Can I solve two problems with one circulator pump?

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I find myself living in a new home that has two problems with the domestic hot water system, and I’m hoping for either encouragement or else alternative suggestions on my ideas about solving these problems.

We moved in less than a year ago, and what we’re not liking is that the on-demand hot water heater seems to add more time and gallons to the amount that we need to flush from the hot water pipes in order to get hot water in the kitchen sink, shower, etc., and that it delivers a “cold water sandwich” if we’re in the shower or shaving and try to conserve water by cutting back the flow for any period of time.

We’re in drought-parched southern California and shaving or showering with taps wide open doesn’t seem like the right solution. On the other hand, gas is cheap and this particular house comes with a small amount of “free” electricity (maybe 100 kWh per month – long story, don’t’ ask unless you want to hear my rant about Solar economics). I’m inclined to minimize my energy consumption, but I want to do that in a “smart” way and the current on-demand technology seems to have a questionable degree of “smartness”.

We recently moved from a rental home that had a conventional hot water tank, but it was a modern tank (very well insulated) and it was fired by natural gas, which is very inexpensive in this part of the country. My gas bill was approximately the same, but we didn’t have cold water sandwiches. We did still have the problem of having to flush 1.5 gallons to get hot water in the kitchen sink. It’s up to 2 gallons in the new house and the on-demand system is probably accounting for about 0.5 gallons (ballpark guess) by insisting on a good flow and a good exhaust draff before it will ignite and then additional time before it reaches max temp output.

Some people claim to have solved the “cold water sandwich” problem that is unique to on-demand heaters by adding a small storage tank and a circulation pump. Other people claim to have solved the problem about needing to flush cold water from the pipes between the heater and the sink by installing a system that circulates hot water throughout the house by momentarily using the cold water supply line as a temporary hot water circulator return line. The Watts 500800 is an example.

A competitor to the better known Watts solution is They lack the reputation that Watts enjoys, but they have an appealing idea that I might borrow from in my own final solution for my home: they use a flow switch to start the circulator pump. It means that the pump is normally off, but I can engage it from any hot water tap in the house by simply opening the tap. This might not be optimal for everyone in the world, but people in my household could easily get used to the idea of “open the hot water for a few seconds then shut it off and wait for 60 seconds, then open it again and the water will be hot”. This would be an improvement over the current system of “open the hot water, fill all of the empty jugs that are at hand and once those have filled to full, let the water continue to run down the drain until it gets hot”.

I’m tempted to try a solution that involves a small (less than 10 gallon) storage tank that is in close proximity to the on demand heater. A circulator pump(maybe the one that comes with the Watts kit) causes the on-demand hater to maintain the temp of the storage tank. If I add the thermostatic “bridge” valve under the kitchen sink, and if I also position the pump such that it is downstream of the heater outlet (and upstream of the tank inlet, the heater inlet, and the bridge valve under the kitchen sink), then when the bridge valve is thermostatically closed, the system would reduce to a simple on-demand heater with a small storage tank. When the bridge valve is thermostatically opened, the pump would be pushing water through two distribution loops instead of one, and water would flow in inverse proportion to resistance, but I don’t know now to accurately calculate the resistance. Would this design work, or should I invest in two separate circulator pumps in order to solve the two different problems?

Part of me feels like I’m building a Rube Goldberg contraption. Surely there’s a better way?

Creative ideas (and also conventional thoughts on this problem) appreciated. Thanks


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Northern Idaho
I noticed that no one has responded to your question. I don't think you've provided quite enough information about your existing tankless system to provided a helpful answer. It appears you have a natural gas, tankless water heater (TWH), with a conventional "racetrack" pipe system (i.e. versus a manifold distribution system), and without a re-circulation pump. Are your pipes copper or PEX? How old? 3/4" pipes to 1/2" at the fixtures, or 1/2" throughout? 65 psi or greater?

You seem to have two issues:

  1. The length of time for hot water to travel from the TWH to the fixtures, and the water wasted in the process.
  2. A "cold water sandwich". A cold water sandwich (CWS) usually means hot water is in the line, turns cold for a variable period, and then goes back to hot.
Let's start with #2.

In non-recirc systems, CWS issues usually occur with electrical TWHs. Warm water might be retained near the fixture (e.g. shower head) from prior use, then the water is turned on, and cold water briefly slips by the heat exchanger before the heater kicks off and again heats the water. In recirculating gas/propane TWH, hot water has been pumped into the hot water line (which is configured in a loop to return to the TWH), something draws cold water into the hot water line, followed by fresh hot water from the TWH after it's activated.

Now there are a bunch of potential causes for the latter phenomenon, but you don't have a recirculating pump in your existing system. So, we'll skip that for the moment.

Let's review your statement: "we’re in the shower or shaving and try to conserve water by cutting back the flow for any period of time".

It's important to understand what creates the "demand" in TWH's. That would be water pressure through an open tap. The manual for my Rinnai unit indicates that it requires a minimum 1 GPM of water flowing flowing through the hot out-take valve to start the water heater, and that was consistent with my testing. (I also just read another FAQ that indicated that figure was 0.4 to 0.6 GPM to initiate, and 0.26 GPM to keep the unit in operation. Not sure of the reason for that difference, and your unit may have different specs.) The first point here is that you will NEVER be able to run hot water through your faucet "reduced", say at a trickle for shaving, without the lack of demand turning the TWH off, and drawing cold water into the line (i.e. CWS). That requirement is beyond the technology. Second, modern faucets put out a maximum of 1.8 GPM (some government or code standard) at (my guess) 75 or 80 PSI. Your hot water fixtures may flow less water than required to activate your TWH due to modern flow restrictors, low water main PSI, calcified and clogged pipes or fixture outlets, and etc.. Regardless, low water flow through hot water fixtures could cause your "cold water sandwich".

