I'm considering getting a "comfort pump" which will pump water from the hot line to the cold line under a sink so that there is no waiting for hot water, as well as saving the wasted water.

My concern is that the hot water is getting pushed back into the cold line and that will get fed into the reverse osmosis filter, and that the filtered drinking water will no longer be as good. I've seen numerous articles that state that contaminants from the hot water heater are not good to drink.

Then again, it might be just such a small amount of this water that gets into the RO filter, and the RO filter might do such a good job of eliminating contaminants that this problem won't be an issue.

I'm considering these 2 models of "comfort pumps":

  • Watts 500800 Premier Hot Water Recirculation Pump, Blue
  • Grundfos 595916 1/25 Horsepower Comfort Series Recirculator Pump

This question is related: Are hot water returns worth putting in?

I got this response from Watts:

The small amount of water from the hot water heater that may make it to the RO system will not matter. The RO does not allow water in constantly, only when pressure has been bled off the storage tank, as when the faucet has been opened, and if it does, the reverse osmosis system will reject the contaminants that may be present, mainly calcium and/or magnesium deposits, which could build up the water heater. The reverse osmosis system should not be a determining factor for installing a hot water recirculting pump, as it will have no effect.

I got this from the Hot Water Lobster. I'm skeptical of the claim that the powered pumps only last 18-24 months. I'm also skeptical of recirculating the water during the middle of the night.

On the other hand, when it's sunny where I live, I see no disadvantage circulating the hot water all day long, as my hot water tank would get unreasonably hot such that I had to add a thermostatic mixing valve.

The Hot Water Lobster Instant Hot Water Valve is thermostatically controlled and opens only when the hot water temperature at the HWL drops below the adjustable temperature setting (allowing thermal convection to slowly move the water through the system until it reaches your set temperature). With the Hot Water Lobster Instant Hot Water Valve, you should get hot water within 4-8 seconds at the faucet where the Hot Water Lobster Valve is installed. Powered pumps circulate the water at a faster rate than the thermal convection generated within your hot water tank. However, the speed the water circulates is indifferent to the user because hot water should always be available when you need it with the Hot Water Lobster Instant Hot Water Valve. Be aware that most pump systems: are restricted to timers or cycle buttons before the pumps will circulate the water. Which means hot water may not always be available during off times or at sinks without start buttons (when you may need it); during pump cycles, your entire cold water pipe is filled with hot water (as the water returns to your water heater), and because hot water is pumping for the entire time period, you cannot get cold water until after the pump stops. pumps often only last 18-24 months before replacement is necessary; initially the noise level of pumps is minimal but over time can become very noticeable (especially when your trying to sleep); pumps often cause electrical interference to televisions and radios; and installation is more complex requiring a nearby electrical source and electrical connections (to power the pump). On the other hand the Hot Water Lobster Instant hot Water Valve: incorporates the flexibility of an adjustable thermostatic control; is built with quality materials and workmanship for a long maintenance-free life, is easily installed (not requiring electrical wiring, timers or perishable pumps); and is completely silent. There are so many variables from plumbing system to plumbing system, calculating a standard energy saving formula would be difficult and very complex. In general, instead of replacing the water (wasted down your drain-while waiting for hot water) with cold ground temperature water (which has to be heated from a low temperature), the Hot Water Lobster Instant Hot Water Valve allows already warm water to circulate back to your hot water tank for reheating-which not only saves the cost of the water, water treatment charges (sewer charges), it also takes less energy to bring the already warmer water (being re-circulated) up to temperature. The more often hot water is used the more energy is saved (compared to not recirculating the water and heating ground temperature water instead). Pipe heat loss is minimal compared to the cost to heat 2-4 gallons of ground temperature water the 18-24 times a day (the average family uses hot water). And in cold months (when hot water is used 42% more often than in hot months) the heat loss is not wasted (any radiant heat provided by the hot pipes transfers to heat for your home). With or without a hot water recirculating system, the better insulated your pipes are the less energy is wasted.

  • I agree with that if your using reverse osmosis filters I'm guessing u don't want to drink the hot water, extra leaching occurs when the water is heated. But having said that what piping are you running through? If it uses lead solder in the hot line I really wouldn't do it.
    – UNECS
    Oct 3, 2012 at 7:26
  • New house, copper lines with pro-press, with a standard solar hot water system with a tank in my basement. Oct 3, 2012 at 18:49
  • 1
    If you're worried about the taste, get a cup of water from the hot side of a tap that hasn't been used in a while (before the water actually gets hot). This is the water that will be in the cold line after you install a recirculating pump.
    – BMitch
    Oct 3, 2012 at 20:27

4 Answers 4


I suspect you have a set-up like this?

enter image description here

You are concerned that when the water gets pumped back into the "water heater" it could possibly pollute your main water line?

