Pump type, HP, Filter, and plumbing size please.

this is a new remodel with spa addition, so I am new to the whole heater experience and otherwise, I know just enough to be dangerous at best.

pump is 1 hp pentair challenger (9 mo's old) and filter is 1999 nautilus 36. old plumbing to pool is 1 1/2 and to new spa is 2 inch. We have kept this pump for now because was advised against 2 hp PB wanted to put in and trying to get a better feel if 1.5 hp will work or if need to go to vs pump. Actually this pump is giving us great circulation, and much stronger pressure from the spa jets than we expected. If/when we switch pumps, I believe we will need to upgrade filter, so if this one is shot or inadequate it won't be a huge deal.
Don't know if pump size is affecting the heater.

I'm thinking now that plumber may not have ran rinse cycle before he put it back into the regurlar mode after it ran in backwash.
This mornng I backwashed again, (it looked normal) and then re added d/e. I didn't see any additional d/e in spa after this. The spa water is still cloudy, but hopefully will clear after it runs awhile.

I guess my other questions are does 10 psi seem low? That is where it is running again this morning. Also does a LO error message on Hayward 400 indicate a low filter pressure? Is that the likely issue?

A couple things:

1 DE in spa only? Bet it's in the pool too. If so, check the grid assembly in the filter. Wash them down with with your hose. Check for tears and stretched fabric. Replace as needed.

2 Check the pump's impeller. Probably partly clogged.

3 LO is low water pressure being sensed by the pressure switch in the heater.

Thank you poolguynj,
I've got a few more questions if you don't mind (or anyone else)
Service guy came yesterday. Everything checks out ok. Filter is good, no problem with grids, and no clogs with impeller. I think plumber did something to shoot d/e out (maybe didn't rinse after backwash?).
when pool is in regular circulation mode (suction from pool and return both pool and spa) filter pressure gauge reads 11 and when in spa mode only pressure is up to 15.
Not sure if this is indicitive of water pressure to heater but this is what plumber was pointing out.
Looking on Hayward site, there a a number of things that trigger the LO error message. Most of them have to do with gas flow, even spider webs can be an issue, and my heater has been sitting outside since Oct.

If I want to try it in spa mode with the higher pressure, how do I clear the error message that is on there now?

OK, I never claimed to be quick.
Just re visited the Hayward site and read on-line instructions. I think the first thing is to determine water pressure to heater. How do I do this?

Hmm, 11 psi is lower that I would expect. I would expect another 5 psi (16 or 17 psi). Take a look at the pressure switch in the heater. It should be set to 2 lbs of pressure. Is the filter on a hill or drop more than 6 feet from the pool's surface?

1.5" or 2" plumbing?

Can you post a pic of the heater's gas line near the heater?

I can take a picture in the morning and can get one of my kids to post it. (Of course they may not get up before noon).
Does the pump impact the water pressure? I only have a 1 (1.25 h.p.) challenger.
I also tried bypassing the filter but that didn't make a difference. I have gotten it to actually show a temp and about the time I try to increase it, it goes back to the LO error with service lit.
All equipment is basically level (I'm in Houston).

I don't see pressure switch.

Could this be a spider web issue I have read about?

Not necessarily trying to solve the issue (although that would be great) but would like to understand what it could be and how to determine what it is. PB is still sending word that my filter is busted and I should just replace it.

By the way, can I just bypass PB and get Hayward involved? I do not have any paperwork, warranty, manuals etc. but should be able to find a serial number.