Replies

1. GBA Editor
   Martin Holladay | Jul 04, 2016 05:40pm | #1

   Richard,
   If you are talking about hot water systems that include a thermosyphon loop that encourages hot water to flow continuously in a circle (in a long loop from the water heater to a distant bathroom or kitchen), the main disadvantage of these systems is that they waste energy. Heat is continuously lost from the tubing that makes up the loop, and the water heater needs to make up for that lost energy.

   With a demand circulation system, the pump is almost always off -- it runs for a few seconds or minutes each day -- so heat loss from the hot water tubing is much less than with a thermosyphon loop. (Demand circulation systems include a check valve to prevent thermosyphoning.)

   For more information on this issue, see Hot water circulation loops.

2. 
   Plumbserve | Jul 04, 2016 07:47pm | #2

   Martin
   Yes, I'm taking about a thermosyphon system. The advantage to this system compared with pump systems is the amount of fluid circulated and the ambient heat lose comparison. A well insulated gravity loop will circulate about 50 gallons a day with the returning water a consistent 95 degrees and above. A pump running just 15 minutes daily will meet or exceed this gallon comparison. I see your point of the pump runs only when demand is needed, most aqua-stats will cut off at 95 degrees. Every installation is going to be different but we have found the gravity lines installed in the joist and well insulated will run at 1/3 the cost of a pump design. I could see your design working very efficient if all the lines are above slab and well insulated and the pump is only on when the customer goes to use it. I have a 75 gallon water heater natural gas with a gravity loop and I average $ 14.00 monthly gas bill. Compare a gravity loop system to a pump in a home, I know you will be surprise. I agree after market is a tough one for gravity systems but all new homes should have above ground well insulated hot water lines. Hot lines in contact with the soil kill the utility bill.

   I will check out your hot water circulation loops site.

   Thank You, God Bless

   Richard Hugo

3. 
   user-5946022 | Jul 04, 2016 08:18pm | #3

   Richard
   A few questions, as this is an issue I'm trying to learn more about.
   1. Are you saying that you can do a gravity loop on a one story house? How? Do you install the water supply lines in the attic and feed them down to the fixtures? If so, aren't the supply lines in the attic riskier than in the crawl, should they break?
   2. Are you saying that the thermosyphon system, even though it runs constantly, uses less energy because the amount of heat loss is so small there is less energy used on the reheat than the amount of energy a pump uses?
   - The thermosyphon seems appealing because it is simple and provides hot water to every fixture.
   - The pump system seems appealing because the water is only recirculated when hot water is needed; so if for example the household is on vacation, the pump does not operate.
   What would be the method of measuring or calculating the amount of energy used by each?

4. GBA Editor
   Martin Holladay | Jul 04, 2016 08:59pm | #4

   CL,
   All the thermosyphon systems I've heard of are installed in homes with a basement. The water heater is in the basement, and the bathrooms and kitchen are upstairs.

   While a thermosyphon system doesn't require you to purchase a pump, it will waste more energy than an on-demand circulation system.