Theory Behind Clay Ovens

The heat pump part of this seems to have little relation to the clay oven part. I like tandoori cuisine, but I don't understand what is being asked here. Some more information would help.

The air to air heat pump in colder environment tends to produce frost on the heat collection coils. This requires the pump to work harder and be inefficient and it also goes into self defrosting mode which again loses energy further lowering heat input to the room being heated.

However if the temperature of the air input to the heatpump is warmer - it works much more smoothly needing less energy to extract the heat and warm the room. I burn some wood and garden waste during the year and thought came to mind that this heat should be captured and used to heat the house. So I believe that the clay oven or similar device could be used to store the heat (and eat tandoori, pizza etc.) and when and as required feed this heat to the heat pump to transfer it into the house.

I have shown that warmth from the ashes of burnt fire also helps but of course the main heat loss is when the fire is burning and I am sure that can be stored in a mass of clay (or some other storage device) and over time released to feed it into the input of the heatpump and then a more efficient transfer of heat would take place.

The clay oven seems to be a very attractive way to go as it can be placed outside (where the heatpump is) and it can be used for cooking and keep the memory of people who have used it for melleniums. One can add as much clay for heat storage as necessary and modify it as desired as one builds it. But I am sure there are good and bad designs for this purpose and that is the reason for my interest.

The history of wood fired bread ovens is resurfacing these days. The shape of the fire box relative to the door size is about 1/3 above the door height. The curved shape and cylindrical heat exchange is completed most evenly when this is taken into consideration, The radiant heat from an evenly heated oven cooks "just Right" when heater right. There is a great book out called The Bread Builders, and they discuss the shape door ratio, and fire box profile, to great detail. I'm planning my oven for a spring release. It only takes someone willing to share the bread to make a producing oven a joyous place for friends to gather for a nice "bake off's" abundant yield.

If you were to place your thermal heat generating and storage device below your room being heated, and control the air movement with heat exchangers and routing controls and pumps on the exhaust you could be heating stored water even still. The retained heat cooled by fresh air as needed during the low energy demands that seasonal use might require. remember heat transfer conduction,radiation,convection

If you want to focus the heat on one point, then refractory clays molded into a parabolic shape would work best.

Looking into few designs of the ovens and assuming parabolic shape, the focal point is very close to the top of the dome (origin of the parabola). I do not understand why this would help retain heat in the oven and also provide heat to the pizzas on the base of the oven? Unless the heat from the initial fire warms the floor and direct the heat to the sides and finally focussing it at the top and during the process heat the sides too.

The point of interest is that a developer claims that the oven door height to the maximum oven height ratio MUST be 63%, (and that the base diameter of the oven should lie between 60-75% of the maximum height of the oven),no more and no less to avoid smoke pouring out from the front and also maintain maximum heat inside the oven. However he has no evidence to support this except his experience!

I am very interested to find out if this is so and why and also find out if the shape allows recycling of air to reduce smoke - if the oven has been used for more than 5000 years I would assume it may have more advantages than meets the eye.

However he has no evidence to support this except his experience!

Isn't that how we get our evidence?

But I agree, you should try to corroborate his claims. This may sound an odd suggestion, but I make it seriously: have you tried contacting your national museum - or in this day and age any national museum through their website - or the archeology department at a suitable university? They probably know more about the shapes of historic clay ovens than most engineers.

You may have to do some statistical analysis, or you may find a PhD student already has.

Please: remember to post your findings back here, it's a facinating subject.

I have not persued the pizza oven (hob) but found that a mass burner technique shows promise and is simpler for my use. This is a burner that has an internal flue and an external flue. Fire is lit at the inlet to the internal flue, hot air rises and hits top plate, air then displaces cold air at the top and after about one minute the smoke appears at the external chimney some distance away.

I have tried two models - the second one is more professional and has four interesting properties

1. It is very responsive to the wood burning heat and produces a hot plate where sausages, bacon, egg, toast, stir fry etc can all be cooked at the same time (plate is 24" diameter only about 1-1.5mm thick)

2. The outside of the internal chimney has bricks and concrete annular store and does store heat. I have recorded temperature of the top plate at 27C higher than ambient of 23C (and lower through the night) after 15 hours.

3. The external flue can be very long and even below ground. Some people have recorded 30feet horizontal flue that heats certain required areas on the way.

4. The outer casing is steel and is lagged and even after about 2 hours of cooking the flue temperature is about 28C - 34C hence all the external chimney and extension pipes are plastic (underground water pipe 6" diameter).

As I had not realised that the exit temperature will be so low I in fact tried to store any residual heat in another mass store. As I know now this would require the internal mass store to be extremely hot to let the temperature of the exhaust increase sufficiently to be usable elswhere.

I need to now remove the heat from the internal store and feed it to the heat pump - cooking on the top plate has taken priority as we enjoy the good weather.

It does sound strange that the focus is up top. But maybe that manages to make some of the gasses combust completely before they are gone up the chimney? I wondor what difference a short or long flue would make.

Brian

From the old ones I have seen, they are shaped like an igloo.

http://www.okaccommodations.com/naramata/parks/pics/rock_oven_park.jpg

The term 'Clay oven" is your problem. They are called 'cob' or 'earthen' ovens. Do a google search and you'll find what your looking for. But I think what you really are looking for is a 'rocket mass heater'. This is the combination of a rocket stove and a thermal mass heater. The rocket stove burns material very efficiently, where its exhaust is diverted through a large thermal mass (clay floor, cob bench, etc...) I think I remember reading something at www.green-trust.org cheers, Rodger

Thank you sir - you have opened a new avenue. The information is in form of books and videos.