Heating Hot Tub using household hot water.

Posted by Bob on July 18, 2005, 10:17 am

I have a hot tub that is currently heated by pumping hot water from the
house gas water heater through a copper pipe installed under the seats of
the tub. I have been told that this does not meet code, because of the risk
of a leak allowing "dirty" water to be sucked back into the city water
system if the city water is shut down for some reason.

If someone out there is familiar with the code requirements for such a
system, I would appreciate consideration of the following questions.

1. Is my current system in violation of generally used plumbing codes?

2. Would any of the following make it compliant?

a. Installing a check valve on the water heater input to prevent
backdrawing of water. (With associated expansion tank)
b. Installing a check valve on the house water input line.
c. Adding a simple counterflow heat exchanger. I see this as a 20'
section of 1" copper turing with 20' of 1/2 inch copper tubing inside it.
This would be coiled above the water heater, with 7-8' of it straight down
along the heater. The bottom end of the 1/2 in tibing would go into the
water heater drain hole, and the top end would connect to the hot water out
from the heater. I am hoping that convection would produce enough water
motion, so that an additional pump would not be needed. The current pump
would pump water from a closed system through the 1|" outer pipe and the
loop through the tub, going up through the added heat exchanger.

Any other ideas greatfully accepted.

In Seattle.

Bob

Posted by SQLit on July 18, 2005, 1:16 pm

> I have a hot tub that is currently heated by pumping hot water from the
> house gas water heater through a copper pipe installed under the seats of
> the tub. I have been told that this does not meet code, because of the

risk

> of a leak allowing "dirty" water to be sucked back into the city water
> system if the city water is shut down for some reason.
> If someone out there is familiar with the code requirements for such a
> system, I would appreciate consideration of the following questions.
> 1. Is my current system in violation of generally used plumbing codes?
> 2. Would any of the following make it compliant?
> a. Installing a check valve on the water heater input to prevent
> backdrawing of water. (With associated expansion tank)
> b. Installing a check valve on the house water input line.
> c. Adding a simple counterflow heat exchanger. I see this as a 20'
> section of 1" copper turing with 20' of 1/2 inch copper tubing inside it.
> This would be coiled above the water heater, with 7-8' of it straight down
> along the heater. The bottom end of the 1/2 in tibing would go into the
> water heater drain hole, and the top end would connect to the hot water

out

> from the heater. I am hoping that convection would produce enough water
> motion, so that an additional pump would not be needed. The current pump
> would pump water from a closed system through the 1|" outer pipe and the
> loop through the tub, going up through the added heat exchanger.
> Any other ideas greatfully accepted.
> In Seattle.
> Bob

Custom made tub?

Pumping the water into the tub? Never heard of such an animal. Where exactly
does the water enter the tub? above or below the user water level. If below,
then a check/one way valve should work.

Heat exchanger is just going to complicate the system. IMO

Best ask a local plumber or an inspector. Fix it right now so when you sell
the place you will not be forced to fix it then.

Posted by Stretch on July 18, 2005, 3:03 pm

I believe that you need a double wall heat exchanger. In between the
two surfaces is a rifled space that is vented. We used to use those
when installing solar water heating systems with glyclo to prevent
freezing. I don't know where to get one now.

Stretch

Posted by Bob on July 18, 2005, 5:44 pm

> I believe that you need a double wall heat exchanger. In between the
> two surfaces is a rifled space that is vented. We used to use those
> when installing solar water heating systems with glyclo to prevent
> freezing. I don't know where to get one now.

Ahh! That helps a lot in the search engine. Thanks.

Bob

Posted by on July 18, 2005, 9:12 pm

>I believe that you need a double wall heat exchanger...

A backflow preventer might be simpler. As found in washing machines.

You might replace tub water with cold fresh water using a slow counterflow
heat exchanger. It could be very efficient, at less than 0.1 gpm. How many
gallons of fresh water per person-hour of use, to avoid using any chemicals?

Nick

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