Motion Sensing for Residential Hot Water Demand Systems
Using a motion detector to start your hot water demand system can have unexpected results.
Demand systems are meant to be activated and then used quickly. Hot water in your piping cools off quite rapidly. Since a demand system shuts off the pump when hot water reaches it, if you don’t use the hot water quickly it will soon become “warm” water and to get hot you will still need to run more water out of the pipes and down the drain.
How the hot water demand system behaves a short while after being used depends mainly on how long since the system was last used. If the water hasn’t cooled to below 96 degrees the pump will be locked out and won’t start until the water in the pump cools to below 96 degrees F. You will be forced to purge the warm water from the pipes to obtain hot water and end up running that water down the drain.
If the water has cooled to just below 96 degrees… say about 90 degrees, then the pump will run again until it sees a sudden increase in temperature or if it senses water hotter than 96 degrees and then shut off again. Now however, there will be 90 degree water filling the cold water line.
If you begin using the hot water mixed with cold, after a short time the warm water will be purged from the cold water line and you may need to re-adjust the temperature to add more hot and less cold water. More details about how the hot water demand system behaves in different situations. It pretty much depends on the water temperatures in the piping.
Using a Motion Sensor
As you can see, any significant wait after you have tripped the motion sensor can cause an inconvenience and or end up wasting water.
Another thing to consider when using motion sensors to control your demand system is what happens when someone enters the bathroom while you are taking a shower? Depending on your plumbing layout and how long since the pump was last run, it could cause a fluctuation in water temperature like flushing a toilet sometimes does.
This is especially true when you have a tankless water heater. Since tankless water heaters take time to heat the water, typically 10 to 20 seconds, if the heater has been off for a few minutes you may experience a brief “cold water sandwich”.
Before you decide you want a motion sensor to activate your demand system you might consider how often you enter your bathroom when you don’t want hot water. Each time the pump runs and you don’t use hot water you are wasting energy and increasing your water heating bill. This is especially true with tankless water heaters. It doesn’t take many firings of the heater to use more energy than a standard tank type water heater loses in standby loss.
The same things hold true when using a timer based system. You can have the demand pump turn on automatically at say 7:30 a.m. every morning, but if you push the snooze button a time or two you are still going to need to run the pump again when you get to the bathroom.
Since the piping is warm you will get hot water faster but you pay the price in extra energy consumption.
For some people motion sensing to control the delivery of hot water is a welcome convenience, but for many others it would just be a wasteful nuisance.
Demand systems are meant to be activated and then used quickly. Hot water in your piping cools off quite rapidly. Since a demand system shuts off the pump when hot water reaches it, if you don’t use the hot water quickly it will soon become “warm” water and to get hot you will still need to run more water out of the pipes and down the drain.
How the hot water demand system behaves a short while after being used depends mainly on how long since the system was last used. If the water hasn’t cooled to below 96 degrees the pump will be locked out and won’t start until the water in the pump cools to below 96 degrees F. You will be forced to purge the warm water from the pipes to obtain hot water and end up running that water down the drain.
If the water has cooled to just below 96 degrees… say about 90 degrees, then the pump will run again until it sees a sudden increase in temperature or if it senses water hotter than 96 degrees and then shut off again. Now however, there will be 90 degree water filling the cold water line.
If you begin using the hot water mixed with cold, after a short time the warm water will be purged from the cold water line and you may need to re-adjust the temperature to add more hot and less cold water. More details about how the hot water demand system behaves in different situations. It pretty much depends on the water temperatures in the piping.
Using a Motion Sensor
As you can see, any significant wait after you have tripped the motion sensor can cause an inconvenience and or end up wasting water.
Another thing to consider when using motion sensors to control your demand system is what happens when someone enters the bathroom while you are taking a shower? Depending on your plumbing layout and how long since the pump was last run, it could cause a fluctuation in water temperature like flushing a toilet sometimes does.
This is especially true when you have a tankless water heater. Since tankless water heaters take time to heat the water, typically 10 to 20 seconds, if the heater has been off for a few minutes you may experience a brief “cold water sandwich”.
Before you decide you want a motion sensor to activate your demand system you might consider how often you enter your bathroom when you don’t want hot water. Each time the pump runs and you don’t use hot water you are wasting energy and increasing your water heating bill. This is especially true with tankless water heaters. It doesn’t take many firings of the heater to use more energy than a standard tank type water heater loses in standby loss.
The same things hold true when using a timer based system. You can have the demand pump turn on automatically at say 7:30 a.m. every morning, but if you push the snooze button a time or two you are still going to need to run the pump again when you get to the bathroom.
Since the piping is warm you will get hot water faster but you pay the price in extra energy consumption.
For some people motion sensing to control the delivery of hot water is a welcome convenience, but for many others it would just be a wasteful nuisance.
Labels: electric water heaters, energy efficiency, environment, plumbing, remodeling
posted by Hot Water Guy at
2:41 PM
0 Comments:
Post a Comment
<< Home