project heating hot tub with boiler

[webie]

Summer planning project here on a cold winter night .
OK guys I need a plan and a grocery list to hook up heating my hot tub year around from my wood boiler heating system .
Here is my set up , I currently have a Solo 60 and 1200gal unpressurized storage . Boiler is in attached garage storage tank is in basement , I am heating with 5 zones , basement , first floor , upstairs 2nd floor and a large 640 sq ft sun room which is like heating a corn crib , it has 2 zones in floor radiant and 2 water to air heaters ( this room we only heat on the weekends and when it is above zero , hey and don't laugh I also have a pellet stove in here too yet ). The system runs a glycol mixture , the storage tank is currently charged with 3-3/4 loops and I have 0ne 3/4 loop for on demand DHW.
The hot tub is in the sun room which I maintain a minimum of 35 up to normal 72 -75 on weekends . The hot tub is 500 gal 7 person insulated outside tub .
It costs me about 25 a month just to keep this thing hot .
Now that I have this boiler and setup this is an after thought . We currently use this year around for DHW and now after the fact I am thinking of using it to heat my hot tub also .
OK here is my plan sort of and that is to use my DHW coil for heating my DHW and a heat exchanger to heat hot tub , and I may only be able to run 3/8 pex to the hot tub
Does anyone have an idea what I use for pumps , heat exchanger ,and controller any other ideas are welcome and if you have any questions about my set up .
I don't want to waste money on an over kill and don't want to fall short on a system either .
Thanks for all your help before hand .
Steve

 

[sgschwend]

Sounds like you have lots of options. I would think you could come up with a simple one that would not be too expensive to install. Also you have a handle on the heat load so that should help you determine the btu daily load.

I would pick a load value and then see if you already have a circulator in the tub that you could append to.

Like a said you have lots of options, such as:
use a differential loader to hold the hot tub at a set temp, but only move heat if the reservoir can provide it,
use a mixing valve and an in reservoir heat exchanger made from copper tubing,
use a clamp on thermostat to open run a checked valve circulator pump, wire the pump from the boiler controller so that it can only run when the boiler is up to temp,
add an immersion thermostat in the reservoir and use it to allow hot tub heat exchanger circulation if its thermostat has demand.

 

[Bad Wolf]

Steve A lot depends on how your system is plumbed. My tub had a 5000 watt heater with 2" fittings. I added a homemade heat exchanger in series with the existing heater. I basically took out a 90 and added added my heat exchanger and plumbed back into the heater. I disconected the heating element and wired it into a relay that acts like a thermostat that turns on a circ pump just like another zone. This way I'm using the existing controls. My heat exhanger is 4" PVC with about 13 ft of copper pipe in it. My tub is outside and its 10 degrees tonight and I keep it at 104. The tub cycles every 20 minutes or so but only runs for a minute and a half each time.
I figure I'ver saved over $320 so far since May and the season isn't over yet, we've still got some of the coldest months yet to come.

 

[Gooserider]

Haven't done it, but I would definitely suggest doing a search, as many others have. Also check out NoFossil's personal site (in his sig on any of his posts) as he's done it quite nicely and has a good description there.

Essentially you will need a heat exchanger, probably a sidearm type would be best, between the hot tub and the heat source, and a pump on each side of it.

The control circuitry can be based off the existing hot tub controls, just route the signals from the existing heater to the alternate supply...

The pump on the tub side of the HX can probably be the existing circulator for the tub - just tie into it's plumbing so that the water goes through the HX - maybe put the HX in parallel with the existing heater, with valves to select one or the other... Note that you will need to be careful about picking an HX that is made from materials compatible with the tub water chemistry - it will have lots of O2 plus your sanitizing chemicals, which can make for a really corrosive mix... Have never done business with them, but I've noticed that Outdoor Furnace Supply (one of our banner advertisers) has some pool and spa HX's that look pretty nice on their website.

On the boiler side, you could possibly run a separate zone for it, or possibly tie in to the DHW zone, depending on what would work in terms of your plumbing, heat loads, etc... I would be leery of running DHW through the tub HX for fear of water contamination if you had a leak, plus the lowered efficiency of having two stages of heat xfer rather than one, but I don't see a big issue with using the same zone plumbing for both jobs if you have enough capacity.

