1. You're saying you have hydronic flooring heat. Do you mean just the convection of warm air off the oil burner in the basement, or do you have water circulation lines in the floor of your house as many new homes do?
If you're calling the hydronic flooring heat the convective air current off the oil burner, then I'd think that I'd put the heating system into operation and see if the bathroom feels uncomfortably cold. If there's an exterior wall in that bathroom, then I'd say you need to have a radiator along that exterior wall. If there are no exterior walls in the bathroom, then I'd probably wait to see how well your existing heating system works at keeping that bathroom warm before adding a radiator to it.
2. Balancing Valves:
What most home owners with hot water heating systems often overlook is that there will be different radiator trains (or "loops") throughout your house, but water don't got a brain. So, the lion's share of the water flow is gonna be through the shortest straightest loop. That's gonna result in the radiators along that shortest straightest loop having the highest heat output, and the other radiators on the longer, more scenic routes having lower heat output. So, any heating contractor installing a hot water heating system will put "balancing valves" on all but the longest and most tortuous loop. The purpose of these balancing valves, which are normally gate valves, is to add resistance to flow in all but the longest most tortuous route so that the flow through all the various loops will be about equal, and the heat delivery about uniform throughout the whole house.
So, if you're going to add a radiator to an existing loop, then you're going to change the flow resistance of that loop, and so that means you'll need to map out the radiator loops in your house and find the balancing valves. The additional radiator will have an elbow at each end, and so you'll probably need to open that loop's balancing valve a bit to compensate for the additional resistance to flow offered by the new radiator.
3. "Heating Specialists"
Look in your local yellow pages under heating and plumbing wholesalers. Each of the major plumbing wholesalers that sell heating equipment will have a "heating specialist" on staff. This guy won't be an engineer; he won't even be a plumber. He's just a guy who's worked his way up in the company and been sent on courses that taught him how to work out heat loss from a building (through the walls, the roof, the ground, the windows, etc.) So, if he knows how much heat is being lost from a building with an indoor temperature of 75 deg. F and an outdoor temperature of -30 deg. F., then he knows how much heat needs to be replaced, and therefore what size of boiler and how many feet of radiator is needed to convect that heat into the building at 75 deg. F.
If you phone any plumbing wholesaler and ask to speak to their heating specialist, he'll be happy to talk to you about your plans. If you bring a $5 box of donuts with you, you're likely to have the entire heating department at your service. Whenever a new building is built, or an existing building is modified sufficiently to require changes to the heating system, it's these "heating specialists" that work out the length of radiators that need to be provided/added, and they're the best people to talk to at the planning stage you're at right now.
4. Controlling the radiator. You might want to install a zone valve on the bathroom radiator to control the water flow through the bathroom radiator depending on the temperature in the bathroom. That would require that you add a thermostat to the bathroom, and a transformer to convert 110 VAC to 24 Volts AC which the bathroom thermostat would run on. When that thermostat calls for heat, it opens the zone valve, and if the circulating pump is running, then water starts flowing through the bathroom radiator. You can also get zone valves with end switches that close an electrical contact once the zone valve is open. This end switch might, for example, complete a 24 VAC circuit to the same relay in the oil burner that fires up the oil burnerboth and completes a 110 volt circuit to the circulating pump. That way, the heating system comes on whenever EITHER the house thermostat OR the bathroom thermostat is calling for heat.
I very much like Honeywell zone valves, and the standard zone valve in the Honeywell line is the V8043 series of zone valves. A V8043C has sweat connections for 3/4 inch copper pipe. V8043A, V8043B, V8043D, V8043E, etc. will all be similar, but will have threaded connections, with end switches and without, wires coming out of the actuator or screw terminal connections on it, etc. Any company selling Honeywell zone valves will tell you what zone valve you need in the V8043 series to accomodate your pipe and wiring connections.
You don't need to have a zone valve on the bathroom radiator loop, but I'm thinking there's gonna be a big temperature change inside the bathroom when showering or bathing, and without some temperature control, you could end up with a steam bath bathroom.
5. Shut off valves, air vents
There are different schools of thought on shut off valves. You need to isolate a particular radiator to replace an air vent (say) so rarely, that there is some merit to the view that having a shut off valve at both ends of every radiator is just adding potential problems. That is, you're more likely to have problems with the isolating valves leaking than you are to have a problem with the radiators themselves.
Air vents are a different story. You want to have an air vent on every radiator in your house, but the ones on the top floor of the building are most important. The reason for that is that the safest and most reliable way of putting water back into your heating system after you drain it down for repairs, is to simply siphon it back into one of your top floor radiators through an air vent. That way, the siphon stops when the system is full, no sooner, no later. You just install a 1/8 inch nipple, then a 1/8 inch ball valve where the air vent would normally go, and then screw an air vent into the top of that ball valve finger tight. When you want to bleed the air off, just open the ball valve and open the air vent. When you want to siphon water back into the heating system, you simply replace the air vent with a hose barb fitting, get the siphon going, slip your siphon hose over that hose barb fitting and open the ball valve.
Never ever never use automatic air vents. Stick to the coin air vents (so named cuz they're meant to be opened and closed with a slot screw driver or coin. Automatic air vents have a tendancy to stick open when you drain the heating system, resulting in the automatic air vent leaking water once you fill the system back up. Often you don't know about the problem until there's water leaking from a ceiling or saturating a carpet.
6. Draining a heating system down
Your oil burner should have a valve on it suitable for connecting a garden hose to. Typically a plumber will direct this hose into a floor drain and then open that valve to drain the heating system down to the elevation of the work site. You don't have to drain all the water out of a heating system if you're replacing a zone valve on the top floor of the building. Just don't turn the circulating pump on when the system is open.
A heating contractor will never save the water for re-use because it represents time and work he won't be getting paid for. But, when you add new water to a heating system, you're also adding some hardness in the form of Calcium, Magnesium and Iron ions. Also, the new water is cold water, and it has plenty of dissolved oxygen in it. The hardness forms scale at the hottest point in your heating system, which will be inside the oil burner, and the oxygen in the water will react at higher temperatures to form rust with any iron it encounters, which is also inside the oil burner. So, if I wuz doing work on someone else's heating system, it's faster to dispose of the old heating water down the drain and fill the system up with new water. But, if I wuz doing that work on the heating system in my own building, I'd save the old water and siphon it back into the system after doing the repair/work. That old ionically dead and oxygen depleted water is healthier for your boiler/oil burner. But, if you don't drain your heating system very often, this point may be to small to make a difference. After all, iron water supply piping lasts 50 years with NEW water in it ALL OF THE TIME. So, expect to get some stares from plumbers who normally just drain and refill with new water if you tell them you save and siphon.
Dunno how much this helps.
