Advice on improving flow / pump placement

[tribly]

I've built an in-ground concrete hot tub (850 gallons) (more info on it here: DIY Concrete Block Soaking Pool - In Progress, Advice Welcome!!).

I have a Balboa M7 Heater (Balboa VS500Z Spa Control System Complete 54219-Z. Balboa 54219-Z) which requires 25 GPM to operate.

My original pump was a 1hp waterway that came bundled with a 50sqft filter (Waterway Plastics | Waterway 50sqft Cartridge Filter System with 1.0hp Pump | Discount Pool Kits). 1hp proved to be way more than I needed (and way more than I wanted when the first electric bill came ) so I got a smaller circulating pump. A 1/8hp Waterway Iron Might which pumps 40 GPM (Waterway Iron Might Circulation Pump 3410020-1E. Waterway Iron Might Pump. Waterway 3410020-1E.).

The trouble is now I occasionally get a Low Flow error from the heater which could indicate several things, but probably means I'm not pumping enough water through (here are other possible reasons: Spa Panel Messages - Balboa Direct
).

Right now my system pulls water from skimmer and goes:

1hp pump (usually inactive, but hooked up as the Jets pump on the heater/control panel)
50sqft filter
6' of flexible corrugated pump hose (don't need that much, but just what I had)
1/8hp circulation pump (probably should be before the filter/jet pump, but was easy to quickly add it after for now)
6' of flexible corrugated pump hose (again, don't need that much)
Heater

There's probably 30' of piping 1.5" piping from skimmer to heater (18' from skimmer to equipment, 12' of that flexible hose pipe) with 5 90 degree elbows in there.

My guess is that either one (or both) of these are the issue, but wanted your input:

1. Having the circulating pump (that's almost always the only thing running) after the filter means I'm 'pulling' water through the filter as opposed to 'pushing' it into it (pressure on filter when circ pump is running is 0). Probably less efficient?
2. The 'feet of head' of the pipes / filter / corrugated hoses means the actual flow of circ pump is below 25 GPM.

It doesn't get the error if I remove the filter completely. If I freshly clean the filter it'll work for an hour or three but then I get the error.

Two more variables of note:

• The pumps are not self-priming, the hot tub is in ground (with 1' above ground), the pump station is dug down so the pumps are probably 1.5' below the water line, but top of the filter is probably only a few inches (3 or so) below the water line. Not sure if that has any effect, but thought I'd mention it.
• The heater is about 8" above the pumps in the pump station (so pump has to push water up 8" vertical). Could conceivably get it on the same level if you think that'd make a big difference.

Anyway, my plan right now is to:

• Move the circulation pump to the beginning of the pump station flow.
• Replace 12' of corrugated pump hose with 2' of flexible pvc (not corrugated, hopefully more efficent) which means I'll only have 20' of piping.
• Reconfigure layout to reduce elbows by 1 or two.

Am I way off base here? Any other ideas to improve flow? Time to just break down and buy a decent 2 speed pump instead?

Any advice you have is very, very welcome! Thanks!

 

[ozdiver]

A picture of the layout would help.
I may be misinterpreting your description, but it seems your 1/8hp circulation pump is pulling water through the inactive 1hp jet pump.
This would be a major load on that little 1/8 hp pump and would reduce flow significantly.

 

[tribly]

Included some pretty crude / quick drawings to illustrate it (pipe with lines is the corrugated stuff I'll be getting rid of).

Been reading more about Feet of Head. My heater requires 25 GPM. For the 1/8 HP pump, looks like it hits that at about 13 Feet of Head. From what I found (http://blog.poolcenter.com/article.aspx?articleid=6383 ) looks like my future plan would have:

2' - skimmer
1.5' - 15' of return lines at 1' / 10' of 1.5" pipe
2' - 2 elbows
1' - flex pipe between pumps / heater
1' - elbow going into heater
----
7.5'

Now, of course, that's not including the two big question marks:

1. Filter - haven't had the 1/8 hp pump pushing into it yet so don't know PSI. PSI on 1 hp pump with slightly dirty filter is 14 so, using the x2.31 the previously referenced website mentions, that's a big ol' 32'!
2. As you suggest, @ozdiver, pulling water through the mostly turned off 1hp pump might be a big draw. I assumed it'd just be like a 90 degree turn (add another 1' of head), but is that way off? Assumed it'd be fine to have a 2nd pump in the line, but am I terribly wrong?

Anyway, the crude drawings I promised.

Current

Plan

 

[WASP]

Centrifugal pumps need a good unrestricted suction. Perhaps you can plumb it parallel to your main pump.