Filter pressure increase in "Spa" mode normal?

[txborn]

Was running spa last weekend and noticed the filter pressure gauge was 10 pounds higher than when in pool mode. Pool mode is about 19 pounds and spa is around 29. The top of the filter actually visibly expands when pump is turned on in spa mode.

Had PB out today and he said that a 5 - 10 pound increase is normal in spa mode and my Hayward 60 DE filter can handle the pressure. He backwashed (which I already did on Friday) but it didn't make any difference. Spa has 2 1/2 inch pipes, 2 drains and 5 returns. Pump is 2HP Northstar.

Anyone else experience this?

 

[JasonLion]

That all sounds fairly normal. As long as your pressure isn't getting above 40 psi I wouldn't worry about it.

 

[txborn]

Thanks Jason, he did say that the filter I have is rated up to around 50psi. I guess I never noticed how the filter housing swelled up in spa mode before.

 

[repair_guy]

That 2.0 Northstar is oversized for a 2.5 line. While most builders plumb it on 2.5PVC, it really needs 3". In fact, Hayward will tell you that the pump is going to be redesigned with flange fittings soon when CA TITLE 24 plumbing standards kick in.

If we assume an ideal 7ft/sec pipe velocity in todays standards that 2.5 pipe should handle 102gpm max. L

The pump curve for your pump puts you at a limitation of no more than 60' of head to fall into the 117gpm range.

With your 29psi, you've already told us you have 67' of discharge head and that does not account for suction head. So you're pushing at least 70' of head and your pump is rated for 111gpm at that rate. You're exceeding your efficient flow rate and there is no room for a dirty filter to even come into the picture.

Looks ok on the pool side so if the spa is oversized, it appears your 5 returns are too little for that pump. Evidence the filter lid is swelling up on start. That is an indicator of excessive backpressure and yes, it has an operating psi of 50 but it will fail prematurely and if it does under manufacturers warranty period they will certainly deny it as the system is operating under backpressure.

There really isn't a poly tank out there that can stand up to this in the long haul. Most crack within 4 years. You might ask the builder to downsize that impeller to a 1.5 or if it's new enough, pop in a 1.5 pump.

Installing a lot of variable speed pumps, I can tell you your jets will increase in flow with less gpm than you are giving them now.

 

[JasonLion]

Keep in mind that PVC pipe doesn't have a fixed maximum flow rate. As the flow rate increases the system slowly becomes less efficient. You need to go way over the design flow rate before there are any mechanical problems. The "ideal 7ft/sec pipe velocity" is a design guideline, not a hard limit.

Assuming you have a Hayward DE6020, the filter is within it's design flow rate and pressure. The flow rate is probably near the limit for that filter, while the pressure is comfortably under the max pressure. Poly filter tanks do have a slightly shorter lifetime at higher working pressures and there have been some problems with premature tank failure caused by high yet within spec operating pressures in the past (supposedly fixed now). I am not that familiar with Hayward warranty practices, but if there was a problem I don't see any reason why it wouldn't be covered under warranty other than the usual reluctance of some manufactures to pay out on warranties.

Switching to a two speed, or variable speed, pump could save a dramatic amount of electricity. The pump is presumably sized for spa mode, where high flow rates are required to operate the jets. Running the pump at a lower speed when filtering the pool would save money, probably quite a bit of money. However to get the flow rate that you are currently getting in spa mode the working pressure and flow rate would need to be the same as it is now regardless of what pump you get (so long as nothing other than the pump is changed).

Switching to a 1.5 HP pump or a two speed pump or a variable speed pump would only help in reducing the system pressure and flow rate if you don't actually need your current flow rate in spa mode. Figuring out what flow rate you require in spa mode is a bit complex and depends on the number and specific models of jets you have plus how much "action" you like to have in the spa when the jets are on. If you are getting more than enough jet action in the spa right now, getting a smaller or multi/variable speed pump could have several advantages beyond the savings in electricity.

 

[txborn]

Thanks for the additional info. The spa actually has 6 jets; I miscounted the first time. And yes, the spa does plenty of "dancing" when on. PB wanted to use the 2HP because in the past people complained when it wasn't a boiling cauldron.

The pool is small (just under 7,000 gallons) but has 6 returns and 2 skimmers plus a 21 ft weeping wall and a spa return (fountain) in the bottom center of the spa so I guess that's why the pump turns the water so fast and why PB used that size pump. I only ran pump for 1 1/2 hours in the morning and same time in the evening all season with no problems whatsoever. I think a 1.5 HP pump or a 2 speed might be better, but would only running that 2HP monster 3 hours a day use more electricity than a smaller pump or 2 speed pump twice that long?

