Hi all - we're new here and new owners of a mass storage passive solar home (built in 1979). We've only been in two weeks and still trying to figure it all out, as the previous owner only provided us with minimal information. Our main concern is the mass fan. It's been running 24/7 (even when we moved in and it was 50 indoors due to being closed up for several months). We were told to "just leave it", but are wondering if it should be shut down at certain times or if the true function is to constantly circulate air thru the system. We have gotten a few books on the subject, but nothing as specific as we're looking for/needing. Any information you can provide or book ideas or website links regarding the details and what to expect out of a mass storage passive solar home would be appreciated! Di & Ben

VERY interesting topic. I'll also look forward to reading replies to this one!

I think you need to explain about the mass and the fan a bit more before anyone can help.

Is this a separate area with stones or something for heat storage?

The house is built on "mass" consisting of 6 feet of concrete with air ducts running through and a gravel top. We have a pellet stove (originally wood) in the center of the house and housed within the chimney are also air ducts (air exchangers?) and somewhere in the midst of all that is a fan. There are vents throughout the top portion of our walls/chimney. The idea being (as it was briefly explained to us) that hot air (due to either solar gain from the south facing windows or the pellet stove) gathers at the top of the ceiling and the fan pulls it into the ducts and pushes it down below ground through the mass which should store the heat and also releases the warm air into other vents which come into the floors of the house. I'm thinking if there is little or no hot air gathered near the ceiling (no sun/not enough output from pellet stove) there is no sense in drawing in that air and pushing it down into the mass for storage. But since we don't know that much about it, we might be told there is a reason for that fan to run 24/7 as it has been. Thanks

Some fan assisted air/masss systems contain an air filter that either needs to be replaced or cleaned on a regular basis.  You might want to ask the previous owner about preventative maintenance such as replacing the air filter or oiling the fan motor.

I'd say:

1) if the house is too hot and the mass is colder, then run the fan (to store heat).
2) if the house is too cold and the mass is warmer, run the fan (to extract heat).

Otherwise, it should be off.

Mass storage takes a long time to accumulate heat. If the house has been shut down with no heat running through the mass then it is probably cold. I would also be concerned that the pellet stove might not be providing enough heat to the chimney. Wood stoves are less efficient ( for the most part) but put more heat into the chimney, if the system was designed for a wood stove then the pellet stove might be part of the problem. If the air coming out of the ducts is cold, then I would shut it down till summer. You may want to turn it on intermittently, just to make sure the air doesn't stagnate in the system.
This summer, when the house is hot run the fan. This will cool the house and over time warm the mass. Mass storage systems don't just turn on and off like a normal heating and cooling system. Thay act as a thermal flywheel, in that the system stores and releases energy at a very slow rate typically over several months of operation. If sized properly to your house, cimate and heat loss and gain calculations a system like this could provide a good protion of your cooling load. While doing this it will store the heat in the mass and over the course of the summer should raise the temperature of the mass. I think you will find that the mass will help more for cooling than heating, although it should moderate the house well into the fall.

I'm with Jonr on this one for now, if the house is hot run the fan, if cool air is coming out of the vents then the system is working. Setting it up on a thermostat might help but if the house is calling for heat and the system isn't providing any, the temps in the house will drop as a result and the thermostat won't turn off and the system won't be supplying heat but actually removing it. I would recommend for now that you monitor the system for performance. This could be as simple as place a thermometer near the supply line or lines entering the house. Turn the fan on and see if the air coming out of the system is doing what you want. If your in heating mode and the air coming out is colder than desired turn the fan off and probably wait till summer. Once the summer months approach and the house requires cooling, turn the fan back on and see what happens. Once again monitor the temps near the supply lines. Once the air coming out of the system isn't cooling the house turn it back off. What ever the case I do not believe the fan was designed to run 24/7 and if your cooling the house when you need heat then it is definetly not working and should be truned off.

IMHO I do not beleive that you will see any benefit from this system until your home calls for cooling and you will most likely have to wait until summer months before running it. However I do believe that will also see some benefit in the fall months from the heat stored over the summer. I built an ICF house that also had some slabs creating a large thermal mass. The house was still cool from the mass effect into the begining of August and continued to warm into early September. It also moderated the temperature swings within the house into the winter months. From my observations of this house I beleive that the flywheel effect can take as long as 3-4 months to reverse cycle. This of course based on one specific house containing mass in a New England climate. Designed for passive solar gains, to a mass floor system, ICF walls and foundation, SIPS roof, High solar heat gain windows with shading/insulating system, radiant floor tied to a high efficiency on demand boiler. No cooling system was installed and the house did not experience overheating issues, except for a few hot days in August. I am sure the flywheel effect will be different from region to region, so there is no set answer as to the best use of such a system. It will most likely take some human intervention to operate this system, turn it on only when it is giving you the desired results, for cooling and heating. Occasionally operate the fan just to move the air but keeping this on 24/7 does not make sense if it isn't providing what you need.

Good luck, with what sounds like a one of a kind system.
Tom Pittsley

[email protected]
www.eebt.org

To give a rough idea of capacity, a 1000 sq ft (for example) by 6 foot deep concrete mass that varies by 10F can store about 1.7 M btu. How long that will last is highly dependent on the weather and the house.

Thanks so much for the responses - they were all very helpful!

Ben - It sounds like you have a very interesting and unique home.  Can you post some pictures to make it a little less abstract to us? 

Thanks,
Ed

Constant running of a '70s vintage blower motor is likely to be a real power hog. At the very least differential thermostatic controls could be integrated into it to insert a significant dead-band to keep it from running when it's already comfortable.

If you wanted to re-engineer the thing with a much more efficient & variable speed blower with ECM motor drive it could probably provide the same technical function(and more evenly/comfortably) with 1/4 of the power use. I have an old print article stashed away somewhere about a house in ID in the late '70 ore early '80s that utilized rock beds for thermal storage, and had not one but two 500 watt air handlers driving it all. A full kilowatt is a not-inconsequential fraction of my design-day heat load, and renders the term "passive", in "passive solar" a bit of marketing fluff at best. If it takes 500W or more of blower to redistribute the heat in a "passive" house, that's considerably more power than that used hydronic pumps running flat panels. The only difference is the working fluid (is now air), and the collector is building-integrated. (A kilowatt of ground source heat pump would be enough to run egouin's house in the shoulder seasons- would that be called "passive solar"? :-) )

Pictures & plans, with real dimensions & specs would be useful here...

Sounds like it might be similar to the designs in this book:
http://www.amazon.com/gp/product/0930031970/ref=ox_ya_oh_product