Rheem RTEX-18 electric hot water heater in Montana

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Small Cabin Forum / Off-Grid Living / Rheem RTEX-18 electric hot water heater in Montana


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spikemarie Member	# Posted: 30 Mar 2021 08:49am - Edited by: spikemarie Reply Hello! Just wondering if anyone here from Montana or another northern state has a Rheem RTEX-18 electric tankless hot water heater in their cabin (or another brand of tankless). I am building a 16 x 24 cabin. I designed it myself and made the rookie mistake of making the bathroom too small to have a regular tank water heater. My plumber talked me into a tankless heater and has already done the rough-in to install it in the loft, which is above the bathroom. My electrician installed more power specifically for this tankless heater. It sounded like a great idea to go tankless for our situation: it's a small cabin, there are only two of us, we only need to heat the shower and the kitchen and bathroom sinks and not at the same time. But now, I'm freaking out that because of the cold ground water here in Montana, I might only get lukewarm water that comes out in a trickle! Which would be cruelly ironic, because we have just insanely good water pressure coming out of our well (100 gpm). My plumber kind of hems and haws when I ask him, "So this will work, right? The water will get hot? I'll have enough water pressure to get the shampoo out of my hair?" So if anyone from MT or another northern state has tankless in their small cabin, I would greatly appreciate hearing about your experiences with tankless water heaters. Any reassurance and/or advice would be greatly appreciated! Thanks in advance!

ICC Member	# Posted: 30 Mar 2021 09:14am Reply So you determine what the inlet water temperature is. Then you look at the specs (here) and determine if at the rate of flow the shower head you choose needs to see the number of degrees of temperature rise needed. Do the math and you will see if this is capable of doing what you need.

ICC Member	# Posted: 30 Mar 2021 09:24am Reply Water pressure is not rate of flow. The 100 GPM is not the pressure, it is the how many gallons per minute can be pumped from the well. Unless it is an artesian well and then it flows itself with no pump. The pump supplies the PSI, pounds per square inch, and that is the pressure. It is the pump you choose that will provide the pressure and volume of water needed to wash your hair. The shower head can be designed to use a low flow and still supply enough water to wash with. The heater volume output of HOT water decreases as more volume is used and as the inlet water temperature falls. Water heaters do have a minimum flow rate (GPM) to turn themselves on. There is also usually a maximum and maybe a minimum pressure it can operate at. 40 PSI is pretty much all the pressure that is needed, but some folks run on less. I run 30 PSI to most fixtures in the house.

spikemarie Member	# Posted: 30 Mar 2021 09:57am Reply Thank you so much for your response! From maps I've seen online, it looks like our groundwater temp here in Montana is 42 degrees. Looking at the specs, it looks like if I can live with 1.5 gpm showers, it will be more than hot enough (42 degrees + 82 degrees temperature rise = 124). To get 2.0 gpm showers, the water temp would be 104 (42 degrees + 62 degrees temperature rise = 104). I think that would be hot enough for a shower, but I'll have to check with a thermometer and see what 104 degrees feels like in a shower. Feeling a bit more reassured, though, so thank you for the information!

ICC Member	# Posted: 30 Mar 2021 10:06am Reply Is this a part time or full time residence? If part time the heater will need winterizing and some are easier to do than others. Most warranties will not cover freeze damage. Some water heaters have a freeze preventer system but those need AC power to operate. So even if full time, is the power supply very reliable? Perhaps you should research and install a suitable low flow head at home and try it out when you also check what temperature you like? If the water temp, flow chart puts you at a just barely good place you might not be happy as those numbers are always a best case, tested in ideal conditions.

spikemarie Member	# Posted: 30 Mar 2021 10:09am Reply Thank you again! Yeah, my well guys and pump guy (well is in, but pump is not installed yet) educated me on the 100 gpm not being what would be coming out of the taps, which made sense after I pictured what 100 gpm coming out of a sink faucet would look like!!! But still, I was thinking how disappointing a trickle would be when I do have the potential to have great water pressure. I see on the last page of those specs that you linked to that there is an "Optional Inline Flow Regulator." Do you think that would be a good idea? Thanks!

spikemarie Member	# Posted: 30 Mar 2021 10:16am Reply It's a part-time cabin, and we would be winterizing it. I installed three Cadet wall heaters to keep the cabin above 40 degrees during the "iffy" months in early spring and early fall. But of course, if the electric goes out, so do those heaters. I'm thinking about getting the smartphone app that lets you know when your power goes out. The cabin is about an hour and a half away, so I could zip over there and get a fire going in the wood stove if we get a weird cold snap in May or September.

