Delay of boiler firing?

Once you get back to your equipment location, we can get down to the specifics. That is when you can get all the information needed. I believe that @hot_rod will have joined by then. I think that this will be a HX that will work for you. B&G BP400-20LP should do a modest size radiant floor system. It will handle whatever the 80K boiler can do. Your Oil heater needs to have the capacity, And I believe it is at least that big or more!

Agree with your information.

So will 120° for the radiant floor system work with that HX? 140° is a minimum or Low Limit setting for that system assuming that the oil boiler has a L8124 aquastat relay. then I would set the high limit to 160° or 170° That means the circulator from the oil boiler to the HX will not operate unless there is a call for heat at T T and the water temperature is above 140°. If I understand how that control works after over 45 years of working on them.

If @nate379's heating system required a higher temperature for baseboard or CI radiators, I might recommend a higher minimum temperature. What do you think the minimum temp. should be for the oil boiler in this case, and is the 160° high limit too low @GroundUp?

That will work, however the control you have on your boiler will determine whether you have a low limit (like the L8124) to maintain boiler temperature or if you need to add a low limit aquastat (to a L8148) to maintain boiler temperature. Once you get back to the boiler and send the specifications, we can address the needed controls and wiring you require. I am just hoping that you have the two stage heat T6 thermostat. If your thermostat has only one stage of heat, then you may need get a second thermostat as you suggested in an earlier post.

Once I know how the thermostat is connected to operate the boiler, I can finish this diagram to see if the control logic will work.

This is the HydroStat 3250 added to the diagram with the L7284U to operate the burner.

OK. You have a T9 thermostat that has at least 2 stages of heat. Right now that T9 is operating the Vesta boiler. Where is that T9 Red R wire connected to, and where is the T9 White W wire connected to, and where is the T9 C wire connected to? I can not see anyplace on the Vesta Boiler to connect a T9 thermostat to.

Vesta wiring diagram: Shows no place to add a room thermostat.

"It's odd thermostat isn't listed, not sure of an application that wouldn't need a thermostat."

Yes and you have a T9 thermostat connected to it somehow? So when you get there, can you look at the actual wires from the T9. R W C and tell me where they connect on the other (boiler room) end?

This is the diagram you need to use. There is one set of thermostat wires that I do not know where they go to on the Vesta VRC 120. But it is easy to tell you what to do. There is a two conductor low voltage wire that comes from the Honeywell HPSR 101 that connects to the Vesta VCR120. That wire should be connected to the Com and NO on Relay Two of the HPSR 101. You will remove them and connect Com and NO on Relay Two of the HPSR 101to the HydroStat 3250 T T terminals as shown

The wires you remove from Com and NO on Relay Two of the HPSR 101 will be connected to the Com and NO wires on the RIB U1C as shown below. The RIB U1C will be powered by W2 terminal of the T9 thermostat as shown by the green wire on the diagram connected to 10 to 30 VAC on the RIB U1C . The COM wire will be connected to the HPSR 101 transformer as shown by the blue wire on the diagram.

This diagram should be printed and placed with the both boiler manuals near the Vesta boiler

Since you are using a 120 size boiler, and we do not know the heat loss of the central heat area, then you may need a larger heat exchanger. The one I specified earlier was based on an 80 size boiler. The piping may also need to be 1-1/4" because 1" can only handle up to 80,000 BTUh under normal conditions.

Once you are ready to commission the oil boiler, you will set the low limit on the HydroStat 3250 to 140° and set the High limit to at least 160°. Pay close attention to the wiring of the circulator pumps on the Oil Boiler. I have the antifreeze boiler circulator pump operating constantly whenever the oil boiler is powered to operate. The HX to System pump will only operate when there is a call for heat from the thermostat. I believe this will be the best design to keep the outdoor boiler heating water mixture constantly moving when it is cold out.

The CH piping looks fine. I have replaced the picture of the Vests boiler with the control inside the boiler. I had a feeling that the (Option) RCOP jumper in the diagram was the thermostat location, but I needed you the confirm this. The Honeywell HPSR101 relay two C and NO terminals are connected to RCOP on the Vesta.

When you are ready to do the control wiring you will purchase the RIB U1C relay and connect the C and NO from the RIB U1C relay to the RCOP on the Vesta VRC120. as shown in this diagram.

That will leave the HPSR101 relay two C and NO terminals availabe to operate the oil boiler as the primary heat source. If the Oil Boiler system fails to maintain the desired set point temperature, then the Stage 2 of the thermostat will operate the RIB U1C relay and the Vesta VRC 120 will operate to maintain the necessary heat for the building.

If any of this information is unclear please Private Message (PM) me and I will be happy to go over all the details and the control logic, sequence of operation and how to connect all the wires.

You will want to print this diagram and keep it with your boiler room manuals for both boilers and other documents.

One final note. this transformer is rated at 24v. 15 VA and may not be strong enough for the Honeywell thermostat, the Honeywell HPSR101 relay and the RIB U1C relay. If that happens you can purchase a 20 VA transformer that will fit in that same position. Amana 24v. 20VA Transformer from SupplyHouse.com. I think you should be fine, but if the transformer fails after a few months of use, get the stronger transformer.

You can try it with 1" but you may need to increase if you need more than 80,000 BTUh to heat your home.  That 1" or 1-1/4" is not written on stone tablets that were delivered to a mountain in a desert.

The pipe sizing is based on keeping the flow speed below 6 feet per second in order to keep the velocity noise below a noticeable level. Above 6 feet per second will become loud enough to hear. It is not that bothersome when water moves faster than that 10 FPS when you open a faucet because the noise stops when you close the tap. With a heating system that noise would be unacceptable.    The next thing is the amount of gallons per minute will flow thru the pipe and this is directly related to the FPS number.  And finally the temperature drop (∆T Delta T) of that water flowing thru the boiler or the radiator as the amount of heat that is added or removed from the water.  With the industry standard of one gallon per minute for every 10,000 BTU based on a 20° temperature change (∆T), we can come up with some rules of thumb for pipe sizing based on the amount of heat you need to transfer.  

If you change the flow rate or the ∆T, you can also adjust the pipe size. So it is possible that 1” can be made to work for you by FPS or GPM, or by changing the ∆T in order to get the system to work.  

For example you can get 100,000 BTU to go through 1” pipe if there is a 25° ∆T  in the design.  But when the water gets to the emitters (the radiant floor for example) it must move slower in order to release more heat to the floor.  The longer the water stays in the emitter the more heat it will release.  You also need to let the water stay in the boiler longer in order to change the ∆T in the boiler.  So slowing down the water may be able to move more BTU through the smaller pipe.  

The best person to make those calculations would be hot_rod if he is interested, since he does that stuff all the time, and I have stopped designing systems since I retired and might make mistakes and don't want you to rely on my numbers where dollars and cents are involved.  But It may work with 1” with the proper flow and ∆T.  It may also be just fine if you only need 80,000 BTUh on the coldest days.  Without a proper load calculation, we may be discussing a point that does not really matter.