Replies

1. Expert Member
   Zephyr7|Oct 05, 2020 12:40am|#1

   This would be of limited usefulness since it there is already flow due to demand (like an open faucet somewhere), then there is no need for the circulation pump. The purpose of circulation pumps on hot water lines is to keep the lines warm so that you don’t have to run a lot of water before you start getting hot water. This means the pump needs to run BEFORE you need hot water somewhere. Typical systems either run the pump all the time, or have a timer or button that runs the pump before hot water demand is anticipated.

   The usual way to put in a recirculating hot water loop with an on demand water heater is to use a very small tank-type water heater after the on demand water heater. The recirc pump works in the usual way, circulating hot water through the small tank water heater. When someone opens a faucet or turns on a shower, the on-demand water heater fires up to supply that hot water. In this way, the on-demand water heater is essentially isolated from the recirculating system and provides only hot water when there is demand, and the small tank type water heater acts to keep the recirculating loop warm in cooperation with the recirc pump.

   Bill

2. 
   DCContrarian|Oct 05, 2020 01:02am|#2

   Thanks Bill.

   What I've seen are recirc systems that don't run continuously, but put a flow valve on the water heater. When the flow valve detects flow the recirc pump kicks on and gets hot water to the tap faster than just letting it run. The recirc pump turns off when hot water gets back to the end of the recirc circuit.

   It's not instant hot water, it's faster hot water. It's a compromise between running the recirc continuously. It's like having a button, but more automated. In fact people can learn to turn the tap on for a second, and then turn it off and wait a few seconds.

   My thought was that a tankless probably already has a flow valve, maybe it could serve both purposes.

   I assume in your example the very small tank-type water heater is a resistance electric heater?

   Thanks.

1. Expert Member
   Zephyr7|Oct 05, 2020 04:35pm|#4

   There are also timer systems that only run the pump around the times of the day you’re likely to use hot water.

   I’ve heard of the “hot water faster” systems but they’ve never sounded like they would be worth much to me. I can’t say I have experience with such a system. My own system is a thermosyphon system, which has no pump, so no electricity use, but it runs all the time and the tradeoff is a little more gas use to make up for the heat loss of the loop. I insulated the entire loop, and see no real difference in gas use but I know there is some.

   Yes, the small tank water heaters are typically electric resistance units.

   Bill

3. Expert Member
   Akos|Oct 05, 2020 10:43am|#3

   What you are looking for is an in-line pressure booster pump. These are sold for places that have undersized piping and come with an integral flow switch.

   不需要额外的控制,只添加一个of those thermostatic (sensor valve) return manifold they sell for recirc systems. The pump will push the hot water through the system until it gets warm enough for the valve to close. Once the valve closes, there is no flow and the flow switch opens shutting the pump off.

   I have tried a similar on-demand setup for my recirc and home (pump flow switch with one shot timer). It helps but not as good as the timer/motion sensor based setup. When you turn on a tap, you want hot water there, otherwise what is the point of recirc?

   Also watch for pressure drop across your tankless. These are pretty high and when you already have a draw across it, it will take a pretty hefty pump to push extra water flow. The small pumps typically sold for recirc application will not budge the flow.

4. 
   Walter Ahlgrim|Oct 05, 2020 05:44pm|#5

   I built a system much like you described in my home.

   The challenge in my home is that the 2 baths are about 90 feet apart and I wanted one heater. My heater is a 36kw electric unit. I did not want to tap into the flow sensor as it is a delicate circuit. I put a current sensing relay on the first circuit that draws 28 amps when the unit is heating. I built a control box for that input. The second input is a sensor for the water temp returning from the hot water loop. Whenever hot water is drawn from any faucet that turns on the water heater then we turn off the faucet and do something else for about 90 seconds. The control senses the heater is running if the loop is cold it starts the pump the pump will run this will keep the heater running. My control will turn off the pump when the water returning from the loop is warm or after 3 minutes.

   Walta