RayPak RP2100 to Pentair MasterTemp 300 - back pressure significantly higher now


Oct 27, 2011
I have a 18,000 gallon pool with a spa. I have a Hayward EcoStar VS 2.7hp pump and have been using it for years with a RayPak RP2100 (Model P-R335AL-EN from 1998)
and I was able to run the VS pump at 1000rpm and have 3psi at the filter and have water flowing down from the spa to the pool.

The RayPak was getting pretty rusty on the inside and I've had to replace parts on it over several years, so I knew it was getting old,
but then the heat exchanger started leaking, so I decided I needed a new heater.

Luckily my neighbor was tossing his Pentair Mastertemp 300 from 2013 which he had been using on his 600 gallon hot tub (using bromine incase that matters).
He didnt have any issues with the heater, he just got rid of his hottub so he didnt need it anymore.

So I decided I use the MasterTemp to replace the RayPak and got it all installed, and have been successfully able to heat my hot tub.
However, I have noticed that the water flow is significantly less compared to when the RayPak was install.

Pressure Observations:
  • I'd say that 2000rpm now is equivalent to 1500rpm previously
  • 1000rpm, the filter reads ~0 psi, and it isnt enough to make the hot tub waterfall into the pool anymore, it used to be able to.
  • I used to run the pump at 1000rpm 24/7, and that was enough to circulate the pool and hottub. But now its not enough pressure to create flow in the hottub. I have to run at 1500rpm now.
What could be causing the extra back pressure?
Could the MasterTempHeater exchanger be clogged up with Calcium deposits?

Other notes:
  • The pool pump pad is about 3 feet below water level in the pool.
  • The RayPak heater exchanger was near water level, versus the MasterTemp heat exchanger is now ~2 feet below water level.
  • added 2 new 90 degree 2" PVC elbows get the plumbing to the MasterTemp
  • He only used his Hottub about once/twice a month, over the past 6 years.
  • There was a rat's nest in the MasterTemp. It was at the bottom, below the heating chamber, and I cleaned it out.
  • He used Bromine as sanitizer in his hottub, and I use Chlorine in my pool.
Here is a picture to show you the difference in piping.

Last edited:


Oct 27, 2011
I decided to remove the manifold and see if any of the tubes look bad or clogged up.
I did the hose test on every tube and water flowed freely (see pic).
I looked inside the tubes and there was some calcium, but not terrible.IMG_0767.jpegIMG_0764.jpeg


In The Industry
Jul 14, 2014
Have you checked the internal bypass valve. I have seen those break and the "puck" block some of the water flow. While it should have caused a heater problem, it wasn't discovered until the thermal regulator failed.

I replace both every time I work on one of those heaters. Generally, if the thermal regulator or its spring have an issue the spring on the bypass valve will also be a problem. Harder than just a thermal regulator replacement but no call-backs.


Oct 27, 2011
I checked the internal bypass valve and it looks ok.

I ended up putting in a H2 Flow Flowvis flow meter and a manual heater bypass valve.
Here are the flow rates I recorded for my EcoStar 2.7hp and DE filter and MasterTemp 300 heater:
  • @1000rpm - bypass mode = 22gpm - heater mode = 4gpm.
  • @1500rpm - bypass mode = 34gpm - heater mode = 11gpm.
  • @2000rpm - bypass mode = 46gpm - heater mode = 30gpm.
  • @2500rpm - bypass mode = 58gpm - heater mode = 46gpm.
I'm not sure if this is normal for a MasterTemp 300, but I'm ok with it now that I can bypass the heater for normal daily filtering.