
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