So you plan on direct firing the kettle to adjust mash temp?
The recirculating process will loose heat.
Do you have a 3 vessel system or 2 vessel system?
With a 2 vessel system, you would have to do a recirculating no-sparge mash at full volume. Few people use this method as it decreases mash efficiency; a little more grain will offset this.
With a 3 vessel system you will still need to increase the initial mash volume enough to compensate for additional water needed in the boil kettle. Probably a gallon or more.
There isn't a high risk of scorching the wort where it changes the taste unlike that of directly firing the mash tun. But you will run the risk of denaturing enzymes if you are heating a small volume in your boil kettle.
To increase the mash temp the kettle will have to be raised higher than that of the mash tun by just a few degrees. This can be hard to control with a big burner set up to boil 5+ gallons; if the wort in the kettle gets too hot the enzymes will denature.
Coil water heat ex changers like a HERMS system gradually raises the temp eliminating the risk of both scorching and denaturing at the cost of a slightly higher volume mash.
Lower volume RIMS systems have localized boiling at the element surface that increases this risk of scorching and denaturing; but only in the event of low flow / stuck mash.
Suggestion: Try it with a low OG brew that you have done before at full volume (no-sparge method).
Keep the kettle volume equalized; roughly the same level in both the kettle and mash.
Use a low flame on your kettle; hold kettle temp at mash temp.
Record mash efficiency and compare to previous batch. If OG is lower than previous batch you should be able to calculate how much extra grain you need to compensate.