Back to the #1.

This is a feature of either TWHs or tank systems. Moreover, the design efficiency of your plumbing, pipe size, and size of your house can play a major role in how long it takes water to reach a given fixture. Short of repiping, the only solutions that will reduce the time from demand to delivery include:

1) Adding a re-circulation pump in the TWH system.
2) Additional storage tanks and/or water heaters, tankless or otherwise, throughout the entirety of your hot water system.
3) Other, more esoteric or exotic technology (you listed some, e.g. additional tanks/TWH on the line, boosters, etc.).

This is the point where any solution proposal for your situation requires guess work, because we haven't sufficient information about your system.

Adding a recirculating pump in a retrofit situation may provide limited benefit, or be inefficient. However, it SHOULD save you water, since you'd be pouring less cold water down the drain waiting for the hot water to travel from TWH to tap. Then again, it COULD be burning off a lot of gas creating what is in essence a large hot water tank out of your entire hot water line. The system might have to run full time to keep water hot throughout the line. Hot water pipe insulation, or lack thereof, can be another issue. It all "depends" on your situation.

Let me give you my brief example: New construction (but retrofitting an abandoned built, so we modified but did not replace all piping). Plumber installed my TWH (RUR98i) as a "cross-over" retrofit style. That forced the hot water through a 1/2" line on a faucet at the end of the hot water line, rather than using the 3/4" dedicated hot water return line. It took 10 minutes to circulate hot water from the water heater to the last bathroom on the hot water line. I modified that to use the 3/4 dedicated line configuration, and we now have hot water everywhere in under two minutes. While we have a built in pump scheduler in our unit (e.g. to schedule hot water for a certain period such as 5 AM to 9AM, etc.), since we run on propane which is BTU for BTU much more expensive than natural gas, we flip the recirc switch on the controller in the hot water closet whenever we want hot water. Regardless of which tap or shower we're using, the water is hot at the fixture before we return to the fixture. (Eventually, I'll add a remote switch so we can trigger it at the bathrooms in the morning.) The recirculation pump is automatically scheduled (via the dip switch settings) to run every 20 minutes, to reheat the water in the line. is It's a very energy efficient and water efficient solution for us. Our water is always very, very hot, even in sub-zero Idaho weather.

The first configuration (cross over) is likely what a plumber would use to install retrofit TWH on a pipe system with out a dedicated hot water return (i.e. hot water line that connects in a loop back at the TWH). If you have this configuration, well, "it depends". If you have 1/2" water lines, that's tough to improve without new pipe. If you have, as in our case, two major branches that you can "connect" in the middle, to create a natural hot water line loop, it might be worth breaking out some drywall to modify your piping.

Adding storage tanks might help, but unless they are actually "hot water heater" tanks, they'll eventually just be a store of cold water. The nice thing about my TWH, however, is that it uses a PVC exhaust. If you can get a gas line to a location on the far end of your hot water line, such in the case of an expansive, single floor home layout, you might add a very small closet for a second TWH, venting to the side of the house (plus maybe a wet deck, drain, etc.). It could reduce the cold water "travel" from minutes to seconds.

Bottom line: If you're in a retrofit situation, it may be a significant challenge, or a weekend modification. Generally speaking, a recirculation system should help with both water conservation and "instant hot water". I would have installed a recirc/condensing TWH in my last California home because I knew where the water lines ran, could have accessed them easily to modify them, and the retrofit PVC venting would have been relatively inexpensive. They were, I believe, 1/2" copper, so that would have been a limiting factor on efficiency. I would probably never install an electric TWH anywhere (my bias).

As far as which solution you opt for, I always vote for "simpler", because "more complex" usually means "expensive and inconsistent".

Sorry for the long speil. Tough to describe this stuff in few words. Some pro's on the forum may correct my above understanding and assumptions. Good luck.
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Retired Defense Industry Engineer xxx
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New England
NOt all on demand systems allow recirculation, or if it is used, it shortens the warranty. Some are setup from the factory to allow it. YOu need to first know what your system allows.

If there is one 'critical' place where you want hot water immediately, a local small tank in-line with the tankless system will provide hot until the line from the remote tankless system shows up. Putting in multiple ones, probably isn't what you want to do, though.

If you can put in a return line, then a small tank near the tankless, you can run the return line back into the tank. During use, the hot water from the tankless will keep the tank hot (but maybe not high enough to be safe long-term), and a recirculation system will keep the supply lines at least warm, depending on your control system. A tank should be at least 120-degrees, and that's probably hotter than the tankless, so while you're getting water out of the tank, it will be hotter until flushed with the cooler tankless' output, then rewarm itself to the set point when things stop. If the tankless has or is designed for recirculation, that makes things easier.

I have a retrofit unit that's been in for about 12-years (RedyTemp) that works well for me. Simple install, all-in-one box with everything. It tends to run about 30-45 seconds, maybe 4-5x per hour to maintain the water temp. The first time in the morning it tends to run about 2-minutes. I have a large indirect tank, so that's not an issue. I have it on a timer so it only can turn on during likely times of use. I figure that the pump only uses maybe 20w/hour since it's small, and only runs a short time.
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