Greetings-I am a new member to the site. I have enjoyed the advice I have read and now I have a question. I have installed a wood boiler and my existing boiler has a limit control which has a 180-240 degree range. I would like to get the burner on the boiler to turn on at a lower temp than the existing limit switch will allow. Lets say for example the lowest limit setting allowed is 180 so the boiler is turning on at 165. I would like the boiler to turn on at 155. I have a strap on aquastat but I am not sure how to wire it into the system. Any advice is much appreciated.
Mike G

 

Mike, if the goal is to prevent the oil burner from firing when you are burning wood, and the wood boiler is piped in parallel, then I think you might be interested in the thread: "WIRING AN AQUASTAT" by Dirtsnowman ...

If you install an auxiliary thermostat in the home, and wire it to the TT terminals on the oil burner primary control, and set it to a temperature BELOW what you maintain with the wood boiler, it will disable the oil burner when using the wood boiler. In the event the fire goes out, the oil burner will be re-enabled...

check that thread out...

 

NJTrooper--
Thanks for your advice.
My system is a little different and I like to be confident in my knowledge before I start messing with control wiring. Let me explain what I have.
Existing system is a hydro-air with Honeywell trol-a-temp air zone damper system. I have a Peerless boiler 182k btu which feeds 2 air handlers and a Phase III triangle tube 40 gal indirect water heater. The boiler system has 3 zone valves in the piping--1 for the hot water 1 for each air handler.
Existing aquastat on the boiler is a Honeywell L8148A which has limit setting from 180 - 240 degrees.
The house is well insulated, all 2x6 framing, 3700sf + a 1000sf heated garage.

The wood boiler is a Woodmaster 5500 which has 200 gal storage capacity. It is piped into the house to a flat plate heat exchanger. I installed the heat exchanger into the return on the boiled close to where the return connects to the boiler. This is after the 1 circulator pump in the return.
So the wood boiler pump is always on providing 180 degree water to the exchanger. Whenever there is a call for heat or hot water my pump turns on until the oil boiler internal temp drops below the low limit and then the oil boiler fires as it should to make up what the wood boiler cannot handle or if the wood fire goes out I still have oil heat. My problem is in my existing limit control in the L8148A aquastat. Since the lowest I can set it is 180 degree and the highest I can set my wood boiler is 180 the 2 settings are to close so the wood boiler really does not get a chance to dump its heat to the system before the oil boiler fires. I want to lower the low limit on the oil boiler so it fires at 160 instead of the 180 minus 8. The wood boiler does not go below 175 so with losing a few degrees through the transfer it should produce no lower than about 170 to the oil boiler.
The t-stat idea you suggested looks good but I need a little more info to go that route.
I have a l6006 aquastat which I have installed per the instructions but I am not sure it is doing what I need. I installed it on the return pipe to the oil boiler after the heat exchanger. I wired it ;
R on 6006 aquastat to L1 on L8148A .
W on 6006 to 3 on the L8148.
B on 6006 to B on L8148.

I think this lets the 6006 control the lower limit so the oil does not fire until the 6006 mounted on the oil boiler return drops below the setting-I have it at 160. If I install the new thermostat do I need a new transformer for it?
Do I connect it to the tstat screws in the L8148? It already has wires connected to those lugs.
You showed connecting them to the burner directly and eliminate the jumper-will this work with the system I described?
The L8148A is available with an optional 140-240 limit switch-maybe I should bite the bullet and buy one of those.
Again--thanks for the advice-much appreciated.

 

I think I understand the problem, good description!

What you did with the 6006 wiring is essentially creating a "WARM START" boiler. Those connections that you made will do nothing to change the high limit setting. What will happen with the control wired as you have it is if the wood goes out, and there is no call for heat from the home, the boiler will fire up without the circulator and heat up the oil boiler ... you don't want that. You have in essence created a 'triple aquastat' which are what is used on boilers with an internal domestic hot water coil. In those systems the boiler is kept warm to provide hot water to the home on demand, REGARDLESS of the thermostat settings or calls for heat.

If you choose to go with the auxiliary thermostat, no, you do not need another transformer. Here is how that setup would work:

There would be NO CHANGE to the wiring of the aquastat on the boiler. NO extra connections to the TT terminals on the AQUASTAT. The TT terminals referenced below are for the PRIMARY CONTROL on the OIL BURNER.

What brand oil burner is installed on your boiler ?

If it's a BECKETT, above the motor, on top, there is a gray or black box. This is your PRIMARY CONTROL. On that control there is ANOTHER set of TT terminals. There should be a short jumper wire across them.

If you remove that jumper wire, the burner will NOT fire under any circumstances.

If you run a thermostat wire inside the home, to a central location and install a thermostat there, and connect those wires to the TT on the PRIMARY CONTROL, you will have control over whether the burner fires or not by the setting of that thermostat.

Example: You set the normal room thermostats to 70° and have a nice fire going in the wood boiler. When the room cools below 70° it will signal the zone valve to open and the circulator on the oil boiler to run. If you have the auxiliary thermostat set to something like 65°, there will be NO CALL from this t'stat, and the contacts will be OPEN, meaning that the oil burner will not fire.

