4h2c heat pump with electric backup and oil boiler

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Old 04-10-20, 06:10 PM
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4h2c heat pump with electric backup and oil boiler

PJmax Me again. This time we are installing a Bosch 2.0 system 3 ton AH and HP with a 15kw heat strip. My puzzle is, I would like to use a 4h/2c stat to take full advantage of the new unit, but then after a certain outdoor temp say 40 degrees I would like to automatically switch from the HP/backup to strictly oil heat. What is the easiest way to do this?
I also want to utilize the DeHumidification setting of the HP/AH.
Can I use a Tstat with a w3 terminal to energize a dpdt relay to switch R from the HP to the boiler and at the same time break Y to open so the compressor can't run.
Or should I switch W1 to boiler as well instead of worrying about Y1
Or would that still energize W1 and W2 to call for the Heat strip?
Aquastat??
Thanks in advance, Joe
 
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Old 04-11-20, 02:53 PM
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A little confusing.

Question..... why not use the oil in place of the electric reheats ?
It's got to be less costly or are you getting a steal on electric rates ?
 
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Old 04-11-20, 04:45 PM
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Yes my electric rates are good up to a point which why at the lower temps I want to auto switch to the oil.
Is switching the R and W1 on the right track?
I guess I forgot to mention I was trying to do it all with 1 Thermostat
 
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Old 04-11-20, 04:56 PM
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Basically you want to make your own fossil fuel kit.

It's not as easy as 1-2-3.
You need to consider things like hysteresis. For example..... if the temperature gets to 40 and stays there. What will the system do..... keep toggling from electric to oil ? You need a way to incorporate a deadband.... a set of points where the system stays in one mode.
 
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Old 04-11-20, 04:59 PM
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Yes , point well taken.
Maybe 2 stats is just easier with a manual switch over.
Just ugly,clunky, and not automatic.
Have been revisiting this for several days, before hitting you up.
Still curious what you might do?

With a W3 stat I believe I can set a deadband point, Have 2 c check
 
 

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