We had to replace the motor/pump on our spa, which of course has necessitated rebuilding the plumbing to the pump. I intended to get it all done today, but what with three or four trips up the hardware store, each marked by increasing cursing, the last by cursing at the bad advice I got on an earlier trip, it’s not. So I’ve got time for a couple of questions (well, three).
1. The old plumbing array was white PVC, which worked fine for the fifteen years we’ve owned it and who knows how many before we bought it. One opinion at the hardware store is that this could be trouble, as PVC is said to shrink in hot water. Experiences?
2. I’ve got a steel heating element – looks like a water heater element – and am using some brass pipe for, among other things, the “well” that the thermocouple for the thermostat rests in to be near the heating element. I’ve had two opinions about whether or not I should use a zinc anode to prevent corrosion of the heating element: yes and no. Anyone have experience with this?
3. The new plumbing arrangement, in improving on some really funky stuff in the old, winds up with an air pocket directly above the heating element, because I’m using a cross (like a T but with four “legs,” in case this isn’t the correct term), one leg of which is facing directly up. I’m thinking to put in a bleed valve, since this is where the thermostat thermocouple hangs down into the water. Can I use the shutoff valve for a natural gas furnace pilot light, which I happen to have hanging around, attached to our old no longer usable heater, as my bleed valve? This will rest about 6″ below the surface of the water, so won’t be under tremendous pressure.
Thanks for any assistance on this.
Replies
I've never heard of PVC "shrinking" in hot water. I'm a mechanical engineer in a manufacturing plant and we've used several different kind of plastic piping, including PVC, in some fairly difficult environments all over the factory. While I've never measured any piping to see if it shrinks, none of the technical literature discusses this, and we've never had anything go wrong over many years that might be related to shrinking.
PVC is rated to 140°F, CPVC is rated to 180°. But, a spa would never get to 140°, because that's dangerously scalding hot. It's possible near a heating element, if somehow the element were still hot and the pump was shut off, the water could get hotter than 140°. I would prefer CPVC if given a choice only for this reason.
Though your heating element has the color of steel, I doubt that it is steel. Heating elements are generally sheathed in inconel or some other very corrosion and heat resisting metal. It should be fine near brass.
It's important for good temperature control that the tip of the thermocouple be touching the interior surface of the well. In industry, we would use either a spring loaded holder which holds the thermocouple to the wall with slight pressure, or a small dab of heat conducting goo. Normal grease and such won't work. The goo is made especially for this, with a very high conductivity. I can't say that your control won't be satisfactory without either of these, but you should make sure your thermocouple is touching or as close as you can get it to the metal of the well.
A bleed valve will certainly work to release the air, but I would not consider it a good design. You may remember to bleed the air every time you fill the spa, but your family might not, and a subsequent owner certainly will not. If it were me, I'd change the design to eliminate the need.
Having said that, an alternative is an automatic bleed valve, such as is used on a hydronic heating system. They're not expensive. They bleed air without letting water out. I would not use a gas valve unless it were rated for water service and were made of something corrosion resistant, like brass. I would not use steel, nor a valve not rated for water.