Gooserider

 

[webie]

Ok guys the ideas are all great so far . But really I want to keep the actual boiler out of the picture as I want to use this all year around to heat the tub , so there for I want to heat the tub via my storage as I only fire the boiler every 7 to 10 days dureing the shoulder season for DHW. I am not to worried about leaks or cross contamination from DHW to tub water because of an HX as these HX's are used all the time for heating DHW from boiler water . Speed at heating the tub up is not so critical because all this is about is maintaining the tub temp .

 

[webie]

Steve A lot depends on how your system is plumbed. My tub had a 5000 watt heater with 2" fittings. I added a homemade heat exchanger in series with the existing heater. I basically took out a 90 and added added my heat exchanger and plumbed back into the heater. I disconected the heating element and wired it into a relay that acts like a thermostat that turns on a circ pump just like another zone. This way I'm using the existing controls. My heat exhanger is 4" PVC with about 13 ft of copper pipe in it. My tub is outside and its 10 degrees tonight and I keep it at 104. The tub cycles every 20 minutes or so but only runs for a minute and a half each time.
I figure I'ver saved over $320 so far since May and the season isn't over yet, we've still got some of the coldest months yet to come.

I like your approach to this as a keep it simple and cheap .
I know the savings will be great that's why I want to do this as soon as we get closer to spring .
If i can I will try to maybe be able to get the cover off of my storage tank and maybe throw a small coil of pex in there and use that as an exchanger , but I doubt I will be able to get it off as I silicone it on so I think it probably would end up damaging the liner. So I am thinking a different route an that is an exchanger off of my DHW coil because I really don't want to cut into my heating system as it has glycol in it and besides being expensive to just dump its pain period .

I figure I can supply the HX with the heat supply from my DHW coil before it goes thru my mixer ( house supply ) then back to the cold side of my DHW coil to get reheated again.
I am sure I have way enough capacity from the DHW coil as I can fill the tub from well water to usable 101 temp in 40 minutes. It cools the bottom of the tank but barely tickles the top when the storage tank is fully charged.
I like your idea of a homemade heat exchanger and could use more explanation on how you built this and sealed it up , How well will the PVC hold up to 180 temps or less OK ?
I really could use more information on pump size and type , this is a very grey area for me .
My biggest problem will be the actual pipe to and from the tub itself as i think I can only get 3/8 pex back and forth under the floor thru the same pvc that is supplying the power for the tub
I like the idea of useing the tub controls by diconnecting the original heater . Do you still have the filter pump run also yet to circulate the tub when there is a call for heat or not ?

 

[Bad Wolf]

OK I'll take a stab at showing you my system.
On mine I have a circ pump with 2" PVC pipe that goes through the electric heater. A sensor monitors the water temp and turns the 240v heating element on and off. I removed the two jumpers that connected the 240 to the heating element. I ran a wire from one side of the 240 to a 110volt relay and a wire from the neutral to the other side of the relay. So when the controller thought it was turning on the heating element it was really turning on the relay which is acting just like a thermostat. Word of caution here. On my system only one side of the 240 was switched, the other leg was always hot. make sure you get the switched side. (see bad drawing)

Next I built a heat exchanger similar to Nofossils. I took 24" of 4" pipe found, some connectors and threaded them through the the wall. I then made the heat exchanger out of 1/2 copper pipe. using a bunch of 90's and connectors to fit the pieces I threaded through the 4" pipe.
I'm having trouble putting the pics in the right order, but you can see a before pic. two of the assy, and one of the final unit installed.

I sealed the unit up with 4" to 2" reducers. I took out a section of pipe from the pump to the heater.

Using the appropriate number of fittings a pipe cobbled up the finished unit pic 4 and installed it back between the pump and heater.

Finally I ran 3/4" pex from my boiler to the heat exchanger just as if it were another zone.

My thinking was that I had not done any irreparable damage to my hot tub I could always put everything back the way it was if I wanted.

Everything except the relay came from Home Depot or Lowe's.

I insulated the pex portion that ran outdoors. As I said before it only runs about 90 second if the boiler is hot (170). longer if things have cooled down or if its competing with 3 other zones at the same time. The heat exchanger is too big to fit under the tube so I have it under the steps with plenty of insulation.
I'm sure half of the cycle time is just heating up the pipe.

I'm sure there are things I could have done better, most of it was cut and fit as I went but it seems to be work OK. It was 1 degree out last night and the tub is still 104. I'm sure it has increased my wood consumption but the $600+ I'm saving buys a lot of wood.

