Heat Pump

[Farmerkid]

My AquaCal SQ125 heat exchanger failed for the second time within three years (while operating in Spa mode). I have a 1.5hp Pentair Whisperflow Pump and my water pressure runs about 12psi in Pool mode and 20psi in Spa mode. I The good news is that both hp failures were covered under warranty.

The service technician was concerned about the nearly 10psi difference between these modes. He suggested I contact my pool installer and have him check for any possible blockage. I removed all six Waterways mini spa jets and ran the pump in Spa mode, pressure dropped to about 17psi with six unrestricted 1” openings.

While I’m considering installing a variable speed pump, my query to the group is whether this pressure difference is reasonable? In reviewing the heat pump specifications, it should be good up to 50psi although not ideal.

Any thoughts?

Farmerkid

 

[CJadamec]

It's not a pressure problem it's a water flow problem.

Your heat pump is rated for a Max of 70gpm.

At 12 psi of system backpressure your pump is moving over 100gpm, I think it's close to 120gpm.

You are wearing out the heat exchanger with excessive water flow. You need a heater bypass loop to control the amount of water flowing through the heater, or you need a variable speed pump.

You were lucky to get a second warranty heater I doubt you will get a third.

 

[Farmerkid]

I did forgot to mention that I installed a 5psi bypass in front of the heat pump after the first failure (per service technician’s advice).

http://www.hxworldwide.com/webimage/pdf/aquacal/aquacal-technical-bulletin-bypass-check-valve.pdf

So you would suggest a VS pump or smaller impeller for my existing pump?

Farmerkid

 

[CJadamec]

I would always suggest a VS or 2 speed pump to replace a high hp single speed pump.

The bypass might not be opening if there is less than a 5 psi pressure drop through the heater.

 

[Farmerkid]

Thanks... will investigate VS solution.

Also, I looked at the WhisperFlo pump flow curves and it seems I need a much smaller pump to meet the 70gpm flow limitation... seems this limitation would significantly effect the spa performance.

This becomes an interesting plumbing system challenge. Maybe heat the water at a low flow rate. Then once the water is heated (104 degrees), isolate the heat pump, then command an increased flow rate for therapeutic spa session.

I can’t imagine this would be an ideal solution. Maybe a more robust heat exchanger design.

Farmerkid

 

[Farmerkid]

Follow up... I found my WhisperFlo pump as build had a 1.5 hp (073129) impeller. Measured total dynamic head (TDH) was 56ft. The pump curve predicts 100gpm at this head level. I changed to 1hp (073128) impeller and found TDH was lowered to 46ft. and predicted flow was 95gpm... hoping for a bigger change.

Next, installed a 3/4hp (073127) impeller. Measured TDH was 41ft. and curve suggested about 75gpm. There was a very noticeable reduction in the impeller slot size. Pump seems happy (lower pump current) and I still have decent waterfall flow. Hopefully, the heat pump with bypass will like this new operating condition.


The final test will be the subjective spa jet flow test tonight!

Farmerkid