Will a 1.5" tube fix this?

So here's my problem with the new system. When I am recirculating my 2" RIMS works great. When I am stepping temps and recirculating fast it works great. But when I am heating my on demand sparge water when the ground water is this cold, or when I'm stepping temps on a sticky (I love rye and wheat) mash the RIMS boils.

From what I can tell through some experimentation with plain water it's a function of the water in the tube not mixing well enough. At higher flow rates the PID's PV pretty much exactly matches the output temp, but at lower flow rates the PV is way below the output temp when trying to make a large (>10 F) step. I have a suspicion that the real cause is a lack of turbulent flow at the lower speeds, leaving a pocket of cool water around the temp probe.

I believe that the 1.5" Brewershardware.com triclover RIMS would solve my problems. On the other hand, I think that stepping down to a 1.5" DIY setup in the same style I have now would do the same thing. Also, I was contemplating putting a T on the RIMS output, running a line back to a T on the pump input, essentially creating a loop that would allow full pump flow over the RIMS, but allowing me to throttle the output. So far I have read nothing but great things about the triclover RIMS, and it seems to be a much better design than any of the DIY setups I've seen.

I'm totally conflicted on spending $90 for the fittings for the return loop, $100 for smaller fittings for the RIMS, or just bucking up the $200 for the triclover RIMS. I don't want to go with a new RIMS if the return loop will fix the problem, but then again, I don't want to waste $$ on fittings that aren't going to fix my problem. If any of you have experience with this please let me know what you think.

Without doing any real analysis I would say that the loop back concept will be nearly impossible to balance in terms of flow rate through the system since you would basically have to get exactly the same Pin at the pump inlet from the loop pressure drop and the tun feed or water in line at exactly the flow rate you are looking for.... The math would be interesting given all of the variables.

Have you thought of moving the PV probe further down the return line, away from the heat source? That allows the output to more fully mix and you don't end up with measuring the temp at some random point in the bulk flow.

As far as the return loop, I don't think that there's a need for perfect balancing between the two branches. As long as there's positive pressure from the MT/LT or the water supply vs. the return to the MT or kettle this should still output fluid at an adequate rate without any super delicate pressure balancing, I don't think. This should only require a small restriction on the return line via a ball valve to create (since the height of the fluid column returning to the vessels will always be larger than the height of the fluid in the vessels). I understand that I won't get maximal flow, but I'm OK with that, as long as the flow through the RIMS is fast enough for turbulent flow.

I like the suggestion about moving the temp sensor downstream. I may stick a brass T in after the RIMS output valve temporarily since they're relatively inexpensive and would allow me to test your theory. I imagine that this location will provide the best balance between mixing and RIMS temp in my current setup. My only concern would be that at the low flow rates the temp response would still be slow enough to allow the RIMS to boil since the probe will be even further downstream.

I hate to say it but I'm getting an unexpected check tomorrow and I'm kinda fishing for a reason to buy the triclover RIMS tube. That being said, I was really hoping to buy some new skis...

The response time will be slower with the temp probe further down stream so you will need to re-tune your PID. If the PID response is too fast you will slam the element and have the boiling and constant cycling that you alluded to.

I'd hate to give you a reason not to buy a nifty new toy, but it sounds like you have the right set-up, just need some tweaking and tuning to get it humming like a porche.

And besides, the snow is incredible this year.

Ok, so I just found Sizz's post and read through to where he added exactly the same type of loop I'm talking about. Apparently it worked great for him, but I'll have to add a second pump to do it, so cost wise it'll be a wash with swapping out to the triclover RIMS...

And yes, the snow was great but it's raining on the mountain here this weekend...