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Most recent update 4-15-2012

Rebuild of the Hot Tub Heater Control system --
Out of the Ashes

April, 2012

Here's where I was on January 11th -- my hot tub was a smoking ruin due to a roaring fire caused by the burning up of a Solid State Relay (SSR):

Morning after

The burning epoxy case of the SSR caught the plastic electrical box holding the controller system on fire, which in turn smoked and charred some of the underside of the tub. I detailed the causes and possible solutions to avoid this problem in the future at the bottom of my original Hot Tub Heater Control page, here.

The first decision was whether to try and rebuild or to just buy a commercial tub. I really thought hard about this, but given that the original worked really well -- until the fire -- and given that a replacement was going to set me back about $3k, I decided to take a good look at the damage and see what it would take to get back in business.

I started by removing the charred wood skirting and door, and that revealed a lot of burned urethane foam insulation under the tub. But the tub itself appeared to be fine, despite some permanent discoloration of fiberglass on the front edge. The water filter housing was partially melted, so that needed to be replaced. The controller enclosure was toast, as was the PVC heater-element housing -- but the heater element and the over-temp thermostat were fine. The air blower motor was slagged, but since we almost never used it, that wasn't a problem. The jet pump looked bad and needs to be tested -- this could be expensive. The fiberglass shell that held all of the original parts of the tub was also badly damaged; but I had previously removed parts of it to get new stuff to fit anyway, so if more of that had to go, OK. Finally, the new and very quiet Grundfos circulation pump was just fine. Considering all of this, and given that replacement parts wouldn't run to all that much money (except for a jet pump motor), it seemed to me that it made sense to try a rebuild.

I ordered a new heavy-duty (read high flow-rate) American Plumber water filter assembly, which turned out to be larger in diameter than the original Hayward, but shorter -- it's girth means that it would have to go lower under the sloping overhang of the tub, but its shorter height would make that doable. I ordered a new heater-element housing, and I found a slick translucent plastic box with a hinged lid to put all of the new controller parts in -- except the SSR and its heat-sink!! I had spares for nearly all of the controller parts, so all that was needed was some wood and molding to finish things off -- that part will come later.

In January, I recommended putting all electrical components in metal enclosures to lower the risk of a burning component causing catastrophic damage to the tub or anything else, like the house. Yet here I am putting the PID controller and an air switch back in a plastic enclosure. I'm doing this because i cannot find a suitable metal enclosure, at least at reasonable cost. And the few I did find were all the wrong sizes and full of knock-out holes for conduit connections -- just no good. So, I decided to mount the SSR in a separate metal box, since experience has shown that this is the component most likely to fail. The only other part likely to have a failure causing combustion is the PID controller, but I gauge this to be unlikely to happen, since no large currents are handled, used, or controlled by the PID unit.

Putting the controller system together was easy -- I'd done that before. But cleaning out the charred urethane insulation and fiberglass was just nasty. The new water filter housing was a challenge due to it's mounting and larger girth, so some more of the fiberglass shell went away, which just made access to everything else easier. I cobbled together a crude mount for the filter and also figured out how to hang the controller box from the tub's frame -- once those jobs were done, it was just putting pieces together. I decided to use standard AC plugs and chassis-mount sockets for the circulation pump, heater element, and the jet pump, just in case I needed to take anything -- or everything -- back out. Finally, I redid the plumbing, replacing a ball valve and a union at the output of the water filter, and just not replacing the valve/union combo that used to be at the input of the filter -- there just wasn't room, and it was redundant anyway.

I bench-tested the controller using a table lamp for the load, and that all checked out, so I installed it all in the tub. Then I filled the tub with water, started the controller, and after a few hours of trouble-free operation, went to bed.

Next morning, I saw that although the water was heating nicely, the water level was a lot lower. Peering under the tub, I could see water dripping freely under a place where the air-blower channels were molded into the bottom of the tub. When I had drained the tub in January, I didn't have time to use a wet-vac to suck the water out of the air-blower channels, which in this tub don't drain through the main drain valve -- just a poor original design. We were leaving for an extended winter vacation the next day, and there was just too much else to do. I guess the periodic freezes that occurred regularly after we left froze the water in the channels and burst one somewhere.

Since we never used the very noisy air-blower anyway, I just epoxied shut all the little air holes in the bottom of the tub, and prayed for success. I re-filled the tub, started the controller, and waited. After half a day, no water loss and no obvious leaks under the tub. That was a big relief. Unfortunately, the jet pump motor won't turn, and replacing or repairing it means draining the tub -- this will have to wait for generally warmer weather, when we won't be using the tub quite so much.

Here's how the inside looks now -- not pretty, but all except the jet pump working fine. The new controller assembly is very similar to the original, except for the SSR box and mount. The SSR and its heat-sink are housed in a steel single-gang outlet box with a steel cover -- if this SSR burns up, I don't think anything else will burn up with it. I cut a slot in the side of the outlet box to fit the width of the heat-sink between two sets of fins -- this puts the SSR inside the box and leaves most of the heat-sink outside for better cooling:

Rebuilt interior
Note the SSR box with black heat sink at the bottom of the interior. The black hole at the upper right is where the air-blower was. The jet pump is below. The yellow plug is for the heater and the orange plug is for the jet pump.

Rebuilt interior
The SSR box with black heat sink is mounted to a piece of wood that also supports the heater assembly. The Pt100 probe is in the PVC pipe tee to the right.

I want to thank Marco, the owner of, who supplied me with a replacement JLD612 PID Controller, 90A SSR and heat-sink, and Pt100 probe; his help and generosity are greatly appreciated. His web site has a multitude of very interesting project components for diverse and wide-ranging hobbies and enterprises -- take a look.

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