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Author Topic: Thermostat control of forced air and oil boiler  (Read 7039 times)

hondaracer2oo4

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Thermostat control of forced air and oil boiler
« on: September 08, 2017, 07:47:07 PM »

History on the system is that it has been operating fine for the past few years since I built it. I am just curious what others have done and if I could make it operate a little better. I was doing some pre season testing of the oil boiler today and found a flaw that isnt't major but does affect the oil boiler operation a little bit, this wouldn't be a problem for the most part when the owb pumps are pumping water to the house. So here is what I have.

I have a primary/secondary loop system. The owb flat plate heat exchanger is plumbed into the primary loop. The oil boiler is plumbed into the primary loop through a secondary loop. The forced air fan box is plumbed into the primary through a secondary loop.

The house thermostat controls a Honeywell RA89A1074 which is a SPST controller. It provides the low voltage power for the thermostat to operate. The other side of the controller powers the forced air blower. I also needed the thermostat to command the oil boiler to operate. The thermostat wires are tied togeather in the Honeywell controller, it runs over to the oil boiler aquastat which is a Honeywell 8148a. The thermostat wires running over to the aquastat are powered constantly by the ra89a transformer. I placed a relay on the aquastat controlleping the T T terminal. The relay is on n/c but is powered into the open position by the ra89a when the thermostat isn't calling for heat. When the thermostat calls for heat it drops the relay out and the relay powers up the TT terminals on the aquastat so that he oil burner will fire.

The oil boiler is powered through a ranco aquastat. The sensor is hooked to the incoming owb line. It is set to power up the oil boiler if the incoming owb water drops below 155 and turns off when the incoming water hits 163. So here in lies my problem. During the times when the owb isn't running and the owb pumps aren't circulating water through the incoming water lines the temp rises above the 163 mark as the oil boiler circulates water around the primary loop. When the water hits 163 the ranco cuts power to the oil boiler which also kills the oil boiler injection pump. This killing of the oil boiler injection pump allows the water in the primary loop to obviously drop quickly as the forced air pulls the heat quickly out of it but it takes a good 3-4 mins for the owb line to finally drop below 155 and kick the injection pump back on. Like I said during the winter when the owb pump is running then the ranco would quickly see that temp drop because the flat plate would cool down the water but during the off season that water takes a while to cool down.

Is there an easier way to do all this? Like I said it works but is complicated and not perfect.
« Last Edit: September 08, 2017, 07:50:26 PM by hondaracer2oo4 »
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E Yoder

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Re: Thermostat control of forced air and oil boiler
« Reply #1 on: September 09, 2017, 08:05:16 PM »

Is the ranco sending temp on the primary loop or on the incoming owb line? Maybe I missed it. That would make a difference.
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hondaracer2oo4

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Re: Thermostat control of forced air and oil boiler
« Reply #2 on: September 09, 2017, 08:38:45 PM »

The ranco is on the incoming boiler line. I hadn't given much thought to putting it on the primary loop. Now I am thinking about how that would work....
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hondaracer2oo4

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Re: Thermostat control of forced air and oil boiler
« Reply #3 on: September 09, 2017, 08:42:53 PM »

Now that I thought about it I have come to the conclusion that it won't work. The reason why it won't work is that the ranco would never know when the owb comes back up to temp so that it automatically swaps back to the owb and kills the oil boiler.
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RSI

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Re: Thermostat control of forced air and oil boiler
« Reply #4 on: September 09, 2017, 11:33:21 PM »

I just quickly read through it but I am thinking putting a 3 way zone valve on the incoming boiler line to keep the oil boiler from heating up the OWB would be the way to go. You could add another controller if needed for freeze protection.

Or if you don't want to do that, wire in a relay that is powered by the oil burner that keeps it running till it completes a cycle. Then it will drop out and once it hits 155 again it will run for another cycle.
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slimjim

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Re: Thermostat control of forced air and oil boiler
« Reply #5 on: September 10, 2017, 04:38:36 AM »

I think you are overthunking it, first if its warm outside, why run the fan instead of the regular baseboard?
If the Ranco is killing the power on L1 then yes it will also kill the injection pump, try breaking power with the Ranco on either B1 or on the TT terminals at the burner itself.
Is there a ball valve between the plate exchanger and the Ranco Aqua stat, if so close it in warm weather.
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hondaracer2oo4

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Re: Thermostat control of forced air and oil boiler
« Reply #6 on: September 10, 2017, 06:52:44 AM »

Rsi, I wood like to not add a component to the plumbing if possible. Could you explain more of what you are thinking on the relay?

