Flow switch before or after heater?

[maxweb]

I'm running a 2004 Hot Springs Sovereign I in 110v mode.  It's been working great, but recently I'm getting the red/green flashing light condition.  I've confirmed that the circulation pump is working ok but the heater-integrated pressure switch seems to be malfunctioning.  I found a helpful YouTube video showing how to disconnect the heater pressure switch and insert an inline Sundance/Jacuzzi flow switch instead, in order to avoid replacing the entire heater.  It seems to me that installing the flow switch *before* the heater (i.e. between the circulation pump and the heater) would make the most sense, since the flowing water would typically be about 100 degrees at that point.  But in the video and on a couple of spa flow diagrams I've seen, the flow switch is installed *after* the heater, where the temperature would be hotter.  In my situation that would be less convenient, because the heater output hose is farther back and not as readily accessible, but more importantly, it seems like the plastic parts on the flow switch might last longer on the heater input side where the water temperature is lower.

What is the conventional wisdom about where the flow switch should go relative to the heater?  Are there advantages to one side or the other?  Am I missing something?  Thanks for any insights or guidance.

[CanadianSpaTech]

On a Sundance/Jacuzzi it is after the heater

[RDspaguy]

You can test the pressure switch with an ohm meter or temporarily bypass it to verify.

Most of the time, this is a dirty filter. Remove filter and see.

Next check the circulation pump. Just because it's moving water doesn't mean it's moving ENOUGH water. The bubbles should be tiny champaign bubbles shooting out in an upward arc. If they are larger than carbonation fizz or go straight up from the jet you have low flow.

Test sensors (thermistors) with an ohm meter. They will be in thousands of ohms, so get an accurate reading. Post results.

[imlost]

I just replaced the pressure switch with a flow switch on my Caldera Spas hot tub, which is made by the same company. ...After the heater. 

As has been mentioned, this may not be related to your pressure switch, it could simply be low flow caused by a poorly-functioning circulation pump, clogged filter, or air in the intake to the circulation pump. 

If your circulation pump is noisy, it is suspect. It should be whisper quiet. 

[maxweb]

Thanks for the replies!  I'm pretty sure the pressure switch is the culprit.  I replaced the circulation pump a couple of years ago (see here for before and after photos) and it has been running strong and quiet ever since.  I get the same flashing red/green lights condition even if I remove the filters.  I tried bypassing the pressure switch by temporarily jumpering the connector pins on the control board, and that works.  I have a good multimeter so I might also check the thermistors, but I really think it's the pressure switch causing trouble.

I considered replacing the entire heater assembly, since I think it's original (which would make it 17 years old), but the heater itself works well and appears to be in good condition.  My local water supply isn't particularly hard, but I'm guessing there's some mineral build-up inside the heater that could be affecting the pressure switch.  I keep the chemistry well-balanced with proper pH levels and a bromine tab float, and change out the water every 3 months since I use the tub daily.  I was thinking that replacing the heater might give me more efficient heating if there is mineral build-up, but installing a flow switch is about 1/10th the cost of a new heater.

If I was designing a hot tub, I'd be inclined to put the flow switch between the circulation pump and the heater, for temperature considerations mentioned earlier.  But as folks have pointed out, the manufacturers put it after the heater.  I guess I shouldn't worry about the switch being subjected to higher water temperatures there.

I plan to tackle this repair over the weekend and will post a follow-up note here about how it goes!

[maxweb]

I'm happy to report that the repair worked well, and I can enjoy my spa once again!

I checked the thermistor resistance when the power was off and the water temperature was around 88 degrees.  The hi-limit sensor measured 3.86 kOhms and the regular control thermostat was 4.00 kOhms, although the thermistors were still connected to the control board so I'm not sure those readings are totally accurate.

To answer my original posted question, I ultimately decided to install the flow switch after the heater.  Although I still think there could be temperature-related advantages to installing it before the heater, I realized that after the heater is the safest option, and probably the design reason why manufacturers install it there.  (Consider the possibility of a leak or hose disconnect at the heater itself.  If the flow sensor is after the heater, it will turn off, but if installed before the heater it would still detect water flow and keep the heater on.)

I used a trick I learned from somewhere to plug the hoses with wine corks to avoid needing to drain the water before making plumbing repairs.  Water still sprays around while you're fiddling with the corks, but the amount is readily mopped up with a towel.  A wine cork will seem a bit too large for a cold hose, but is the perfect size for warm 3/4" ID (1" OD) tubing.

I've attached a few photos of the operation.  The Sundance/Jacuzzi flow switch comes in 2-connector and 3-connector versions, but I'm guessing you could cap off the smaller 3/8" tee barb on the latter.  It looks like water flow moves a magnet so it approaches a reed switch.  For as long as the switch housing remains clear, it's easy to inspect visually that it's working, which is nice.

BTW, here's the YouTube video that inspired this repair: https://www.youtube.com/watch?v=KtZRyVoorZ4