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it is not always the case that old heat emitters need high temperature supply.

Certainly not the entire heating season , if at all. Running some load numbers is the only way to know for sure

I have run cast radiators it 120f and you still feel the warmth radiating off them

Any time the iron is above ambient temperature it is giving off heat

Agree that there may be a real word scenario that is close to this, but as you said, it is a poor design idea. so the hypothetical may be a good test question, it often backfires with the students of the trade because in makes this bad idea something the the new technicians that are unfamiliar with best practice design believe that this poor design is something that is acceptable or even recommended by the industry. Inefficient and riddled with all kinds of problems, this hypothetical is not in the best interest of teaching our new members of the trade.

This is easy for me and unbelievable. Baxi. I hate to say it but its true, I have had more than a few go 10 + years without touching them. I got a call from what I thought was a former customer because I never heard from them. He let me know, that they hadn't called because there was no need. Maintenance conversation aside, I went over and found it running great. Needed a cleaning, and a flame rod and igniter. Here's the pic. I dont understand it, this thing was on LP, and only did radiant and hot water. It sees the hours, and should need more love. Baxi… I dont like them, but man they have held up for me, so I dont even know why I dont like them.

you must've had a tee and a cap or a male and female adapter or something. the galvanized in my mom's house had a tee and plug for a 90 where there would have been no other reason for it other than they were short a 3/4 galvanized ell.

Whoever gave you the hypothetical scenario does not have a grasp of real world conditions. The fact that you are using steam as the space heating source and a tankless coil as the DHW source eliminates the need for a summer/winter switch. If there is no call for heat the water temperature in the boiler will not make steam to heat the space. The burner will only make enough heat to heat the DHW. Since you need DHW all year by definition, the boiler minimum temperature of 150° with a maximum temperature of 180° will never heat the space and will only heat the DHW. There is never a reason to operate the "Summer Switch" at any time to prevent space heating. Unless. Of course, it could be a trick question to see if you put the switch in there somewhere or explain why you don’t need it.

In order to get one thermostat to operate two boilers without using a 24 VAC source from one of the two boilers requires an independent 24 volt source that is not connected to one of the boilers. The Switching Relay does that by having its own 120 VAC source powering the on board 24 VAC transformer that will provide the low voltage circuit for one thermostat to operate the relay. That relay has 2 sets of contacts that can operate both boilers with separate circuits using separate contacts on the same relay. This isolates the boiler circuits so both boilers can operate independently from the one thermostat while turning off the power supply to the other spoiler for some reason, and will not in any way affect the operation of the remaining operating boiler.

This is always an issue. Customers would prefer to not throw away oil they paid for.

@HVACNUT has the correct answer.

The other alternative is to burn the old tank down to just below 1/4 of a tank and then install the new tank.

On the other hand, how will the tank MFG find out what went in the tank if no one tells them?

The tank MFG are doing a CYA.