Slim, my house is forced hot air, no baseboard in the home. I used to have an old oil furnace. I removed it and replaced it with a boiler and built a cabinet box for my forced air fan. Yes I am breaking L1 with the ranco. the problem that I see with running b1 is that the injection pump is going to always be running when the thermostat calls for heat.
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slimjim

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Re: Thermostat control of forced air and oil boiler
« Reply #7 on: September 10, 2017, 06:56:33 AM »

No, the injection pump should be wired to C1 in the Aqua stat and will only come on after the boiler temp reaches its differential on the high set point, this would require the boiler to first fire up and bring itself up to temp and then the circ would energies.
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hondaracer2oo4

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Re: Thermostat control of forced air and oil boiler
« Reply #8 on: September 10, 2017, 07:38:30 AM »

Right, but won't the aquastat start seeing hot water from the plate exchanger as the owb heats up and the in house thermostat calls for heat it will kick on the injection pump which will pull hot water from the primary loop into the oil boiler. This will cause the injection pump to always kick on when the thermostat calls for heat because the oil boiler will never sit there long enough without the house thermostat calling for heat to cool down below the point that the aquastat will drop out.
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slimjim

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Re: Thermostat control of forced air and oil boiler
« Reply #9 on: September 10, 2017, 07:48:54 AM »

You are not understanding the operation of the triple acting Aqua stat that controls your oil boiler, the C1 terminal only energizes on a call for heat AND the minimum oil boiler temp has been met.
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hondaracer2oo4

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Re: Thermostat control of forced air and oil boiler
« Reply #10 on: September 10, 2017, 10:15:41 AM »

Maybe I am..... but this is what I was thinking.

Owb incoming line drops below 150, ranco kicks on allowing oil burner to fire. Oil boiler comes up to temp and aquastat allows injection pump to kick on. Oil boiler is now heating the primary loop.

Owb is filled back up with wood, temp comes back up and hits 160 and turns off ranco so that oil burner can't fire anymore. Oil boiler temp is still at 180 from heating. House thermostat calls for heat. Aquastat still sees warm oil boiler and kicks on injection pump. Injection pump pumps into primary loop and pulls off primary loop back into oil boiler. The flat plate is pulling heat from the owb and now the injection pump is pulling that hot water back into the oil boiler keeping the oil boiler temp high enough to keep the aquastat up to temp and telling the injection pump to turn on every time there is a call for heat in the house. The cast iron boiler would have to not have the house call for heat for a long long time to drop off the aquastat.
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RSI

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Re: Thermostat control of forced air and oil boiler
« Reply #11 on: September 10, 2017, 11:38:24 AM »

I didn't have time to read through all the details but I assume the oil boiler's aquastat is still controlling the oil burner when the ranco activates it.
If that is correct, the, use a relay with 120v coil. Connect coil to injection pump power. Connect contacts in parallel with the Ranco controller.
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slimjim

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Re: Thermostat control of forced air and oil boiler
« Reply #12 on: September 10, 2017, 11:51:02 AM »

Again, the only time the injection circ will energize is on a call for heat, as soon as the load is satisfied then the oil boiler injection circ power is cut as well as the burner and the oil boiler will be taken out of the loop and in turn should cool down, that should only take perhaps 20 minutes with the cooling power of your chimney.
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mlappin

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Re: Thermostat control of forced air and oil boiler
« Reply #13 on: September 10, 2017, 12:33:55 PM »

Sounds entirely like a whole other kettle of fish, for the most part old school boilers have gone by the wayside unless a person is planning on radiant, most don’t though as they want Central AC as well, if your gonna run all the ducts for AC then might as well heat with em as well. My friend did however install a ductless mini split system in his new house, DHW and his shop floor is heated from the same instant demand LP boiler, just a tiny thing hanging on the wall.
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hondaracer2oo4

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Re: Thermostat control of forced air and oil boiler
« Reply #14 on: September 10, 2017, 05:51:19 PM »

So during the cold winter months my house will call for heat for about 8-10 mins and then it will be ussually another 10 minutes or so before it calls for heat again. That is not enough time for the oil boiler to drop down from incoming OWB temp to the temp below what the aquastat is set to energize the circulate. BTW the oil boiler is power vented which keeps more heat in the boiler. I ran it the other day when I was getting it ready for the heating season and it took 6 hours to drop from 180-120.
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