Remember to keep the design simple, your widow may not find a replacement with your mechanical skills.
Most radiant design programs use the present floor coverings to calculate the heat output of a floor. This can be a mistake.
In a forced air or baseboard heating system, the air is being used to supply the heat to the house, and although the floor coverings may change, the heat loss requirements do not, nor does the ability of the system to deliver heat.
In a radiant system, the heat source will not need to be changed. But the ability of the floor to deliver heat will change. This can be avoided by simply designing the system for the worst case scenario in floor coverings.
In a floor radiant heating system, the floor is your heater. That is why it is called a radiant panel. The panel must always be able to put off enough heat to satisfy the heat loss requirements at the design temperature, regardless of the type of floor coverings.
In the life of the house, it is unlikely that the heating requirements will change, but it is highly likely that the floor coverings will change. It will be very difficult to change the amount of tubing that is installed in the floor at a later date.
Some might raise the objection that the extra tubing in the floor will cause the system to surge or somehow overheat, but remember that except at the design temperature, the system has always had the ability to overheat. It is the thermostat that controls or prevents this from happening.
There is actually two ways to prevent this from becoming a problem later. One is to install more tubing in areas that could be subject to change, another is to design at a lower water temperature with the ability to raise it later.
Another common mistake is to design the system around the pump that is furnished with the boiler. This has a very limiting affect on the design options. Most boilers come with the cheapest pump they can buy installed on them, these pump have less than ten feet of head, they would hardly run a baseboard system let alone meet the requirements of a radiant system. It takes a pump with at least fifteen feet of head to overcome the resistance that the tubing presents. If you design the system with the pump that comes with it, you will have to limit the tubing loops to less than three hundred feet, this requires several manifolds spread throughout the house. By using a higher head pump you can keep all of the plumbing in the mechanical area, this reduces the cost and the chance of damage by a leak.
Most boilers on the market today were designed for base board heating systems, they require a 180° operating temperature to keep them from condensing on the fire side of the heat exchanger, which will greatly shorten the life of the boiler.
New homes today require less than one third of the heat that they did just a few years ago, yet many installers are still dragging in the same size or larger boilers in as they did before. Because of the volume of water they hold, combo-heaters are not affected by over sizing, but over sizing a boiler can have disastrous effects on the life and the efficiency of the boiler. Even though heat loss calculations are very easy to acquire today many installers are still measuring a heating system by the pound.
Total house heat requirements are running about twelve to fifteen Btu. Per square foot in most new homes even in areas with minus twenty or thirty winter temperatures, this means a three thousand square foot home would only require 45,000 Btu to heat it, if your heating system is 95% efficient you would need a 50,000 Btu heat source.
Designing a radiant heating system with a combo-unit is much easier than with a boiler because we can oversize it without causing problems or losing efficiency, it also means you will not run out of hot water even under the most demanding situations. And allows you to run the water in the system at the same temperature as the tank, this eliminates the need of mixing valves and extra pumps.
Designing a radiant system with the loops near the same length eliminates the balancing problems and the equipment associated with it. Placing the tubing under the floor has an overlapping effect that allows most loops of tubing to overlap slightly into other areas without any affect on zoning.