heating the mash with a heat exchanger

So, I know that with a RIMS and HERMs, you are pumping the mash to the heat source, but why not go the other way and put a coil in the mash and pump hot water through it? Bring the heat source to the mash? I am thinking you could even build a "wand" or some sort and stir the mash while heating it up. I have been in a situation before where after striking, I was like a few degrees below the target.

If you put a coil in the mash it's going to be hard to stir completely. Also you have more to clean up later.

If it's only a few degrees, try stirring with a heatstick.

From what I understand the heat exchange from coil to mash is also very localized and its hard to mix the mash enough to get even heat distribution.

I agree with broadbill. The great thing about a HERM's system that is the hot water is placed on top of the grain and has to filter all the way to the bottom before being recirculated. This evens out the temps and evenly distributes the heat. If you just had a heat source on the bottom you would need some sort of commercial auger set-up to constantly move the mash around to allow the heat to be distributed.

You could use a steam wand. Attach a length of hose to a tea kettle or pressure cooker and the "wand" is a length of copper tubing with holes drilled in the side and capped at the end. You stir this trhough your mash as steam passes through the holes to raise heat without adding much liquid. I have seen this online used for step mashes. However it can be dangerous since you are using steam. I all of the holes get plugged you could have a pressure issue pretty quick. If you use a silicon hose to attach to a kettle you simply don't clamp it so if pressure builds it simply pushes the hose off.

When I was mashing in a 10 gallon insulated plastic Rubbermaid cooler, I built a device to add heat.

I bought a presto pressure cooker with a threaded port on the old through with a pressure dial was present. I removed that, and added some brass fittings, a mini ball valve and a QD. Then I bought some flex copper tubing and bent it into shape. I had the pressure cooker on the burner and the cooler next to it on the driveway pavement. I bent the tubing by hand. On one end was a spiral crimped off in the center, diameter enough to rest at the bottom of my cooler and easily get in and out. I think about 8-10". The other end of tubing then rose upward out of the (empty) mash tun and I bent it into a gentle curve near the QD on the pressure cooker. I attached a QD to the copper tube, and drill small holes (1-2/16" ?? Don't remember). Along the top part of the spiral.

When mashing, I could add steam via the copper manifold and pressure cooker into my mash. There are a few folks I found who do this as well. The only downsides I found to doing this were:

The water in the pressure cooker had a funny smell to it perhaps from machining oil on the brass parts? Give everything a good scrub/soak in PBW first and eliminate this problem

The mash tun made of plastic, eventually separated the inner shell from the insulation, causing my false bottom to be displaced. I couldn't get a seal and had to eventually (after months of frustration before I discovered what the problem was), went with a bazooka screen. Problem solved.

The mash itself was prone to developing a large amount of "teig" which is a grayish colored almost gel like material that forms at the top of the grain bed. Prior to recirculation and sparging, this required a cutting into the top of the grain bed a few inches deep in a tic tac toe type pattern to get good flow. I suppose now I could've simply stirred the whole mess back into the mash and the whole mash itself and be done with it. Perhaps stirring the mash during heating would mitigate this as well. Tis caused no obvious flavor flaws that I could tell.

This contraption was very effective at rapidly heating the mash to whatever I wanted without overshooting temps. I wish I hadn't sold the pressure cooker though, but I have since built a Brutus system and sold off all the old equipment.

TD