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chufi

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Posts: 3
Reply with quote  #1 
I have a DIY solar hot water system that is over sized for my hot water use, and was thinking to use the extra heat for bonus house heating.   I was thinking of putting something like: https://www.amazon.com/18x20-Water-Exchanger-Outdoor-Furnace/dp/B00B1GRFIM/ref=sr_1_3?ie=UTF8&qid=1489690557&sr=8-3&keywords=water+to+air+heat+exchanger in the plenum of our furance (return or supply?) and use a circulation pump to pump the tank water through it.  I have no idea what exact pieces I would need for control.  Conceptually I need to get a basic thermostat to kick it on at temp X and also turn on the fan for the furnace, but how to do that?  The furnace already has a thermostat hooked up to it since it is the primary source of heat.  Would have two sets of wires hooked up to it be a problem? The second one would just need ground and fan I would guess.   Any thoughts? Thanks!

stmbtwle

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Reply with quote  #2 
IF the fan is controlled by a relay, I see no reason why you couldn't put a second relay in parallel with the first, so that either relay would turn on the fan.  

If the fan is variable speed and is NOT controlled by a relay but rather a circuit board, this could be dangerous as feedback could damage the expensive board.  I think in that case I'd find a way to install a second fan, dedicated to the solar system.

If in doubt I'd go with the second option as it doesn't require tampering with the original setup.

A third (and probably simplest) option is to turn the HVAC fan to "circulate", and just let it run. Use your thermostat to control the water circulating pump to the radiator.




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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
chufi

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Reply with quote  #3 
I'll have to check the furnace and see how the fan is wired up.  For second idea - any thoughts on how to get a second fan in the system (and or where one might get something like that? I dont have much experience with the hvac side of things.)  And I guess should this go on the supply or return side of the furnace (or does it even matter?)
Rick H Parker

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Reply with quote  #4 
You only need one thermostat and a logic circuit that selects which furnace kicks on and off. The logic circuit could be simple relay controlled by the temperature of the solar hot water.

  1. The relay coil is controlled by the temperature of the solar hot water.
  2. The pole(s) of the rely are hooked to the thermostat.
  3. The throws of the relay are hooked to the two furnaces.

Rick H Parker

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Rick H Parker
Kansas, USA
Electronics Engineering Technologist
pianoman8020

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Reply with quote  #5 
Just a suggestion:
Excess available heat could be rerouted through the coil you found on Amazon and then cooled with a separate 18" box fan which would be typically located in you basement.
Things to consider: Use Pex tubing where possible; Use a differential controller to maintain you water heater temperature; Use a variable fan control to balance/regulate air flows; All could be mounted in a separate box (ie. filter, fan, coil, control); Stay out of your furnace ducting because an extra coil will decrease air flow??; The available BTU's may not be as high as you would expect and quite intermittent.  Good Luck!!

Jim from IL
chufi

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Reply with quote  #6 
Quote:
  1. The relay coil is controlled by the temperature of the solar hot water.
  2. The pole(s) of the rely are hooked to the thermostat.
  3. The throws of the relay are hooked to the two furnaces. 


Can you suggest parts by any chance?  I am familiar with basic home electrical but haven't used relays before.  I only have the one furnace so not quite sure what 3 is?  Thanks, this is all a bit of a learning curve for me.
stmbtwle

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Reply with quote  #7 
Once you install your solar, it will be your second furnace. The relay will allow you to select one or the other.

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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
Rick H Parker

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Reply with quote  #8 

"Can you suggest parts by any chance?  I am familiar with basic home electrical but haven't used relays before.  

You would need to determine the switching voltage for the relay coil, the voltage and current of the loads before we could select a relay that would work.

"I only have the one furnace so not quite sure what 3 is?"

Throws is the number of outputs per input. A normal household light switch is single pole, single throw, one output per input. A two way light switch is single pole, double throw, two outputs per input, it is never really off, one output or the other is always on. In mechanical switches the pole is thrown to the outputs, that is where the term comes from.  The first switches where knife switches. Some had two possible on positions.  "Throw the switch! ... Its double throw, which way?"


SPDT-switch-circuit.png 
SPDT-diagram.png 


If you do this you will have two furnaces. The second one will be your solar furnace. Your need to throw the the thermostat output to the furnace that you want to run.

Double Pole Double Throw (DPDT) would be two SPDTs with the poles link together.

Double-Knife-Double-Throw-Switch-300A.jpg 

Two poles, two ways it can be thrown.

 

"Thanks, this is all a bit of a learning curve for me."

It is good you acknowledge there something that needs to be learned, too many people want an answer without learning anything. I'll try not to throw to much at you at once.

Relays are just electrically controlled switches. Today we have more the just the old electromechanical relays, which type you need is going to depend on the current and voltages you need to switch.



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Rick H Parker
Kansas, USA
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stmbtwle

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Reply with quote  #9 
I'd suggest a double-pole-double-throw relay, with a 24v coil (thermostat is 24v)

You could run a wire from the thermostat power supply in the furnace to a snap switch in your collector, then to the relay. When you have enough heat it will pull in the relay and select the solar collector. Otherwise it would default to the furnace.

I have a similar setup on my houseboat. The A/C thermostat controls a relay which in turn selects the air conditioner OR the water heater. As the A/C normally cycles on & off, I end up getting both, just not at the same time.

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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
Rick H Parker

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Reply with quote  #10 
"You could run a wire from the thermostat power supply in the furnace to a snap switch in your collector, then to the relay. When you have enough heat it will pull in the relay and select the solar collector. Otherwise it would default to the furnace."


That is the same thing I'm suggesting, Just trying to teach relay/switch terminology in my last comment.

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Rick H Parker
Kansas, USA
Electronics Engineering Technologist
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