Hello I have a 70K btu convential wood boiler and works well but for a few cold weeks a year it struggles to heat the house during the couple weeks of -15 or so degree weather but for the rest of the year it easily heats the house and DHW and spends alot of time idling.
With that we got a hot tub this summer and learned how much one REALLY costs to run. On a good month its only $40, but we are seeing it cost 50-60 a month alot, OUCH!
So im thinking it would be worth while to utilize the heat from the wood boiler, Ive read several threads on how others have done it but because I do not have storage nor a wood boiler with alot of extra capacity (IE cant heat quickly) I am looking to do it differently.
If you look at the plumbing part of the diagram you can see what I mean. My plan is to use an indirect DHW tank but plumb it the opposite of normal. IE The tank part will be filled with heating system water and the coil will have hot tub water flowing through it.
Reasons for doing this:
- My wood boiler is small and doesn't have alot of capacity, IE when it is providing heat it only does so for a short cycle, then stops and reheats itself. Having a 'BUFFER' tank would allow to store some heat when its avail from the wood boiler even if the hot tub isn't calling for heat.
- Setting the plumbing up so the hot tub water flows in the HX coil gets rid of the water level issue, and reduces the MASS of hot tub water to heat up.
- Used Indirect DHW tanks are cheap, about the same or possibly less than a HX setup.
I would also plumb in a connection from my DHW tank to allow topping off and filling the hot tub with hot water through the connection coming into the house, no more using a hose in the winter outside.
With that said Id like to hear folks pros and cons comments.
How it works electrically:
1 - Normal Electric Heat operation. In Normal operation without heat from the wood boiler setup. There is an added relay 'Hot Tub Call for Heat' Relay that will be activated by the 120v connection from the hot tub control box that would normally go to the 120v heating element.
When that signal is applied the Hot Tub Call for heat Relay activates. Since this is normal heating without wood boiler assistance, the "Hot Tub Tank Top Aqustat" would be OFF and the NC contact would be closed. This along with the Hot Tub Call for Heat NO Contact2 closed would send 24v to the Hot tub Elect Heat relay and send 120v to the electric heat element. Plumbing wise IM not sure how the water will flow at the hot tub but I would like to have a path where I could close off the tank heating system at the hot tub to allow complete normal running for maint or a failure.
2 - Heating the Hot tub tank - This is pretty standard, the internal aqustat on the tank is set to 150deg, when temp inside tank is less than 150deg it will close. As long as A wood boiler is running (WB On contact closed) and B my main heat zone isnt calling for heat; Zn1 NC contact is CLOSED and then 24v AC will be applied to the relay to turn on the circ to heat the tank.
3 - Heating the Hot tub - When the water in the tank is at min 110 deg, the aqustat on the top will activate and the NO contact will close. Then when the hot tub calls for heat, the 'Hot Tub Call for Heat' relay needs to also activate and close the NO contact 1 which will activate the relay to run the circ to pump water from the hot tub through the coil in the tank through the 3-way valve to a max temp of say 130 deg.
Some questions I have about this setup:
- The circ that is pumping the hot tub water, can I use a normal taco 007 pump or should I use one of the special ones for domestic water? I dont want to introduce contaminates into the hot tub possibly from the standard pump.
- The heating pipes going out of the house to the hot tub, I am thinking of using a pre-fab insulated dual pex setup, is this OK to use? How safe will this be to keep it from freezing when it gets down to -15 or colder outside?
Another Idea I have and would like to implement but probably in the future is to set it up so that the hot tub's built in temp control is set to a lower temp say 85 deg and only controls the electric heat element.
Then have another temp sensor, say a thermistor that would have a digital 'Aqustat' with normally open and closed contacts. This would be used to control heating to the preferred temp say 102 deg via the wood boiler setup. This would allow letting the hot tub electric element only heat to a lower temp to SAVE money and I can build a fire to heat it when we want to use the hot tub.. Its use has dropped off to once or twice a week now.
My concerns here would be What to use for the temp sensor and the "Digital Aqustat" Is there such a thing? Also, now with the huge difference in heating temps, how would I protect the insulated pipes going in and out of the house from freezing if we were away for a long period and no wood boiler heating going on...
