Next project: the hot tub.

[Bad Wolf]

Now that my Tarm is running well I need to move on to my next project.
I need some people to check me on a hot tub HX set up, specifically NoFossil but anyone else that's done this.

Background: I have a fairly large hot tub on the back deck. 500 gallons
It has two pumps: one just for the jets and one that both circulates the water (low) and runs the of the half of the jets (high).
2" piping, inline 4000 watt heating element.
We keep the tub at 104 most of the time (which is a killer on the electric bill, hence the need to switch it over to wood).
When the temp drops a degree or two the pump comes on low and a relay turns on the 230 vac heater.

I was going to put a inline HX on the 2" line and disconnect the heater, instead running 120 from half the heater circuit to a relay and have that turn my 5th heating zone on and off.

I'd like to build my HX along the some lines as NoFossils. I was going to take a 4" dia pieces of PVC 2' long, drill and tap for a couple of fittings through the side for some 1/2 copper pipe. I figure going back and forth in the PVC I can get maybe 14 or 15 feet of pipe.
The tub cycles fairly often so the temp doesn't need to come up more that a few degrees at a time.

I guess my main concern is will this be enough pipe or too much? Heating water will range from 170 to 140 before heat loss along the way.

Also does anyone have a recommendation on a relay? 120 powered with contact to open and close to handle the 24 vac for the zone controls.

Thanks

 

[BrownianHeatingTech]

I guess my main concern is will this be enough pipe or too much? Heating water will range from 170 to 140 before heat loss along the way.

Also does anyone have a recommendation on a relay? 120 powered with contact to open and close to handle the 24 vac for the zone controls.

Hard to have too much heat exchange surface.

A Tekmar 004 relay would work, if you have an enclosure for it. There are many, many relays available from electronics suppliers, both relays with bare bases, and relays with enclosures.

Joe

 

[2.beans]

i used a ranco controller with the ranco temp sensor mounted in the tub. i installed a 2" flow check and a tee on each side of the electric heater then ran pex to a bronze taco that circulates thru my hx.the ranco turns on a zone on my primary loop just like a thermostat and the loop turns on both circs for the tub. the tub can run on its own without the wood boiler going if i went on vacation. im sure theres a better way of doing this , but this is how i did it. http://www.rancoetc.com/

 

[leaddog]

I don't see the need for a seperate relay. Just use one leg of your heater relay to turn your pump or zone valve on.
leaddog

 

[2.beans]

if you did that wouldnt you have to completely unhook the electric element? if not the element would come on when the tub called for heat. on mine theres no way to put heat element on a separate switch.

 

[Bad Wolf]

Actually there are two jumpers on mine that connect the board to the element. I would just remove the jumpers. Or better yet run them thorugh a switch so I could turn the electric heater back on if I wanted to for some reason. Also on mine I need to run the relay to my controller because it opens a valve and turns on additional pumps. I have a big clunky relay I was using for a similar situation with a feed pump for my pH treatment.

I was more concerned with over heating the water as it passes through the HX and scalding my buns in the tub. Going from 2" to 4" will slow the water down for a short stretch and let it pick up come heat.

 

[2.beans]

i have a mixing valve on mine.if i didnt the tub wont turn on because of overtemp. i think my tub supply temp is 120* or so. i like the idea of putting a switch on the element and using the tub controls to run the circs and temp.maybe i will get brave and try hooking mine up like that. i would need high voltage / low voltage relay to activate my taco switching relay.

 

[BrownianHeatingTech]

I was more concerned with over heating the water as it passes through the HX and scalding my buns in the tub. Going from 2" to 4" will slow the water down for a short stretch and let it pick up come heat.

As others have mentioned, you can add a mixing valve to control how hot the water in the heat exchanger tubing is.

The advantage of more heat exchange surface area is that you can operate at a lower supply water temperature, thereby maximizing your efficient use of storage...

Joe

 

[Nofossil]

Using a highly scientific technique called 'just winging it', I arrived at 16' of 1/2" copper pipe as the ideal heat exchanger surface area. My boiler water is inside the copper, and hot tub water is inside the PVC but outside the copper.

My hot tub has a very small circulator that runs 24/7 and circulates water through the heater. I used a generic $3 relay with a 240vac coil. I wired the relay in place of the heating element. My heat exchanger raises my 550 gallon hot tub by 8 to 12 degree per hour - figure 40,000 BTU per hour or more.