 

[webie]

yep ; nice and simple and reversable that is what I am looking for . My only question is I see a fitting running thru the 4" pvc for your copper ,how is this done ? How did you get this sealed or what did you use to seal this up ?
Your heater and control box look the same as mine . I think I may be able to figure out how to get some pex in and out of my storage tank . My biggest problem will be ; I can only run a supply line of 3/8 I hope this will supply enough heated water . I still dont have an idea yet on a pump to use ? I dont think we even need a relay off of the one switchable leg off the current hot tub heater , I should just be able to use that directly to supply power to the circulating pump , what you think ?

 

[Bad Wolf]

Sreve I just went to Lowe's and kept going through fittings until I found one that I could thread thru the 4" pipe wall and sweat a piece of 1/2" copper pipe into. I sealed everything up with PVC cement. Heaven help me if I ever have to take it apart again. I thought about running a loop through the storage unit, but this was simpler.
The other way would be to splice in two "T's" and a pump and run your tub water out through your tank and back again. My concern with that would be you would be getting a slug of cold water, then very hot water until it mixes. I didn't want that coming out of a jet as I was sitting there. My set up just put my heat exchanger in series with the existing heater. What ever the transfer rate I'm getting must be better that the electric heater because it would take at least 5 minutes to cycle.
3/8 line shouldn't be a problem, it will just take longer.

 

[Gooserider]

Steve A lot depends on how your system is plumbed. My tub had a 5000 watt heater with 2" fittings. I added a homemade heat exchanger in series with the existing heater. I basically took out a 90 and added added my heat exchanger and plumbed back into the heater. I disconected the heating element and wired it into a relay that acts like a thermostat that turns on a circ pump just like another zone. This way I'm using the existing controls. My heat exhanger is 4" PVC with about 13 ft of copper pipe in it. My tub is outside and its 10 degrees tonight and I keep it at 104. The tub cycles every 20 minutes or so but only runs for a minute and a half each time.
I figure I'ver saved over $320 so far since May and the season isn't over yet, we've still got some of the coldest months yet to come.

I like your approach to this as a keep it simple and cheap .
I know the savings will be great that's why I want to do this as soon as we get closer to spring .
If i can I will try to maybe be able to get the cover off of my storage tank and maybe throw a small coil of pex in there and use that as an exchanger , but I doubt I will be able to get it off as I silicone it on so I think it probably would end up damaging the liner. So I am thinking a different route an that is an exchanger off of my DHW coil because I really don't want to cut into my heating system as it has glycol in it and besides being expensive to just dump its pain period .

I figure I can supply the HX with the heat supply from my DHW coil before it goes thru my mixer ( house supply ) then back to the cold side of my DHW coil to get reheated again.
I am sure I have way enough capacity from the DHW coil as I can fill the tub from well water to usable 101 temp in 40 minutes. It cools the bottom of the tank but barely tickles the top when the storage tank is fully charged.
I like your idea of a homemade heat exchanger and could use more explanation on how you built this and sealed it up , How well will the PVC hold up to 180 temps or less OK ?
I really could use more information on pump size and type , this is a very grey area for me .
My biggest problem will be the actual pipe to and from the tub itself as i think I can only get 3/8 pex back and forth under the floor thru the same pvc that is supplying the power for the tub
I like the idea of useing the tub controls by diconnecting the original heater . Do you still have the filter pump run also yet to circulate the tub when there is a call for heat or not ?

Note that when I said "boiler-side" earlier, I didn't mean to imply that you had to hook that side up to the boiler, just that it was the side of the HX away from the hot tub - doesn't really matter where in the system you tie in...

I'm not sure how copper would stand up to hot tub chemistry long term, but certainly otherwise I like Greg's HX design - simple and essentially what I was suggesting both in terms of plumbing and control circuitry

I'm still not quite clear on what you mean about tapping into the DHW loop - is this a loop that is carrying DHW between a storage tank and a coil HX in your storage tank, or a loop of boiler or storage water that heats your DHW tank via an indirect coil of some sort? Either way you probably wouldn't want to make a full break in that loop to divert all the flow through the tub HX. Doesn't matter overly much other than that if it is potable water you need to be more careful about what materials you use in the plumbing - no iron, just stainless / copper / brass / bronze, lead-free solder on any joints, etc... I would probably go with a pair of close spaced tees in that line and a small brass or stainless body pump (probably the smallest size made) This would only pull heat to the tub when demanded, and would minimize the disruption to the existing plumbing...

