I have a 1200 square foot second floor apartment that is presently heated using a forced hot water radiator system. The system is called a Paloma "Paloma Pak" heating system. It is basically a tankless hot water heater that they used back in the 1980s to heat apartments. With today's technology, can I replace it with a standard tankless hot water heater for similar results? My biggest problem is I can't find anyone to service the unit. It has an 85% efficiency rating when I check, so I can get no assistance to pay for the replacement. I think the tankless hot water heater may be an option.
Just get a combi if you need both DHW and heating or just an 'only heating' boiler. Few brands names: Immergas, Baxi, Buderus, Bosch, Riello, Junkers.
If you get I condensing unit I suggest you to keep the flow below 55°C, to exploit the condensing capability of the unit.
DHW units usually aren't suitable for space heating because those units lack a feedback control on water temperature (they assume inlet is standard 'acquaduct' range: 5°-20°, so returning with hot water may cause serious issues bringing water to boil in the exchanger leading to steam production and eventual explosion)
I do not think that a standard tankless water heater would work for space heating. A standard hot water heater does not have the feedback controls to keep a room at a specified temperature based on a thermostat.
A standard tankless water heater just delivers water at a temperature specified by its internal controls. It does have an internal feedback to deliver water of a certain temperature for a range of flow rates depending on demand, but I don't see how a room thermostat would control it.
A hydronic heating system pumps water around in a loop, but a standard tankless heater is designed for a single pass--it takes in cold water from the cold water supply and sends out hot water which goes to the drain after use.
There are tankless heaters whose output goes to a storage tank and presumably get recirculated water from the tank, but I don't think even this type is suitable for space heating. However, you should research this further.
See this: tankless water heater to heat home