Hartland Cohousing Project System Design - Memo 2

by Marc Rosenbaum, P.E.

Modeling estimates
Payback Discussion
The Other Factors
A Possible Scheme

6/11/98

From: Marc Rosenbaum

Subject: Systems Memo 2

It feels appropriate for me to add some more detail to the discussion now. I am going to present some estimates with fuel quantities and costs, based on the Hartford, CT small ranch house modeling referred to in my first systems memo, and ratioed to VT climate. Then I'll discuss payback issues. Along the way I will add some thoughts about fossil fuel vs. wood vs. solar heating systems. I will finish with some thoughts about the range of systems possible.

Modeling estimates

Payback Discussion

Here are 7 cases. The first is a conventional "good house" - 2x6 walls, low-e glass, no special attempts at airsealing. The second adds airsealing, at a cost of $500 over non-airtight case (these are guesstimates, folks). This is the real base case, I think. The third adds suntempering, no net additional cost over the base case. The fourth adds passive solar, which means better and more south glass, and some thermal mass, at a (really guessing here) cost of $3000 over base case (mostly cost of thermal mass). The fifth goes back to base case and adds superinsulation (SI) - no solar features - this means fat walls, more ceiling insulation, heat recovery ventilation, better glass - at a cost of $4000. The sixth takes SI and adds suntempering, no added cost over SI. The final combines SI and passive solar, at a cost of $6000 (some duplication in upgrades).

Airsealing pays back quickly. SI pays back over the length of the mortgage, based on propane (29 year simple payback, no fuel escalation), over the already air-tightened base case. Some form of middle ground on the building insulation upgrade probably has a better payback because it saves perhaps 1/2 of the SI savings at perhaps 1/3 the cost. Suntempering has a great payback, because it costs nothing but good planning, but it doesn't save a lot, so other site design factors should be considered as or more important than solar orientation for suntempering (there are other benefits, less quantifiable, of south light). Passive solar has a slower payback than SI in this climate, but added to SI I am guessing that the package has a similar payback to SI alone.

Obviously, once wood is the primary fuel, things shift, because it costs about half of what propane costs. (Note that oil is not considered for these micro-load houses, because boilers are so oversized for the load of a single house, just like the central wood gasifier mentioned in the first systems memo.) This is why designers doing houses heated with wood often don't build to as high a standard as those (few) doing true solar houses. Please note that SI has the advantage of being able to be compromised on solar access to the south wall of the houses (more difficult to achieve in a compact site plan than solar access to the roof), whereas the solar options need full access.

When active solar is added, the savings can be greater yet. Usually we would back off on the passive solar features and shift that cost into the active system, which will probably cost $6000 more than the passive design. An estimate of propane cost with a SI, active solar design in this case is about $55/year, and the house cost is probably $9000 more than the base case. The simple payback is in a similar range (25-6 years), but both the capital cost and the savings are higher. You can begin to see why most people don't opt for the active solar house. What may not be so obvious is the issue of the inter-relatedness of fuel choice and solar design and building insulation levels.

The Other Factors

It costs money to provide a system for heating. That cost is made up of distribution systems cost, spent on blowers and air ducts, or pumps and heating pipes (Dana - they WON'T freeze, honest!), and heat supply system cost, spent on a boiler (heats water) or a furnace (heats air) or a wood stove, plus a fuel tank, a chimney, etc. Not all designs have a distribution system - a woodstove doesn't, and neither does a small cabinet-type gas unit heater. Not all designs have a chimney - many gas appliances, both boilers and unit heaters, can be vented through a sidewall. As the building heat loss is reduced by moving from base case levels to SI, somewhere along the way the need for a distribution system goes away. Heat moves around by natural convection of the air. The least expensive houses, in my experience, are those in which the distribution system went away, and the savings from that paid for the building envelope upgrades. There is no net cost over the base case, so payback becomes irrelevant - cash flow is positive from the start, because the cost of the mortgage and the fuel cost are less than in the base case. I have a number of suntempered houses which are heated by a gas unit heater, at an installed cost of about $1500 (most have woodstoves also). The fuel savings are modest ($148 in the above table - $503 to 355) but it is affordable because no net cost is added up front. The unit heater burns fossil fuels (propane or kerosene), which is something this community would really like to avoid. Of course, the renewably-fueled unit heater is a wood stove, which costs more (needs a chimney) and has more pollution output.

When the load of the house can be reduced so much with SI and active solar, then it is possible to get some upfront savings by eliminating any combustion equipment and fuel storage, and providing the small amount of heat required with electric resistance heat. This is what we did in the house in Hanover - the savings allowed a larger solar system, so the energy load dropped further for the same capital cost. I believe that we did NOT choose the lowest lifecycle cost system - that would have been this very SI house with a unit heater. The unit heater would have saved over $10,000 in active solar and distribution system costs, and the energy bill would have been about $200/year more (because we would change fuels from electric to gas.)

This brings up something else to pay attention to with both active solar and central wood heat - they both typically are implemented with a distribution system. It is possible to do what I mentioned above, and pipe the heated water to just one heat emitter, mimicking the unit heater, but this is much more feasible with the wood-heated water, which is at high temperatures (160-180F), than with solar heated water, which may be a cool as 80-100F. In an active solar system, the lower the usable water temperature, the more efficient the system is, so much more usable heat can be produced by the same size collector. So in practice, active solar systems have been coupled with either a radiant floor or a specially designed air distribution system, both of which add significant cost. if central wood is chosen, there will be distribution to each unit from wherever the boiler is located (much cheaper if housing is attached), and then there will some level of distribution within each unit.

So, decisions will need to made about insulation levels, fuel choice, distribution system type, solar access, cost, etc., all in one big multi-dimensional matrix. Additional concerns about the health aspects of combustion on site may weigh in. And my experience is that people ultimately make their decisions on cost and emotional factors (I did in my own home - I put in a woodstove because I like to cozy up to it), and because it burned a renewable fuel. Once the stove, hearth, and chimney are paid for, I wasn't willing to spend thousands on active solar, to save some fraction of the 1-1/2 cords/year we burn for heat and hot water.)

A Possible Scheme

Peter Forbes has given us a lot to think about, including what he characterizes as some crazy ideas. In that spirit, I offer this:

Part of what makes active solar expensive is expensive storage tanks or rock beds, which serve no other function, and expensive distribution systems. To make a solar house more affordable, this has to be addressed, and it will need to be addressed integrally with other aspects of design. Some people have expressed a strong desire for natural materials. I propose an active solar system in which there is a site-built collector on the roof, and a wall or walls on the main building level which is a radiant massive wall, probably 12-16 inches thick and faced with stone or tile or brick or plastered. It can be concrete or rammed earth. It is built with radiant heating tubes embedded, and it becomes both the solar heat storage, the heating distribution system, and a wall (could be a party wall between units - very good sound insulation.) This system is more "passive" in that it has less user control, but it costs less and gets some nice materials into the homes that might only be decorative otherwise. Properly located, it gets passive gain, too. I estimate that about 200 or more square feet of surface area is required (one side, but both sides are heat emitting.) I don't know if the back-up system would be part of the wall or not.

- Marc Rosenbaum