We are thinking of replacing our current natural gas tank water heater by a high efficiency tank less heater.

However we have a water main issue, the house is from 1930 and the main is partially clogged. We had people from the town water service and a local contractor who does water main replacement come to our house and long story short we have no short term solution to solve the main clogging issue. Anyway, because of that we have good water pressure but low flow. Meaning as long as we only one faucet opened water pressure is great but it drops as soon as we open a second one.

I read several posts online about people complaining of pressure issues with tank less heaters but it seems to usually be because of undersized units for their house. In our case we have only one bathroom and are already use to not using 2 faucets at the same time so I don't think this would be an issue. We are looking at a 9 GPM rated unit.

Now my question (sorry for the long introduction): Not considering the sizing question, do people observe a water pressure drop when installing a tank-less unit compared to a similar flow rated tank unit? I'm not sure how a tank unit works. Does it refill at the same time as you are using the hot water or is the refilling process asynchronous? I'm afraid that the tank less unit would actually behave like if we had 2 faucets opened at the same time...sorry if the question is stupid or doesn't make sense.

I will obviously also ask his opinion to our plumber but I like to get different opinions.


  • How many GPM do you get from a faucet in your house now?
    – Kris
    Dec 8, 2018 at 17:08
  • I assume that you have a natural gas fired tank water heater (WH) right now, right? It is in a basement? A basement is a good location for a tank WH because if it ruptures, the water will be collected in the sump and pumped out. When you get a new tank you can fit a WAGS valve so that the water supply and gas supply (to the WH only) will be shut off automatically. Dec 8, 2018 at 21:48

3 Answers 3


Actually, tankless plays very well with low flow. The achilles' heel of electric tankless is that it can't heat high flow, because of the stupendous amount of power it takes to do it in real time. That's not so much a problem with gas, because burning gas can make that stupendous amount of heat. But regardless, constricted flow on your water supply just makes the tankless' job easier.

For your information, tanked heaters are literally just a tank. The only trick is a long tube so that cold water is delivered to the bottom of the tank, not the top. As you take hot water off the top, cold water pushes in the bottom. The tank is designed so the water stays layered (hot on top) until it is nearly exhausted. This is aided by gravity; hot water is slightly less dense than cold.


In short a tankless doesent require a high flow if anything a lower flow, the problem most folks have is a lower flow of hot water due to the fact the tankless heater can only heat the water so many degrees at a specific rate. For instance if your tap water is 50 degrees and you want a 110 degree water the unit has to be able to heat the water 60 degrees most units restrict the flow to maintain the temp with a max temp of ~130. The nice thing about tankless you never run out of hot water, when changing the water in our hot tub I set the heater at 110 and connect a hose to the hot water line and fill the entire tub hundreds of gallons and it is hot and ready to use when full. Where the problem comes in if you are showering and someone turns the hot on in the kitchen most tankless systems can't heat that much so the shower gets cold if using both hot and cold water, if the heater is set to the temp you want your shower and you are only using hot when another hot tap is turned on the flow drops but the temp stays fairly constant.

  • Thanks Ed. I looked and it seems like water in MA is between 45-50ºF. I didn't really understood the different GPM ratings based on temperature. Now I understand thanks to your comment. Dec 9, 2018 at 0:43

A tankless water heater will not act like you have another faucet open. They do have a lower flow rate than a tank heater because they control (restrict) the hot water flow rate to keep the temperature of the output hot water at the set point.

A gas fired tankless water heater has a very large burner--up to 199,000 BTU/h compared to 40,000 BTU/h for a 50 gal tank. You must have gas supply piping that can deliver at that flow rate. Have you determined that your gas piping system can deliver gas at that rate?

In a hot water tank the cold feed water comes in at the the same volume flow rate as the hot water is taken out. The tank stays full at all times. The two water lines enter the heater at the top; the hot water out tube is a short tube so it draws from the top of the water column; the cold input tube goes to near the bottom of the water column where the heat source is, at least in gas fired water heaters.

I'm not sure about electric tank heaters, but it must be about the same. As you draw water there is stratification with cold water on the bottom and hot water at the set point on top.

You say you are looking at a 9 gpm unit. What is the mfgr and model of the unit you are considering? What is the maximum heating rate in BTU/h?

A tankless water heater is more expensive than a tankless and many people do not need a tankless. If you are not running out of hot water regularly, then it would be cheaper to stay with a tank. Tanks are very low maintenance, but the tankless require some maintenance.

We changed to a tankless from a tank after we had two tank ruptures resulting in damage to the house. I don't think tankess heaters normally fail catastrophically. They start to leak at a drip rate and one can hear the dripping or one could have a water detection alarm in a pan under the heater.

EDIT What is the size of the house and how many people are living there? Are you running out of hot water right now at times when more than one person is using hot water? What size of tank do you have right now? How long has the current tank been in service?

To install a tankless WH you would definitely need a new flu and you would a 120 V receptacle at the location of the WH. Modern tankless WHs require 120 V ac to power the controls on the tankless unit. All this adds up. The total cost of purchasing and installing a new tankless WH could be $2500. It is possible that a tankless WH would not significantly reduce your gas usage. Modern tanks have good insulation.

Is this residence located in Boston MA? I assume then it has very cold inlet water. I do not think a 180 kBTU/h tankless WH will yield 9 gal/min of hot water with very cold inlet water.

  • Jim thank you so much for the comprehensive answer. I hadn't thought of the gas supply issue. Is that something I can check on my own? Do I need to contact the gaz company? We are considering a high efficiency tank less heater because we are getting a $700 eco-incentive from our local provider to do so. The model I was looking at ( although I'm absolutely not set on the unit) was a Rheem - 11k to 180k BTU/h - 5.2 GMP at 67ºF rise - 9 GMP max. I suppose the maintenance can be done at the same time and by the same technician as our natural gas forced furnace. Dec 8, 2018 at 19:48
  • @guillaume.molter still unanswered by you is how many GPM Yiu are getting from your water supply. If you could edit that into your question it would be helpful
    – Kris
    Dec 8, 2018 at 20:51
  • The gas company can easily change your meter to a larger one if necessary. The question is whether the gas lines from the meter to the house and inside the house are large enough to supply the extra 130 kBTU/h that a tankless WH would need. An installer of tankless water heaters could look at the piping and tell you. Note however that they will have an interest in selling you a tankless heater. The gas company might come out and inspect and give you an opinion. Dec 8, 2018 at 21:19

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