The only thing I could suggest is putting a check valve on the "cold" water supply so that it can only supply fresh water. That is helpful if the pumps output pressure is higher than your supply pressure (This could happen on someday with low supply pressure or if you do not have main water pressure regulator)

enter image description here

In this situation the return is a dedicated port on the "water heater/boiler" and there should already be a check valve/pressure regulator/non return valve just before the water heater. So the rest of the house will run just on the cold water supply and never get any recirculation contamination.

  • I don't have any recirculation loop, and regret that. Oct 3, 2012 at 18:48

The best solution that I have found in the market is the Metlund D'MAND system, which operates only on user demand. It can be installed in any standard plumbing system, and uses the cold water line as the return pipe. One of the best features is the fact that it turns off based on a 6 degree rise in temperature so that once you've initiated circulation in your pipes, the pump will shut off before hot water starts to cross over into your cold water lines, yet hot water is only a few seconds away. This is definitely the cadillac of all recirculation pump systems.

  • The price for the Metlund is much higher than the Watts and Grundfos systems. Not sure if the higher price is justified. All are available on amazon.com. Oct 4, 2012 at 6:21

I modified @ppumpkin's image to reflect my understanding of your setup, including recirculating the water over the cold line.

enter image description here

The pump may be located near the water heater with a remote valve, but the idea is the same. When the temperature at the end of the line is low, it dumps water into the cold line until the temperature gets hot. This means it's going to put some warm water in the cold line, but typically not hot water.

One downside of this setup is anytime you open a cold tap, you're going to get some warm water. This includes when the RO system uses water. A couple points about the RO:

  • RO systems typically use a pressure tank to store purified water, and only draw source water when the tank pressure is low.
  • The RO system should filter almost everything out of the water anyway (note some bacteria can get through imperfections in the membrane and imperfect seals around it)
    • Some RO systems additionally have a UV light to kill anything that does get through
  • Hot water does wear down the RO membrane, but it should only be a minimal amount of warm water that the RO system ever gets.

Hot water can leech more eg, Lead out of pipes than cold water, but this is really only a concern if you have a particularly old plumbing system. If you're using anything installed within the last decade or two, you are likely fine.

One concern could be Legionnaires' disease. Legionella (the bacteria responsible) grow at temperatures between 20 and 50°C (68 to 122°F). The recommendation in Canada is to keep your water heater at 60 °C (140 °F) or higher. I actually can't find any research on legionella growth in pipes (somewhere in the temperature gradient you're going to get the ideal growth temperature) but since the water is always moving (eg, due to the recirculation pump kicking on as it cools) it may not be an issue.

TL;DR: It is just about (but not quite) as safe to drink as the regular cold water. Your RO membrane may wear out slightly quicker, but it should not be significant.


Whether water from your water heater gets into your filter system is a question of how these systems are plumbed. An recirculation pump like what you're considering will remove water from the hot water pipe and move it to the cold water pipe. This will naturally create a draw on your water heater. At the same time, your water heater will draw in new cold water.

Is your faucet plumbed to always draw cold water from the output of the RO filter? Because if so, then typically when you use the recirc system, there will be no movement of water through the filter. Only if your faucet draws cold water from a source upstream of the filter, and your hot water heater draws its cold water from a source downstream of the filter, would you have a situation where your recirc pump draws water that had been in your HWH across the filter.

Granted, this is the typical case and it is possible for water to flow backwards into your filter in uncommon circumstances such as a drop in municipal water pressure. However, these are rare enough that their impact on the filter wear and tear is probably negligible. And you likely already have a backflow preventer on the filter to address this issue.

Looking at the message from the Hot Water Lobster company, I would put aside the sales pitch (including claims about the noise and longevity of recirculation pumps). This system essentially incorporates your hot water piping into your hot water heater tank, letting hot water circulate in those pipes until your set temperature is reached. This sounds convenient, but I could see it increasing your energy costs significantly, as you are essentially operating a larger-capacity hot water tank, parts of which are likely not well insulated. Additionally, using the HWL may require you to disable your heater's check valves, and probably wouldn't work with e.g. a tankless water heater.

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