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Community and Q&A

What is the best heating and ventilation system for a passive solar home?

Maria Hars | Posted in Green Building Techniques on

I am designing Eco-friendly passive solar homes in southern New Hampshire. These homes will use active solar as well. I do not want to use fossil fuels on this project. I am leaning towards Thermomax tubes for solar electricity and solar hot water due to the fact that they work on cloudy days. Sizing the system would be for a family of four which would use an 80 gallon hot water tank. I am looking at a Tempra tankless hot water heater as well. Has anyone used these products and what do you think of them. What other systems would work best in this scenario?

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  1. GBA Editor
    Martin Holladay | | #1

    1. The best heating system: either a wood stove, a pellet stove, or a ductless minisplit air-source heat pump. If you choose a wood stove, you'll still need some type of backup system (for example, electric resistance baseboard) to prevent freeze-ups when the homeowners aren't home.

    2. The best ventilation system: an HRV with dedicated ductwork.

    Don't overestimate the contribution that an active solar system can make toward space heating in New Hampshire. Your winters are cold, dark, and cloudy. Even when the sun comes up, its angle is low. When evacuated-tube manufacturers talk about solar hot water production on cloudy days, they are talking about a cloudy day in July in Malibu -- not a cloudy day in December in Manchester, NH. You're not going to get any heat from a solar hot water system on a cloudy winter day in New England.

  2. Matthew Amann | | #2

    Backup tankless water heater would be good idea.

  3. Michael Chandler | | #3

    The Thermomax tubes only generate hot water, not solar electric and solar hot water to the best of my knowledge. I've found tubes in general to be very good for earlier and later collecting but the big panels like the Solar Hot USA platinum series will collect more total BTUs per year, just tend to collect more in the middle of the day and much less on cloudy days and early morning late evening. If you have room for more panels and for more storage big flat plates will get the job done cheaper than tubes. If you have an off-axis or shallow pitch roof with big wind loads the tubes come out ahead so they are ideal for remodeling. Your solar installer should be able to run the trade-offs for you using Polysun or eq. solar modeling software.

    I think the electric tankless water heaters like the Tempra can create issues with time-of day power rates and if you can possibly use a natural gas unit you will be happier. Using an electric water heater is not really the same as not using fossil fuels as much of our electricity is still generated with coal and natural gas, and some is still generated with nuclear.

    If you are committed to using electric as your back-up you could use a solar tank with an electric element in the top of it or you could use the solar to pre-heat water flowing into a Marathon plastic tank, foam insulated electric water heater.

    I like radiant floor as a back-up heat for passive solar but I understand that in some markets that can be a very expensive option. In southern New Hampshire a combo of passive solar and a small wood stove sounds ideal. SolarHot has a pretty good generic design if you want to look into the economics of it for your location.

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