Your HVAC expert who is designing the system should explain the requirements that the manufacturer has for their particular system. In general, in your situation, you will need lower temperature loops closer together rather than the traditional higher temperature loops further apart. The whole system has to be designed for the water temperature that you will need, as well as the PEX placement distance. Floor coverings also has to be taken into account in the plan. Rubber and carpet actually have insulative power (R values), and are not conductive to easily passing the heat through into the home. That has to be calculated when designing the system, and it will increase the lag time to temperature changes, which is already radiant's biggest drawback.

I've known many people who have vacation homes in our nearby Sierra Nevada mountain area. Sub-freezing temps are the norm during winter at the higher elevations. It's standard that all fresh water systems in structures have a drain at their lowest point so that by turning off the water supply, flushing toilets (adding a bit of antifreeze in the bowl) and opening hot and cold taps at the highest locations, water can be completely drained out of the pipes before leaving such places vacant. That's how frozen pipe damage is avoided. Could your cabins be re-plumbed a bit to work that way?

Sizing a Heat Pump Pool Heater. https://www.heating-air-conditioning.org/ You should have a trained pool professional perform a proper sizing analysis for your specific swimming pool to determine pool heater size. Sizing a heat pump pool heater involves many factors. Basically, a heater is sized according to the surface area of the pool and the difference between the pool and the average air temperatures. Other factors also affect the heating load for outdoor pools, such as wind exposure, humidity levels, and cool night temperatures. Therefore, pools located in areas with higher average wind speeds at the pool surface, lower humidity, and cool nights will require a larger heater. Heat pump pool heaters are rated by Btu output and horsepower (hp). Standard sizes include 3.5 hp/75,000 Btu, 5 hp/100,000 Btu, and 6 hp/125,000 Btu. To calculate an approximate heater size for an outdoor swimming pool, follow these steps: Determine your desired swimming pool temperature. Determine the average temperature for the coldest month of pool use. Subtract the average temperature for the coldest month from the desired pool temperature. This will give you the temperature rise needed. Calculate the pool surface area in square feet. Use the following formula to determine the Btu/hour output requirement of the heater: Pool Area x Temperature Rise x 12 This formula is based on 1º to 1-1/4ºF temperature rise per hour and a 3-1/2 mile per hour average wind at the pool surface. For a 1-1/2ºF rise multiply by 1.5. For a 2ºF rise multiply by 2.0.

Old oil boilers are relatively expensive to run, but in most cases they were sized larger than really needed, so you may be able to add a couple radiators and improve the heating in additional rooms. If possible, converting to a modern natural gas boiler would help a lot. If a NG boiler is not possible, you could add an electric heat pump system using mini-splits or a standard ducted HVAC system if you are not too far North in NY. Modern heat pumps are efficient down to about 15 degrees, but then usually have to use electric resistance heat (much more expensive to run) to keep up. However, its really hard to tell the best solution unless we know the design of your house. For example, a ducted heat pump HVAC system might be straightforward to install if you have access from the basement to run ducts. Another task is to improve the insulation and air sealing in your house. Start out with adding insulation to the attic, and get prices on having additional insulation blown into the walls. Bruce