snobdds Member	# Posted: 30 Mar 2021 10:23am Reply I have a Marey 3.1 GPM instant water heater. I have an artesian well that is on the side of a nearby mountain. It provides plenty of pressure to run the heater. The water is cold coming out of the side of the mountain. It heats up fine. I talked to a friend who is a master plumber and he said don't get a high flow efficiency unit. You want a lower flow rate through the heater so the water stays in the coil longer to heat up. I was going to get a 8GPM unit, but the 3.1 has worked well. Plus it was only like 350. So I bought two and have one for a back up. So if your having problems heating up the water, get a lower flow unit and regulate your pressure down to give the water more time in the unit.

spikemarie Member	# Posted: 30 Mar 2021 11:48am Reply Thank you for your response! I'm feeling a bit more reassured from yours and others' responses to my post. My plumber already bought my tankless water heater. There was a holdup last summer with my build, so now it's past the time when I can return it, but I hope it will do the trick. I don't believe it's a high-flow efficiency unit. It's a Rheem RTEX-18 electric tankless. I think we'll be okay with it since we will only be using either the shower or the kitchen sink or the bathroom sink at any given moment, not all three or even two at once. I will repost with a review after I see it in action. Thanks again!

ICC Member	# Posted: 30 Mar 2021 10:15pm Reply snobbds, what that plumber said does not make sense to me. That is a very general statement. What really matters is how well the unit is engineered and if the user is using it in a manner compatible with the low and high design limits. We installed a Ruud 9.5 GPM high-efficiency propane on-demand water heater in the bunkhouse in the fall of 2019. It does not say high flow anywhere but I believe the rating of 9.5 GPM would qualify. It is called a high-efficiency unit; uses schedule 40 PVC for the exhaust. It does not matter if one shower or all 4 are going at the same time. Plenty of hot water in any situation and the hands have never run out of hot water with a full house. No problems when there may be only a single user. We have seen large savings in propane use compared to when we used tank-type storage water heaters.

snobdds Member	# Posted: 31 Mar 2021 11:54am - Edited by: snobdds Reply It made sense to me. The high efficiency units that have high flow rates use large tubes for the heat exchanger to transfer heat. The lower efficiency units use smaller tubes, but more of them, to transfer heat. The smaller tubes have lower flow rates than the big tubes, but longer time in the heat exchanger to account for a larger temperature differential changes from inlet and outlet temps. J Just like a radiator in a car, which is just a heat exchanger. The high flow (high efficiency) radiators use one big core and the old 3 core brass radiators are considered lower efficiency as it takes longer for the coolant to pass through a 3 core radiator. However high efficiency radiators don't really cool the coolant as well as the multi core radiators because the coolant does not stay in the radiator as long to transfer heat. There was a period when street racers switched over to high efficiency aluminum radiators to deal with the heat only to figure out they ran hotter than the old non efficiency radiators. Time in the heat exchanger turns out to be pretty important. Maybe a picture will help...can you pick the high efficiency and low efficiency hot water heaters?

ICC Member	# Posted: 31 Mar 2021 09:09pm - Edited by: ICC Reply My point of contention was the blanket condemnation of water heaters labeled as high efficiency. My experience with the Ruud water heater which is labeled as a high efficiency model has been nothing but stellar. It works down to less than 1/2 GPM and up to the top rating of 9.5 GPM. Even with a full crew there is neverending water and with just a single person on site they have hot water whenever needed. We are using it within the design criteria for flow and water temperatures. I am not sure if a comparison to auto radiators is fair. Cooling system engineering is more than having a rad with more tubes. I'm sure there is a lot of science behind both residential water heating and automotive engine cooling. Lots of crossover but probably differences too. When speaking about water heaters high-flow means how many GPM can be heated and delivered to the point of use, which will vary with the inlet water temperature and the desired outflow temperature. The Ruud I mentioned can deliver up to 9.5 GPM and the output water is still as hot as it is at 0.26 GPM. It needs a 0.4 GPM surge to start it but it can keep the water heat at a lower flow. It works. The camp has neverending hot water if it is full or if there is only a single occupant. With water heaters the high-efficiency phrase is all about how much heat is extracted from the burning fuel to make the water hot. That high-efficiency has little to do with flow. A normal residential water heater must extract 80%, have an efficiency rating of 80% minimum, according to US law. A high-efficiency water heater must be at least 90% to gain that label. The Ruud I mentioned is 96% efficient. That is how it can use plastic PVC pipe for the exhaust. Two automotive cases of my own. (1) I have a hot rodded 1960 Volvo PV544 with a modded later year 16 valve turbo engine. Originally I used a larger than stock 4 core radiator. Mostly it stayed cool. But not always. About 10 years ago I had a new radiator built by a shop in AZ. Only two cores, but wider tubes and about the same overall thickness. They explained that gave more tube to fin contact and hence better cooling. It worked. (2) My CJ6 has a small block Ford V8. It had trouble keeping cool when crawling on slow uphill slopes. An Edelbrock high flow water pump solved that issue. I forget the percent increase but for the same RPM's it pushed more water volume. That too has never overheated since. And a trail buddy solved his overheating on his CJ3A by driving the water pump with an overdrive pully ratio. The water pump spins faster and pushes more water. More tubes also means more resistance to the airflow, I was told. Lots of variables in play so I don't think it wide to make blankets statements about something being the best thing or the worst thing. That was the point of my original note.