I believe this is a good plan for a few reasons.

If you lower the temperature on the aquastat, when the oil burner DOES run, the water may not get hot enough to heat the home, and you also may run into flue gas condensation troubles... you don't want that.

If the fire DOES go out for some reason, when the room drops to the setting of the auxiliary thermostat, the oil burner will AUTOMATICALLY re-enable itself and maintain that lower temperature.

When you intentionally don't have a fire going, and you WANT the oil burner to fire, you won't need to go to the boiler and fiddle with the aquastat control. Simply turn the auxiliary thermostat a few degrees ABOVE the temperature of the room thermostats. The auxiliary thermostat will never be satisfied, and keep the oil burner enabled for normal control by the room thermostats.

It's a very simple solution, and cheap too... you don't need a fancy thermostat for the auxiliary. It doesn't need to be programmable, won't need a battery, etc...

I hope I haven't 'over explained' it ...

 

There is another possible way to use the TT terminals on the PRIMARY CONTROL, that involves using the 6006 aquastat.

You said that the outdoor boiler is constantly circulating, and that the controls on that boiler maintain 180° water to the FPHX (flat plate heat exchanger).

If you installed the bulb of the 6006 inside the insulation of the pipes to the outdoor boiler, and set it to say 160°, the R and W contacts on the 6006 would 'make' and the R and B contacts would 'break' at 160° plus or minus the diff on that control.

You could wire the R and B contacts on the 6006 to the T T terminals of the PRIMARY control. In this way, when you are burning wood, R and B would be OPEN, the oil burner would be disabled, but all the other controls would continue to function, opening and closing the zone valves, running the circ, etc...

This may even be a BETTER choice than the aux t'stat, because once it was set up, you wouldn't have to even diddle the control on the thermostat... no pulling an extra thermostat wire... nothing ... just start a fire and run. As soon as the wood boiler water went above 160° R and B would open, and the burner disabled. Fire goes out, boiler water drops temp, boiler re-enables... no intervention required.

 

This is such a great simple idea!
The burner is a Beckett and I am farmiliar with the primary and the jumper on the TT so I am very comfortable with this set up.I am sooo happy I am practically giddy! I will make these changes and let you know my results. Knowing that the aquastat is just a temperature activated switch and using it to control the burner is perfect.
This forum is truly empowering people to do and learn and that is a great thing. It is a good feeling to have an understanding and do something for yourself with success.
Thanks--

 

Please come back and let us know how it works!

Make sure that the temperature sensing bulb on the aquastat is making intimate contact with the PEX tubing inside the insulation. You might even consider adding some extra insulation around the bulb and secure it all with zip ties.

You'll also want to experiment with the temperature setting on the 6006 to find the best results. The correct setting will be that which isn't so low that there is a period of time where the home doesn't have enough heat, i.e. fire goes out, water cools, but oil burner hasn't re-enabled yet... and not high enough that the burner fires on occasion. Experiment, I'm sure you'll find the 'sweet spot'. If there is a differential setting on your aquastat, keep that at the MINIMUM.

 

Update--
I checked the oil boiler temp today when I got home. The oil boiler was at 160. As I was there the hot water heater called for heat and the circ pump came on. I watched with great pleasure the oil boiler temp rise to 172! This was while it was satisfying the hot water demand. I think I have a good set-up which should provide plenty of heat. I did leave the aquastat on the oil boiler return for now with the thought that it will better sense if the wood boiler is not meeting demand. It is essentially the same routine to find a sweet spot on the setting to meet demand without the oil firing. If I put it on the supply from the wood boiler the oil would definitely not fire with the wood boiler going, but it may not sense if it is keeping up with demand.The aquastat is only inches from the heat exchanger so even if there is no circulation on the oil boiler side it is still very hot so close to the exchanger. I have it currently set at 155 which I believe is still an acceptable temp to draw some heat. I know some boilers operate at 145. Ill keep you posted.

 

I thought about on the HX ... and on the return pipe ... but the reason that I decided on the runs to the woody was because you mentioned that the wood pump was constant circ when it was on. My reasoning was that in these 'shoulder seasons' the return pipe might actually cool off too much and re-enable the boiler. But if it's real close to the HX, I guess it will still 'know' when the woody was burning. As long as it works, that's what's important.

Yes, some do... and as long as you are on the return line, that will be fine. With an oil boiler, you want to strive to keep the return water temp at least 120, but 130 is better. However, this is only when you are actually BURNING oil. It keeps the flue passages from condensing the flue gases. Since you aren't actually burning _anything_, my guess is that you could run that all the way down to room temp and not have a problem.

No ... wait a minute ... there WAS another reason that you can't use the HX or the return line ... here's why:

Say you want to burn oil. The fire is OUT. The 6006 will cut the boiler off too early ... you don't want that. I think you want to stick with the initial plan. Put it on the line either from or to the wood boiler, far enough from the HX that it won't pick up the heat when you are burning oil.

 

Good point. It will be a 2 minute fix. Thanks--
Also Is it OK to wrap some kind of insulation on the HX? It is obviously very hot. Insulate for efficiency and also safety.