Best way to hook up hot tub heat exchanger

[jpowell1979]

I finally have all the parts I need to hook up my heat exchanger for my hot tub. This weekend I buried the insulated PEX and now I need to figure out the best way to install the heat exchanger. I bought an 80,000BTU SS heat heat exchanger with 3/4" and 1 1/2" outlets. The hot tub is only about 600 gallons so this should easily heat it, even with my boiler temps at the usual 145 degrees.

I was originally going to install the heat exchanger in parallel with the existing plumbing but I'm worried that going from 2" to 1 1/2" will restrict the flow too much. This seemed like it would be easier at first, but I'm leaning against this idea now.

My second idea is to install two new jets with a small taco circulating pump and aquastat. This way the heat exchanger plumbing would be completely separate from the existing plumbing and controls. I'm sure that a small taco 007 would use much less electricity to circulate than the low setting on the big pump.

Either way I'm going to have the boiler lines constantly circulating hot water to prevent any possibility of freeze up.


Has anyone else done this and what did you do?

 

[Gooserider]

No personal experience, but... Given the O2 in the water from the hot tub, I would certainly say to make any added pump one with a stainless or brass body, but I think you have a good idea on keeping the two plumbing setups separate. Another thought for doing the pump install is to use the existing jets and returns and just tee the hot circulation pump in parallel with the main spa pump, with a check valve in each line to preven short circuiting through whichever pump isn't running...

Gooserider

 

[DaveBP]

And don't forget that the water returning from that heat exchanger is going to be much colder than typical space heating return water when the hot tub is cold.

Your boiler return temperature protection scheme has to be able to deal with that. Some posters in the past have reported some serious (but short term) condensation when first firing up a cold system until their storage tank was up to a normal minimum temp.

I don't know if that kind of short term condensation causes permanent damage but it might be alarming if you aren't expecting it.

 

[Nofossil]

For mine, the hot tub has a small Laing circulator (stainless and ceramic) that runs 24/7 and circulates water through the electric heating element and the filter. I just plumbed my HX in place of the electric heating element.

You definitely don't want to run hot tub water through a cast iron circulator. Chlorine / bromine treated and aerated water is not what they were designed for. I got a replacement Laing for around $120.

 

[jpowell1979]

Thanks for the info on the Laing pump. I just ordered a 3/4" circulator off ebay for $100 with shipping. I'm not used to working on spas so I had no idea what to look for.

This pump only draws 98 watts so it will be extremely economical to operate. I still need to figure out the best way to incorporate this into the spa plumbing.

If I tee these 3/4" lines into the existing plumbing where should I put them? If I tee them into the existing plumbing the circulator will probably backfeed within the lines and not circulate within the tub very well. I like the idea of teeing into an existing line but worry about how it would work when the main pump is on high and pumping lots of water. I might be able to place one tee just after the main pump and then tee the supply line just before it goes into a jet. Would the small circular be damaged by this extra water pressure?

The local dealer can sell me new 3/4" jets that I can drill a hole and glue in place next time I drain the tub. This sounds like it might be the easiest way to go. If I run the circulator constantly how do I control the temperature?


My wood boiler is on a secondary loop with an aquastat set to 145 that controls the pump that pulls heat into the primary loop. There is no chance of this dropping the temps to the boiler, but it could drop the temps to the house. I figured the average heat required by the hot tub is only about 30,000 BTUs per day so it will not take much to keep it hot. I really want to get this right the first time to save myself both time and money. With electric rates almost 20 cents/kWh here is Fairbanks, AK this should pay for itself in a few months.

 

[Nofossil]

The hot tub plumbing varies by model. If there isn't a small low-volume circulator that puts water through the existing heating element, then adding a jet seems like a good way to go.

What I did for controls was to disconnect the wires that go to the original electric heating element and connect them to the coil of a 220VAC relay. I use the relay as if it were a thermostat, and use the contacts to control a 24VAC zone valve. When the hot tub's controller calls for heat, it supplies 22VAC to the relay coil (the relay coil uses a LOT less power than the heating element). The relay contacts close, completing a circuit that opens the zone valve. If I needed to turn on a circulator in the hot tub (your Laing, for instance) I could use a double pole 220VAC relay and control power to the Laing with the second set of contacts.

 

[jpowell1979]

I have run into another potential problem. Talking to the hot tub dealer he indicated that PVC has a max temperature of 140 degrees. Do I have to install a mixing valve on the boiler supply lines to prevent damage to the PVC that will be connected to the spa HX? This would require an additional circulator and mixing valve $$$$

I was going to follow nofossil's advice and install a zone valve and add a bypass loop to provide circulation of the boiler water when there was no call for heat to help prevent freeze up. I have a Tekmar controller so I can lower the max boiler temp if I need to but at -40 I need to have at least 160 to maintain the rest of the house.

 

[Gooserider]

My guess is that there shouldn't be a huge problem - you will lose some temperature going through the heat exchanger - I would expect that if you have 160* going in on the boiler side, you aren't really going to get more than 140-150 out on the tub side just because of the heat transfer math.

Second thing you could do is put a partial bypass on the tub side HX piping so that only part of the water circulating through the line goes through the HX, and the other part goes around it to the output side in order to mix in with the HX output and keep it from getting overly hot - probably not a bad idea in any case as you really don't want potentially scalding water entering the tub...

i.e. -----|---HX---|----
|--valve-|

The throttling valve probably wouldn't even need to be thermostatic...

Gooserider

 

[Nofossil]

I have 16' of 1/2" copper in my HX - boiler water is inside the copper, and it's in a bath of hot tub water. I built it with a tempering valve on the hot tub water outlet, but it's not necessary. I never get more than about 125 degrees out of the HX.

 

[jpowell1979]

I think that I finally have it all figured out. Instead of circulating the boiler water outside, I'm going to circulate the pool water inside. For now I'm going to wire the SS pool circulator to run 24/7 so there will not be any freeze potential at 98 watts this will not cost much and I will have constant circulation in the spa to keep the water clean.

Inside the garage I will plumb the PEX from outside into the HX with a bypass and a valve. I will install a temp gauge going to the tub so that I can monitor the outgoing temps and adjust via the bypass valve. There will be no PVC inside the garage so I will not have to worry about temperature limits except for the heater water that I can adjust via the bypass.

For now I will run full boiler temps and use a zone valve and relay like nofossil is doing. I will just pull some thermostat cable inside the 4" conduit that the insulated pex is in for this control. I will also be installing a bypass on the boiler side of the heat exchanger to help keep return temps from dropping too low.

 

[jpowell1979]

I finished my project and I got some pictures to prove it. All that is left to do is hook up the aquastat this weekend. Right now I just adjusted the temperature with the ball valves and I got the hot tub right at 102 degrees. I installed a 95 watt, SS Laing pump outside inside the spa cabinet. I did not feel like draining the tub in the middle of an Alaskan winter to take apart the plumbing so I hooked the supply into a fitting where there was a drain valve and I hooked the return into a 3/4" line that was going to the air intake for the jets. After a couple of days I have not noticed any increase in wood required for the house and now hot tub.

(broken image removed)

(broken image removed)

With our electric rate set to increase to 22 cents a kWh next month I expect to save around $40-$50 dollars per month on my electric bill.