BTW, now I have turned off weeping wall for the winter and only run pump about 45 minutes a day and pool is still crystal clear. That being said, with such short run times I don't know if I would be better off with a different pump other than having the 29psi spa issue. I really don't like watching that filter swell up when I turn to spa!

Edited to add: I have an Aqualogic PS-8 controller and SWG. Can that control handle a 2 speed pump?

 

[JasonLion]

The basic rule of thumb for pumps is that a pump of (EDIT) half the RPM or one eight the HP (/EDIT) will move half the GPM but only use a little more than one quarter of the electricity. If you run it for twice as long it will move just as much total water and still use only a little over half as much electricity. The filter will also work a little better at lower flow rates. However, skimmers sometimes have problems clearing surface debris at very low flow rates.

Given how small your pool is, you could filter the pool with a pump one quarter the RPM and use around one third as much electricity (for the pool filtering part of your electrical usage). Using a variable speed motor like the IntelliFlo or Ikeric would give even greater cost savings because they are also inherently more electrically efficient than the regular kind of pump. Of course they cost far more to start, though you usually more than make it up in the long run.

There are four basic pump configurations for a pool spa combo.

1) What you have now. Inexpensive to install. Wastes electricity.

2) Two speed 2 HP pump. Use high speed for the spa and low speed when the spa is off. Saves electricity, only slightly more expensive to install. Can save 30-40% on electricity.

3) Dual fixed speed pumps, one for the pool and one for the spa. Allows optimal sizing of each pump (two speed pumps are always full speed and half speed, limiting choices). Allows you to not run the spa water through the filter which increases system efficiency and allows you to use a smaller filter. The cost is higher for two pumps and you only save a little on the filter. Can save 50-70% on electricity.

4) Variable speed pump. Significantly more expensive to install. Allows maximum electrical savings and lets you dial in how much spa action you want to have at any given time. Can save up to 90% on electricity.

There are also various combinations of fancier pumps on the dual pump setup, but they wouldn't make all that much sense for your particular situation.

 

[txborn]

Thanks Jason.

Option 3 is out because the equipment pad is maxed out. I have a 1 HP booster pump for the Polaris.

Option 2 - I think a 1.5 HP would be more than enough for the spa but do they make a 1.5 HP 2 speed pump? Also, on low speed would there be enough flow to activate the salt cell and do proper skimming?

I have looked at the Intelliflo; not familiar with the Ikeric. Can I control speed with my Goldline PS-8 or would I have to add additional equipment to interface with either one?

 

[JasonLion]

There are many 1 1/2 HP two speed pumps, for example the WhisperFlo WFDS-6 (just over $600 on the Internet).

The IntelliFlo pumps require the optional IntelliComm unit to interface with control systems that aren't IntelliTouch compatible. The Ikeric units can usually be controlled directly. In either case you will need to use two or more relays (one for each speed you want to be able to set with the automation system).

 

[repair_guy]

The IntelliCom will work with any controls system. You don't necessarily need a physical relay to use it. There are many workarounds that do not need the physical relay spot. For energy use the pump affinity law states:

1/2 the rpm (not HP) = 1/4 the flow = 1/8 the power. So, yes, a two speed pump will still reduce energy dramatically as it runs longer hours than you ran on a single speed.

 

[mas985]

1/2 the rpm (not HP) = 1/4 the flow = 1/8 the power. So, yes, a two speed pump will still reduce energy dramatically as it runs longer hours than you ran on a single speed.

Not quite.

1/2 RPM yields 1/2 Flow; 1/4 HEAD and 1/8 HP of full speed

Flow and RPM are directly related.

 

[repair_guy]

Correct, mis-typed the flow. If you look at pump affinity calculators, the terms HP or WATTS are part of the power factor or P1. While the HP is reduced to those levels (not exactly labeled that) the total wattage consumed is 1/8 of what it was at full speed.

 

[mas985]

The affinity laws are approximations to the hydraulics equations and assume constant efficiency. The actual relationship between RPM, Head and HP are much more complicated. However, they do provide a fairly good approximation. Also, HP reference is actually closer to the pumping HP and not the input power or consumed power.

Pumping HP is expressed as Head * GPM / 3960

So the pumping HP ratio is 1/8 but the input HP decreases much less than that which is why low speed operates at about 1/3 the input power of high speed. This results in a significant drop in efficiency at low speed but the operator benefits with lower overall energy consumption.