Another thing that I see as a possible issue is that I suspect you might have code issues trying to run PEX through the same conduit that is carrying electrical power - worth checking, but generally they tend to frown on mixing electrons and water...

Gooserider

 

[webie]

Thanks for the comeback gooserider , I know there would be issues about pex and electrical running thru the same conduit but the current electrical is underground cable in the conduit not sure if that makes it ok or not . I am not currently under any inspection laws right now not saying that is ok just the way it is . If i don't do it this way it cant be done as the concrete floor has radiant heat in it so there is no other way to and from the hot tub . Like i said this is an afterthought .
I have an unpressurized storage tank ,there is a coil of 3/4 by 60' copper that supplies on demand DHW for my house ,and I use this year around . My idea right now is to T off the hot supply side right out of the storage tank before the mixer valve with 3/4 across my basement about 20 feet then downsize about 15 feet to 3/8 pex thru the pvc electrical conduit to the Home built HX for the tub and then back again in the same order to the cold side of my DHW before it goes thru the coil . I don't believe I will see any effect on my DHW supply at all because we are only talking a few gpm flow and my DHW is usually mixed with my cold anyways to reduce its temp , even at that I am still looking at return temps being well in the nineties reheating wont really take much at all . Not sure also on how long the copper will hold up but in reality hot tubs along with pools are generally only slightly alkaline if you are keeping the chemistry right , besides I can probably build ten of these for the cost of a single stainless HX. Right now I have my eye on a pump a taco 009 stainless, what I have read this should work well 34lbs of head and only 10 gpm . I do have an old B& G 4 speed cast iron I may start with just to see how well it all works first before I drop the $$$ on a new pump.
Just a thought I could probably use pex rather than copper for transferring the heat in the HX but I am sure I would need more feet of that as the transfer rate isn't as fast .

 

[Gooserider]

Your approach sounds like it might work, aside from the code questions... Not sure what to say about that, but the code guys can be pretty stubborn about what they'll allow... It might not be an issue, perhaps some of our electrician types can venture an opinion... If it is a problem, what about re-routing the electrical rather than the plumbing? I'm thinking maybe running across the top of the floor with some of that threshold like conduit they sometimes use in office buildings?

I'd also think that pump is likely on the excessive side. A 009 is mostly for really high head resistance loads, with a high BTU demand... Obviously you'd need to do the math in the Taco circulator selection page I'm always sending people to, but just guessing from the typical pumps I see being used for radiant floor loops, I'd be more inclined to think you'd need a 005, 006 or 007, and could possibly get away with as little as an 003... You don't have a lot of BTU demand, and except for the 30' of 3/8", which is a short distance by floor loop standards, the flow resistance is going to be pretty low...

Gooserider

 

[webie]

Your approach sounds like it might work, aside from the code questions... Not sure what to say about that, but the code guys can be pretty stubborn about what they'll allow... It might not be an issue, perhaps some of our electrician types can venture an opinion... If it is a problem, what about re-routing the electrical rather than the plumbing? I'm thinking maybe running across the top of the floor with some of that threshold like conduit they sometimes use in office buildings?

I'd also think that pump is likely on the excessive side. A 009 is mostly for really high head resistance loads, with a high BTU demand... Obviously you'd need to do the math in the Taco circulator selection page I'm always sending people to, but just guessing from the typical pumps I see being used for radiant floor loops, I'd be more inclined to think you'd need a 005, 006 or 007, and could possibly get away with as little as an 003... You don't have a lot of BTU demand, and except for the 30' of 3/8", which is a short distance by floor loop standards, the flow resistance is going to be pretty low...

Gooserider

I am sure some where in the books its a big no-no about running pex and underground cable thru the same pvc . My first project will be to get a hold of a couple of feet of 3/8 pex and see if it will actual fit thru the first couple of bends of pvc along with the power cable , that may have me changing my mind very quickly.
I have another thought as maybe this tub could use a dancers pole in the corner to the ceiling as I could pull the power in from over head and then just run the pex thru the conduit pvc on its own .
To bad I wouldn't have thought about all this before the construction project it would have been so much easier .
Like I said earlier the pump size is a very grey area for me , I was looking at the 009 due to the high head because of my 3/8 pipe ( besides I usually over kill everything anyways why not this too )
I think i am going to try my old B&G on it first and see how my results ends up and what kind of performance I get .
Its a 4 speed pump so it will give me a bunch of variables .