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Mains pressure boosting and hot water

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franticvike

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Hi,

(Mods, if this should be in another sub forum please let me know)

I've been running a Navien 240S on demand water heater for about a year in my house. Apart from having to wait for the heat at the shower it worked great. I'm doing a reno though and have added a couple bathrooms, and with weak city water pressure (I have 3/4" from the city) I was looking for a way to keep decent pressure across multiple taps (I have a manifold controlling each floor of the house).


After some research and calling around I settled on the Grudfos Scala 2. I've got it installed now and pressure is excellent. However, the pump seems to have introduced a new problem whereby the water temperature now fluctuates quite significantly during a shower. (I should add that I have 20L - 12 gallon - how water buffer tanks for two of the floors in the house and the problem is only noticeable on the floor without a buffer tank)

Previously the hot water was dead consistent for as long as you'd like to run it. Now it goes up and down, like in a wave pattern, easily dropping 10C for 20 seconds or so before returning to set temperature.

I haven't found anything in the literature for either the pump or the water heater so now I'm casting about hoping someone will have some insight.

Many thanks!
 

CT-18

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I would contact Navian a let them know what your set up is. It may be an issue with the pump.
 

franticvike

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I called Navien. Apparently there is a "DIP Switch" setting inside the main cover that needs to be changed when an external pump is used with this unit.

Dip Switch 5 needs to be set to the "up" position. I'm not at home now, so have not yet tested, but will update this post later.
 

Valveman

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The problem is the pump. They are having major issues with that Scala pump. Apparently they spent more on marketing than design and equipment. Here is a little tidbit of info from an engineer who has done autopsies on a few of those Scala pumps.

1) The Scalia uses a magnetic pin wheel to detect flow. Its embedded in the pump and it gets plugged up easily with Teflon tape, minerals or any contaminants in the water. When it fails, the pump fails and it's not serviceable.
2) The Scalia uses a bladder tank, its internal and if it fails, so will the pump and it's not serviceable.
3) The VFD is covered in plastic. Most VFDs are. This is a horrible design since plastic does NOT dissipate heat well.
4) The VFD board is not coated, if moisture builds up in the pump (easy due to plastic construction) the board will short.
5) The motor itself is very cheaply made, not a NEMA frame, not commercial quality, light bearings and light on iron.
 
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Valveman

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They have been recalling or "updating" all VFD type pumps for 30+ years now. It is all part of their planned obsolescence. The more electronic you have in a pump system, the less likely water is to come out of a faucet when opened, and the more it is going to cost you. A dumb pump will last 30 years and they don't like that. They will figure out a band-aid for the thumping on the next generation of VFD. It is always the next generation that is going to solve all the problems of the last generation. However some VFD pumps are in their 10th-12th generation. They solved some of the last problems but developed a "thump" in the process. Very typical.

Some people call those "demand" pumps since they come on instantly when there is a demand. Here is a quote from someone who has been in the same position.

"If I had only found this solution 10 years ago! I've suffered with mediocre water pressure for years from a "demand" pump system where you notice a pressure boost after a water demand is created. This system has given me all the pressure I could ask for and is CONSTANT - you notice no variation in pressure after the water demand begins and the pressure is excellent throughout your water demand. This system just plain works and will completely relieve your issues with your pump cycling on and off while the demand still exists, which creates pressure fluctuations. The quality of materials in this system is excellent with stainless fittings all the way." (from a customer review on the PK1A)

Jet Pump PK1A.jpeg
 
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franticvike

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Thanks for the info Valveman. There always seems to be another solution that I don't quite figure out in time.

What is a VFD that you mention?

Is the system you recommend a pressure tank alone or combined with a pump?


--

For what it's worth, flipping that DIP switch on the Navien seems to have solved my problem, at least for now. Of course, my pump is two weeks old, so time will tell.
 

Valveman

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VFD stands of Variable Frequency Drive, which means it varies the speed of the motor/pump to vary the flow rate to the house as needed. It is a computer controlling the pump speed. Just like any computer, it is temperamental and not really dependable. But when your regular computer flakes out, you are just out of info for a while. When the computerizes VFD flakes out, which happens often, you will be out of water. The post with the bold wording list many, but not all of the problems with that Scala pump.

As I said a regular dumb jet pump will last 30 years, but they designed the Scala so they could sell you maybe 10 replacements pumps during those 30 years.

A regular jet pump with a PK1A kit to control looks like this. And the CSV in the PK1A will give you constant pressure and allow the use of a little 4.5 gallon size tank, the same way a VFD pump does. The difference is a CSV is a simple and long lasting control while the VFD is not.

Sized Jet pump PK1A.jpg
 

franticvike

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I correct my previous post. The dip switch did not fix the problem. Reduced the temperature variance perhaps but hasn't removed it.
 

Valveman

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Check the water pressure gauge while this is happening. If the pressure is fluctuating, so will the temperature.
 
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