Dual Aquastat control scheme for GARN/oil burner

[Jim K in PA]

I do not have a wire schematic, but let me do my best to describe the simple (to me anyway) control scheme I want to build. I need suggestions for a particular Aquastat(s).

GARN will be supplying hot water to a HX in the house via a P/S with circs on each loop.

I want to incorporate an in-line Aquastat in the boiler return loop (before the HX) that will power up the Primary and Secondary circ pumps when the return temps drop to it's set point (exact temp not important right now).

However, if the GARN is not firing, and the supply water temp from the GARN is inadequate to meet load/demand, I want to turn off the P/S circs, and fire the oil burner. That will prevent back-feeding energy into the GARN with expensive oil.

My thought is to use a DPST Aquastat on the oil burner, with the in-line SPST Aquastat wired in series with the DPST stat. The in-line aquastat will be powered up via the normally closed contacts on the oil burner A-stat when the burner temp well sensor is above it's minimum setpoint (which is below the in-line aquastats minimum setpoint). If the oil burner aquastat senses boiler temps below it's minimum setpoint, the relay opens the normally closed circuit, shutting down the P/S circs, and closes the normally open circuit, lighting the burner gun. Both sides need 120VAC.

Sound right?

Can anyone help me decide which brand/model aquastat I need? Or should I just get whatever my local supply shop can get me that meets my criteria?

Also - for my in-line aquastat, I was planning on using the standard 3" temp well mounted in a Tee with a nipple and coupling (3/4 inch) size to the length needed to just get the tip of the well in the active flow. Is that a reasonable plan?

Thanks for the input.

 

[Nofossil]

A picture would truly be worth a thousand words or more in this case. I've also found it helpful to lay out the conditions that determine control of each device. For instance:

- Oil burner is on if there's heat demand and the Garn not hot
- Oil circ is on if oil is hot
- Primary circ is on if there is a demand or if the Garn is hot

and so on for each powered device - don't know if these are right for you, but I like this format for initial discussion. The and/or/not language maps nicely to contact wiring.

 

[Jim K in PA]

A picture would truly be worth a thousand words or more in this case. I've also found it helpful to lay out the conditions that determine control of each device. For instance:

- Oil burner is on if there's heat demand and the Garn not hot
- Oil circ is on if oil is hot
- Primary circ is on if there is a demand or if the Garn is hot

and so on for each powered device - don't know if these are right for you, but I like this format for initial discussion. The and/or/not language maps nicely to contact wiring.

OK, NF. My apoplogies for burying some of those details in the above blather. I will try and post a schematic tomorrow from work. For starters, here is the basic config:

GARN is plumbed with a primary loop and circ feeding three secondary loops/loads. House secondary loop goes from GARN to HX in basement near existing pressurized oil boiler - secondary pump for the house loop is near the supply Tee coming off the primary circuit.

The existing oil fired hydronic boiler has a single 007 circ pump and 4 zones w/z-valves. Existing Honeywell L8124A aquastat controls circ pump and gun. Return leg to oil boiler is diverted to HX immediately after oil burner circ pump. In-line Aquastat will be installed in the return leg after pump but before HX.


Now for the (desired) step function:

Load (t-stat) calls for heat = boiler circ on via existing on-boiler relay (zone valve(s) open);

Scenario 1:

If temp in boiler is ABOVE boiler's aquastat low cut in set point, and return line is BELOW in-line A-stat low setpoint, GARN primary pump and secondary pump for house loop to HX is ON, and heat transfers to oil boiler circuit via HX. As heat demand is met, in-line Aquastat turns off primary and secondary circs, and relay shuts off oil burner circ when t-stat stops calling for heat. All is right with the world.

Scenario 2:

If temp in boiler is BELOW boiler's aquastat low cut in set point, (and therefore return line is also BELOW in-line A-stat low setpoint), GARN primary pump and secondary pump for house loop to HX is OFF, boiler circ is on, and oil burner gun lights to heat water. Oil boiler water is still circulating through HX, but GARN water is not, so waste/loss into GARN is minimized.

Scenario 1 occurs when the GARN is providing enough heat to meet the demand of the house.

Scenario 2 occurs when the GARN cannot meet the demand of the house. I have another scheme for protecting the GARN from freezing that will not affect the above scenarios.

So, for scenario 2 to happen the way I want it to, I need the boiler's aquastat to provide primary power to the in-line aquastat, which ultimately controls the function of the primary and secondary pumps. If the GARN meets demand, primary/secondary pumps are energized and oil furnace is just circulating hot water. If the GARN does not meet demand, the oil furnace reverts to normal firing up to high temp (cut out) set point.

I hope this helps. I will put together a simplified wiring scheme tomorrow.

Thanks.

 

[TCaldwell]

Jim,for what it's worth, I have a aquastat in the garn extra tank bung, temp set to make or break at 135deg, the primary wood loop circ and secondary circ on the oil boiler side of the hx., are both controlled by the garn aquastat. the oil boiler aquastat is set to fire at 120deg. there is no backfeed when the oil boiler fires because both primary and secondary loop circs are off because the garn feed is below 135deg. i am not suggesting this is the best setup only that there is no backfeed through the hx when the oil boiler is firing

 

[Jim K in PA]

Jim,for what it's worth, I have a aquastat in the garn extra tank bung, temp set to make or break at 135deg, the primary wood loop circ and secondary circ on the oil boiler side of the hx., are both controlled by the garn aquastat. the oil boiler aquastat is set to fire at 120deg. there is no backfeed when the oil boiler fires because both primary and secondary loop circs are off because the garn feed is below 135deg. i am not suggesting this is the best setup only that there is no backfeed through the hx when the oil boiler is firing

Gotcha. Thanks TC. I surmise then that your primary and secondary circs are running at all times? I was hoping to avoid doing that with the scheme I am hoping to use. Also, when the tstats are not calling for heat, I would like to keep the GARN side of the HX hot so as to avoid delays in heat transfer/response due to the 125' long slug of (relatively) cooled water in the supply loop that will exist if the primary and secondary circs are off.

Also, my circ on the oil furnace side will only run when the tstats are calling for heat.

 

[TCaldwell]

Maybe you need to shut the oil boiler burner relay off when you are supplied by the garn, because both thugs of return water would make the oil burner call,however you might be able to mitigate some system loss, i am interested in the outcome