snobdds Member	# Posted: 1 Apr 2021 10:50am - Edited by: snobdds Reply I think you're missing the point of my initial post with regards to efficiency. It's a function of flow. So let's step back to the 10k foot level shall we... Let say we have a supply of water from a tap that provides 50psi. It doesn't matter if it comes from a well, a pump, etc...the supply is 50psi. If we take a high efficiency 9 GPM water heater and a normal efficiency 3 GPM water heater and turn them on for one minute, one is going to put 9 gallons of hot water into a bucket and the other is going to put 3 gallons into a bucket. How is that possible with both inlets being 50 psi? This is the efficiency I am referring to. It's also not a mistake the main factor manufacturers make in letting the consumer know about their product is the GPM number, which is a function of flow, efficiency and time. I would also argue the efficiency of heat transfer to the heat exchanger is the same on both units, after all, both units use similar heat exchangers, copper tubes . The difference comes in BTU and design of the heat exchanger. So this brings us to the 2 variables in the problem, BTU and design of the heat exchanger, that gives us different GPM coming out of the heater and into the bucket at a hot temperature. If we acknowledge that BTU's is always going to be variable, but the the heat exchanger design is fixed...well it leads us to the design of the heat exchanger being where the efficiency focus should lie. If you look above at the heat exchangers, the high efficiency heater heats all the water in one heat exchanger with very little flow restrictions. The normal heater has water travel up and down through 3 heat exchangers...this is restriction to flow. The water is being kept in the heat exchanger longer and therefore can bring colder input water up to temperature more efficient than having a larger unit burn more BTU's to get the same temp rise. So yeah, I bet your unit is working and operating just fine. However your burning a lot of BTU's to get high flow through an efficiency heater.

snobdds Member	# Posted: 1 Apr 2021 11:07am Reply Quoting: ICC Lots of variables in play so I don't think it wide to make blankets statements about something being the best thing or the worst thing. That was the point of my original note. Never once did I make a statement that something was the best or worst. I could care less. I'm more interested on the physics of the matter.

ICC Member	# Posted: 1 Apr 2021 12:38pm Reply Quoting: snobdds If we take a high efficiency 9 GPM water heater and a normal efficiency 3 GPM water heater and turn them on for one minute, one is going to put 9 gallons of hot water into a bucket and the other is going to put 3 gallons into a bucket. How is that possible with both inlets being 50 psi? Well, I'll stop right there. We cannot accurately compare two units when the GPM rating and the efficiency rating are different for the two units. And the pressure is immaterial as long as both heaters are designed to work with the pressure.

snobdds Member	# Posted: 1 Apr 2021 12:50pm - Edited by: snobdds Reply Sure we can...we just have to logically see what variables are constants and what ones are not like I did. Break it down to a binary mathematical equation. If the inputs are the same, but the outputs are different...rationally figure out how that is possible. I did.

gcrank1 Member	# Posted: 1 Apr 2021 01:06pm Reply Doesnt what really matter is how far you turn the tap on? With ours when it was getting pretty cool out we just throttled the flow back which kept the water in the unit longer to up the temp a bit.

pabear89 Member	# Posted: 1 Apr 2021 03:25pm Reply So both units have 50psi at the inlet connection. The 9gpm has a larger tubing which allows more volume to pass thru the heater than the 3gpm unit. The pressure will be the same on both but the volume differs. The question should be how much temp increase does one unit provide over the other? If the water temp going in is 50* entering what is the temp as it exits the faucet at full flow and what will it remain at is what I would need to know before choosing which one to install. Then it would come down to the amount of energy each takes to maintain usage. My 2cts

ICC Member	# Posted: 1 Apr 2021 05:03pm - Edited by: ICC Reply Yes! More hot water volume uses more energy to heat that specified volume. That is pretty basic. A high efficiency water heater will be able to raise the water temperature with less fuel than a standard water heater. The same thing applies to a space heating furnace. A high efficiency furnace or boiler can heat the space to the same level as an old unit, but use less energy. Any water heater or space heater will be trouble if not sized correctly to the needs.

