Thursday, April 21, 2011

Heat Distribution

Heat distribution is important when using solar heated water. The heat distribution system must be able to use a low water temperature, the lower the better, so that most of the solar heat can be extracted for the tank, leaving the water close to ambient, and improving the efficiency of the collectors.
I have looked at radiant floors, baseboards, and hydronic radiators. I have summed up the pros and cons of each in the table below:

Radiant Floors:
  • PROs
    Wide heating area than can use low water temperature.
    The floors will be replaced, we can choose what we prefer.
  • CONS
    Very costly solution, labor intensive.
    The floor is an insulator, which offsets the advantage of a wider heating area.
    Some of the heat will be drawn into the concrete slab.
  • Estimated Cost > $10k

Hydronic basebords:
  • PROs
    Cheapest solution
    Heater directly in contact with the air (more efficient).
  • CONs
    Not enough heating area, which will increase the water temperature needed to heat the room.
  • Estimated Cost = $4k

Hydronic Radiators:
  • PROs
    Can be scaled up, to account for lower water temp.
    Heater directly in contact with the air.
    Can use a thermostatic valve, which greatly simplifies system design.
  • CONs
    Somewhat expensive.
  • Estimated Cost = $6k

Based on this table, the choice is hydronic radiators.
Heating needs for our house are between 25 and 50 BTUh/sqft. Aiming at the high end, we will need a total of 3000sqft * 50BTUh/sqft = 150,000 BTUh.

The BTUh value of hydronic radiators is calculated for a water temperature at 180°F. The solar system will be set at 140°F maximum. At a water temperature of 130°F, the derating is already 0.5, which sets the BTUh rating at between 50 and 100 BTUh/sqft.
The whole house will need between 150,000 and 300,000 BTUh.

Our bedrooms are between 120 and 160 sqft. A good choice to stay within the 50 - 100 BTUh/sqft range, and fit all bedrooms is the Myson SD70160G, at 13,000 BTUh, and $525.35.
The biggest rooms are the living rooms, upstairs and downstairs, at 33,000 sqft. Each will use 2 SD70160G, for 26,000 BTUh. Since each room has a wood stove, it is conceivable to put only one radiator in these rooms, and use the wood stoves for supplemental heat during the Winter. The system will be initially designed with two radiators in each of these rooms.
The bathrooms, stairwell and laundry rooms will use a smaller radiator, such as Myson SD6060G, at 4300BTUh and $200. There are three bathrooms, and two laundry rooms.

The total amount of BTUh is: ( 13,000 * 11 ) + ( 4300 * 6 ) = 170,000 BTUh

Because the hallways and other dead spaces are not heated, we fall on the low side of the required range. This is OK, since the solar array is not supposed to provide enough heat through the year. During December and January, supplemental heat from the wood stove will be needed. An additional heat source may also be added to the hydronic system (wood pellet boiler or heat pump).

Next step is to design the system for phase 1, which will heat our hot water and two bedrooms, using two solar panels. If everything goes well, the system will be scaled up to the whole house, and 8 solar panels.

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