I make a lot of money every year around this time from people who are unable to diagnose and fix air locks, so hopefully this will reach a few in need! Apologies if this is already written up somewhere or misplaced.
Whether you're draining and refilling in the middle of the season or filling up for the first time this winter, there's a good chance you're going to run into this problem. Why? Because you're good. You're so thorough at draining your spa that you evacuate every elbow, every little 1/2" line, and every piece of equipment until you know there's not a drop of moisture in the thing. And then you throw your hose in for half an hour, turn it back on and Voila! The jets aren't working! The pump sounds fine, but there's no water moving.
Why? Air. All up in your equipment.
When you fill your spa back up, many times the air that's in the equipment has no place to go, and is pinned in on each side by the water you've added. You've got a tub full of water but you've got a lot of plumbing with a lot of air bubbles trapped inside. What to do? That depends.
Can you access the equipment? Maybe it looks a little something like this:
The red arrows point to unions, the blue arrow points to where there may or may not be a drain plug. If you've got some channel locks or a strap wrench, go for the unions. If not, you may have to use the drain plug or keep reading.
Each union and drain plug represents a spot in the equipment where we can bleed the air from. With the tub off, find one that's easy to loosen. You should hear air or see water coming from it once it's loose. Keep in mind that there will soon be electricity near you. You want the water to drip from the fitting, maybe pour, but try to use one that will keep water away from the control pack and the back of the pump. DO NOT DISCONNECT IT COMPLETELY. Just enough to let some water or air out. Otherwise you will get more water than you bargained for. Got it? Now tighten it up and let's do this thing for real.
Turn the spa on and the pump on high. There's no water in it, so don't go inside and make a sandwich. Go back to your union/drain plug and loosen it again. As water begins to flow in toward the equipment you will hear the force of the air and water coming out increase. That's your cue to start tightening. With a little luck, the next thing you hear will be bubbles coming from your jets followed by a fully primed pump. If the air is not purged within 2-3 minutes, turn the spa off, let the pump cool off, and try again.
Can't access the equipment? Why? Nevermind, this way is just as much fun.
Find your filter. Does it have a lid with an o-ring that gets clamped down on top of it? If so, remember to take the lid off next time before you refill it. With the lid off, turn the spa on with the pump on low (do not hit the 'jets' button). Take the lid and push it into place, and then immediately take it off again. Do not use the lock ring for this, just the lid. Repeat this process, using the lid like a plunger until water starts to come up at you from the filter housing. If the pump is on low, it won't be a disaster, just a little wet. Turn the spa off, clamp the lid in place, spa back on, pump on high and you should be in business.
If the filter just sits in a well behind a hole in the wall your life is much easier. Go find the plunger. Take the filter out. Again, pump on low. Then get to plunging on the hole that the filter threads onto or sits on. Once the water gets going put the filter back in place and turn the pump on high to purge the remaining air. If the plunger gets stuck on the hole, turn the spa off and remove it. It will not filter the water, doesn't look good to guests, and will probably stop you from putting the cover on properly.
Of course, you'd never have gotten yourself into this situation if you'd just closed the valves on either side of the equipment before you drained it. Not a good option if you're going to leave it for a long time, but if you're doing a simple drain and refill close the valves first and you'll always have water in the pump. Just remember to open them again before you turn the spa back on.
Hope this saves somebody a service call!