ICC Member	# Posted: 1 Apr 2021 05:04pm Reply BYW, those huge images mess up reading this topic on small screen devices.

snobdds Member	# Posted: 1 Apr 2021 05:28pm - Edited by: snobdds Reply Quoting: ICC More hot water volume uses more energy to heat that specified volume. That is a "doh". A high efficiency water heater will be able to raise the water temperaure with less fuel than a standard water heater. Same thing applies to a space heating furnace. A high efficiency furnace or boiler can heat the space to the same level as an old unit, but use less fuel, less energy. Any of the above will be trouble if not sized correctly to the needs. Let's look at it from another point of view shall we. If we have 30 gallons of 50* temperature water and we want to bring that to 125* . We have two given ways to do this. One way is a burner under the 30 gallon pail. The other way is 3 10 gallon pails with a burner under each. Well the 30 gallon pail will need a big burner to heat all 30 gallons and a lot of BTU's in a given time frame. The smaller 3 gallon pails need smaller burners to bring that same water up to temp with a lot less BTU. Now what if you connect those smaller 3 gallon pails to flow the hot water into the next pail etc. Each pail acts like a preheater so that one burner is not doing all the work. A forced flow restriction. The 3 smaller burners working in series is always going to be more efficient at heating than one large unit, but at the sacrifice of high flow. The concept of high efficiency in a one large heat exchanger is on par with a traditional hot water tank.

ICC Member	# Posted: 1 Apr 2021 07:37pm Reply Science indicates otherwise. Here in the US we use BTU's as a measurement of thermal energy. A British Thermal Unit (BTU) is a measurement of heat energy. One BTU is the amount of heat energy required to raise one pound of water by 1ÂºF. That's it. More BTU's input means a faster rise in a fixed quantity of water or the ability to heat a larger quantity of water to a higher temperature.

ICC Member	# Posted: 1 Apr 2021 07:39pm Reply ....interesting page of calculators....

snobdds Member	# Posted: 2 Apr 2021 10:17am Reply Quoting: ICC Science indicates otherwise. Here in the US we use BTU's as a measurement of thermal energy. A British Thermal Unit (BTU) is a measurement of heat energy. One BTU is the amount of heat energy required to raise one pound of water by 1ÂºF. That's it. More BTU's input means a faster rise in a fixed quantity of water or the ability to heat a larger quantity of water to a higher temperature. I'll try and do a better job of explaining it next time.

Alpine_Junky Member	# Posted: 7 Apr 2021 05:02pm Reply I live in NW Montana, off-grid, and just went though this exercise. I opted to go with a H20 heater that has a tank for the very reasons you cite (+lack of electricity needed to run) but have several other properties that I have experience with on tankless. The unit will be ok with a 1.5gpm shower head in all but the most extreme cold times when operating efficiently. When the unit gets a little build up, you will notice temps start to drop which will drive you to descale. If you are using a pressure tank that is in a heated area, you can always cycle the tank a bit before needing to take the shower so the water can warm up. IE: manually trip your pressure switch. Or if you have a bath tub, nurse it along with a pot from the stove. You should be able to get most of your money's worth from the unit with a little extra fussing.

spikemarie Member	# Posted: 7 Apr 2021 05:18pm Reply Thank you so much for sharing your experience, Alpine Junky! It helps a lot to hear from someone who is also in Montana. If we were living in the cabin full-time, I would've found a way to go with a tank water heater. The reason we went tankless was because I designed the bathroom too small to do a tanked heater (according to my plumber). The tankless heater and the pressure tank will be installed in the loft right above the bathroom. We're going to add a point-of-use cartridge-type water conditioner to help avoid the buildup you mentioned. And we're going to winterize the cabin and not use the water system at all from about October 1 through May 1, so hopefully, hopefully the water will be hot enough. I did already consider heating water on the stove to help out with filling the kitchen sink with hot water for dishes, if need be. I'm glad to hear someone suggest that as a possibility so that I know it's not a crazy idea to have to do that! Thanks again for your response!

Aklogcabin Member	# Posted: 8 Apr 2021 12:21pm Reply Your water heater should be sized for your needs for best efficiency. If just a couple people you wouldn't want a heater that can heat high volumes. I suspect that your plumber n sparky know their stuff. And their looking at it.

spikemarie Member	# Posted: 8 Apr 2021 12:43pm Reply Thanks for your response, Aklogcabin! Yeah, it's going to be the two of us most of the time, with a couple more people on occasion. I'm feeling better and better about going tankless now. I do like the idea of not keeping a whole tank of water hot all the time when we're only going to be spending a couple of weekends each month at the cabin.