Many detractors of the RIMS concept state that it is impossible to
produce a beer with body because before a high mash temperature is
reached that favors the production of unfermentables in the resulting
wort, the starch would have already been completely converted at a
lower temp while trying to achieve the higher temp.  One of the facts
upon which this claim is based is the oft bandied phrase "American
highly modified malts will completely convert in 15 minutes".

This issue comes up over and over again.  I finally had to put it to
rest or know that the RIMS detractors were right and some design
modifications would be needed to overcome a shortcoming.

An assumption in my testing that is crucial is that the RIMS system is
capable of a temperature boost of 1.5 degrees F per minute.  If the
system in question cannot achieve this rate, it is improperly designed
for the amount of grain that is being used.

I conducted a test of two batches of pale ale.  They both used
sufficient pale malt to achieve an original gravity of 1.045 and also
had 1/2 lb. of flaked barley for head retention.  No other grains were
used.  Both versions were given a 10 min. protein rest at 123F.  The
"dry" version was mashed for 60 minutes at 150F, while the "sweet"
version was mashed at 158F.  Both were mashed out at 168F for 10
min. They were both fermented in primary at 75F in a temperature
controlled water bath using the same Chico Ale yeast.

The dry batch fermented out to 1.06, while the sweet batch only
dropped to 1.013.  The dry batch was so thin as to almost be
undrinkable and the sweet batch had a nice mouthfeel and a good
residual sweetness.  I took a sample of the sweet batch to one of the
very vocal RIMS detractors, a local homebrew shop employee and let him
taste it and he had to agree that it had residual non-fermentables
that contributed both to sweetness and body.

Now why this is possible.  While I agree that it is true that
"American highly modified malts will completely convert in 15
minutes", I have gotten hold of some supporting evidence for this
statement that indicates that this is only so at 158F which is a temp
that highly favors unfermentables.  Mashing at lower temps that favor
completely fermentable wort takes much longer since the enzymes
favored at those temps are much slower workers.  Secondly, the "danger
zone" in which completely fermentable wort is favored is not stayed in
for very long in the boost to a temp favoring unfermentables.  This
danger zone is from about 140F to about 154F, or 14 degrees.  At a rate
of 1.5F per minute, this zone is passed through in only 10 minutes.

Now, if a RIMS system cannot achieve this rate of boost, what can be
done.  Well, of course, you could add a shot of boiling water.  Or you
could put a second heater chamber in series that is controlled by a
manual switch, only to be turned on during boost and turned off a
couple of degrees below setpoint to avoid overruns.

Another method which has just been suggested to me but I have not yet
had the opportunity to test out is to hook the heater up so that it
can either be controlled by the Triac on the temp controller, or by an
on/off switch and during boosts, put it on the switch.  It was pointed
out that when the heater is driven by the Triac, there is a
significant drop in the wattage output, but when driven directly by a
switch, it operates at full capacity.