thermal mass too slow

Stove is drafting great, however my thermal mass is not soaking up heat correctly.
1.) Its taking about 8 hrs to penerrate about 4in of cob and broken concrete mix.
2.) A thermometer read 148 degrees exit temperature yesterday,
My duct is 20 feet 10 from core to tee 3 feet accross and  and 10 feet going back toward the core.  I have an 8" system no smoke. 20 foot stack outside to compensate for my tall house. My mix was in most cases 2 clay and 1 sand to fill, and 1 to 1 everywhere else. Could it be too heavy with clay? For duct, i used sch 40 carbon for two feet, then 8 in stainless steel thin stove pipe, then spiral duct for the last 5 feet. Gave as much info as I can think, thanks.

Philip, is your heater still wet? That could be the reason, damp clay will take forever to warm up. The cure is here: keep firing the thing, drying out completely will take the best part of a fortnight and in some cases even more. A related problem is condensation fluid in the bench in most cases, that'll stop also when the whole bench is bone dry.

It could be, we have been burning it about a week, we have seen the last wet spot dry, but there could be moisture within the thermal mass. Is there an amount of hours that would satisfy you in terms of having been given enough time for it to be classified as a malfunction?

Hi Philip ; My mass took well over a month to dry out. Not seeing wet spots on the outside is not an indicator of how dry the inside is. The more cob you have versus rock the longer it will take to dry out. Your numbers and temperatures all sound good , I think you just need to burn burn burn , the mass will dry out and it will warm up, it just takes time.

After completing the thermal mass bench fill, my stove had a whole summer in which to naturally air dry, in a 45% RH environment. By the time Fall arrived, all the brick mortar and cob (3.5:1, masons sand to fire-clay) appeared totally white in color and dry to the touch. Even so, it took several days of daily firing before the stove stopped making condensate, and then another full week of daily firing to be dry enough so that the mass (6 inches thick) would come up to temperature normally.

So keep the stove going and give it a few weeks of daily firing, of something like 2 hour sessions or longer. When it's fully dried, you'll notice a sizable performance increase in the amount of radiant heat from the mass.