With that we got a hot tub this summer and learned how much one REALLY costs to run. On a good month its only $40, but we are seeing it cost 50-60 a month alot, OUCH!
So im thinking it would be worth while to utilize the heat from the wood boiler, Ive read several threads on how others have done it but because I do not have storage nor a wood boiler with alot of extra capacity (IE cant heat quickly) I am looking to do it differently.
If you look at the plumbing part of the diagram you can see what I mean. My plan is to use an indirect DHW tank but plumb it the opposite of normal. IE The tank part will be filled with heating system water and the coil will have hot tub water flowing through it.
Reasons for doing this:
- My wood boiler is small and doesn't have alot of capacity, IE when it is providing heat it only does so for a short cycle, then stops and reheats itself. Having a 'BUFFER' tank would allow to store some heat when its avail from the wood boiler even if the hot tub isn't calling for heat.
- Setting the plumbing up so the hot tub water flows in the HX coil gets rid of the water level issue, and reduces the MASS of hot tub water to heat up.
- Used Indirect DHW tanks are cheap, about the same or possibly less than a HX setup.
I would also plumb in a connection from my DHW tank to allow topping off and filling the hot tub with hot water through the connection coming into the house, no more using a hose in the winter outside.
With that said Id like to hear folks pros and cons comments.
How it works electrically:
1 - Normal Electric Heat operation. In Normal operation without heat from the wood boiler setup. There is an added relay 'Hot Tub Call for Heat' Relay that will be activated by the 120v connection from the hot tub control box that would normally go to the 120v heating element.
When that signal is applied the Hot Tub Call for heat Relay activates. Since this is normal heating without wood boiler assistance, the "Hot Tub Tank Top Aqustat" would be OFF and the NC contact would be closed. This along with the Hot Tub Call for Heat NO Contact2 closed would send 24v to the Hot tub Elect Heat relay and send 120v to the electric heat element. Plumbing wise IM not sure how the water will flow at the hot tub but I would like to have a path where I could close off the tank heating system at the hot tub to allow complete normal running for maint or a failure.
2 - Heating the Hot tub tank - This is pretty standard, the internal aqustat on the tank is set to 150deg, when temp inside tank is less than 150deg it will close. As long as A wood boiler is running (WB On contact closed) and B my main heat zone isnt calling for heat; Zn1 NC contact is CLOSED and then 24v AC will be applied to the relay to turn on the circ to heat the tank.
3 - Heating the Hot tub - When the water in the tank is at min 110 deg, the aqustat on the top will activate and the NO contact will close. Then when the hot tub calls for heat, the 'Hot Tub Call for Heat' relay needs to also activate and close the NO contact 1 which will activate the relay to run the circ to pump water from the hot tub through the coil in the tank through the 3-way valve to a max temp of say 130 deg.
Some questions I have about this setup:
- The circ that is pumping the hot tub water, can I use a normal taco 007 pump or should I use one of the special ones for domestic water? I dont want to introduce contaminates into the hot tub possibly from the standard pump.
- The heating pipes going out of the house to the hot tub, I am thinking of using a pre-fab insulated dual pex setup, is this OK to use? How safe will this be to keep it from freezing when it gets down to -15 or colder outside?
Another Idea I have and would like to implement but probably in the future is to set it up so that the hot tub's built in temp control is set to a lower temp say 85 deg and only controls the electric heat element.
Then have another temp sensor, say a thermistor that would have a digital 'Aqustat' with normally open and closed contacts. This would be used to control heating to the preferred temp say 102 deg via the wood boiler setup. This would allow letting the hot tub electric element only heat to a lower temp to SAVE money and I can build a fire to heat it when we want to use the hot tub.. Its use has dropped off to once or twice a week now.
My concerns here would be What to use for the temp sensor and the "Digital Aqustat" Is there such a thing? Also, now with the huge difference in heating temps, how would I protect the insulated pipes going in and out of the house from freezing if we were away for a long period and no wood boiler heating going on...