Understood. I am still in the “due diligence period” so I wanted to make sure this wasn’t a major issue I was going to have to deal with once I moved in. I will live in the house for a few weeks before I do anything to see if the problem is as bad as I think. Maybe I’ll just get used to it taking a while to get hot water. Having never had a tankless water heater before this is all new to me. Live and learn.
From an energy point of view I question this statement.
I meant that there won’t be a short cycle of the heater as soon as the water starts running. Obviously the heat still has to come from somewhere.
IF the house is only one year old, it was likely plumbed with pex or pvc, not copper. Neither is particularly difficult to work with and if you have access to the lines it should be supremely easy to adjust the pipe routing such that you get hot water at the kitchen faucet more quickly.
A couple things to note: Almost all faucets these days are flow restricted to 1.2GPM or lower in order to save water and energy. However when my kitchen was remodeled and the faucet was installed, some debris made its way into the piping and was making the flow even worse. Test how long it takes to fill a gallon jug, if it’s longer than a minute, it could simply be a flow restriction. I had to replace the faucet in order to resolve the issue.
According to my liquid volume calculator, 100 feet of 1/2 in ID pipe only holds 1 gallon of water…and 100 feet is a good bit, unless your house is pretty big, so if you are low-flow faucet (1GPM) and you have a really long run of 1/2 inch pipe. it could take a couple minutes to deliver the hot water, but this is kind of an unlikely case.
The homowner in me would want to fix the actual problem unless it’s truly difficult to fix (ie pipes in finished walls, no access from basement or attic, etc) as the booster heater is more of a band-aid solution.
Good luck to you!
A mini-tank will not increase energy usage by very much. Possibly not at all depending on the tankless water heater. I’m familiar with idea of a tankless water heater used in conjunction with a furnace of boiler, I don’t know what @dolphinboy has. If it is part of a heat system you’ll use less energy in warm weather because your heating system won’t have to turn on.
If you get the mini-tank you’ll be happy with the volume of hot water available. However, before doing that, if you have the space for it consider getting a larger hot water heater. They are very economical and provide plenty of hot water throughout the house. You don’t need a heating system to come on in the summer time, and when the heating system is on in the winter the water is preheated and the hot water heater will use very little energy. If you can’t do that, get the mini-tank. You might even end up routing the hot water for the bathroom through that once it’s in.
It’s true that a booster hooked up to the hot water line will cause both heaters to run (at some point). However, there’s some scenarios that I think would be interesting to explore.
There are basically three temperatures of water in the pipes.
- Cold water coming in from the street, let’s say 50 degrees F
- Cool water that’s been sitting stagnant in the pipes and has equalized to room temperature, whether in hot water pipes or cold water pipes, let’s say 70 degrees F
- Hot water from the water heater, let’s say 120 degrees F
Water heaters are usually near the main water service so they mostly get fed cold water. Also, I’m assuming an instantaneous water heater at the sink rather than a small tank just for illustrative purposes, you’ll see why later.
Without a booster, and with a long run of piping to the main water heater, if you must have hot water to wash your hands, then you have to flush out all the cool water from the pipes before starting your hand washing. Let’s say a normal hand washing takes 1/4 gallon, but it requires flushing 3/4 gallons out of the pipes before it gets hot. So one gallon of cold water is entering the water heater and needs to be brought up to temperature. That’s 584 BTU.
If you have a booster at the sink and just wash your hands, the booster will fire up immediately. The main hot water tank in the basement will also take in 1/4 gallon of cold water from the street. The main tank will have to run sooner to bring its temperature back up to setpoint, so there is some double heating going on. You’re heating 1/4 gallon from cool to hot (104 BTU) and another 1/4 gallon from cold to hot (146 BTU). That’s a total of 250 BTU, so you use a quarter as much water, and a bit less than half as much energy.
Without a booster, if you’re doing dishes that require 20 gallons of hot water, and you flush that first 3/4 gallon down the drain, you’re heating 20.75 gallons of cold water which takes 12,119 BTU.
With a booster, it will cut off after hot water from the main tank reaches it, so of the 20 gallons of hot water, 3/4 gallon is double heated. That works out to 11,681 BTU for the main tank and an additional 313 BTU for the booster for a total of 11,994 BTU. So that’s a small win for the booster by 125 BTU.
If you only have an instantaneous water heater at the sink supplied by a cold water line, then it can heat 3/4 gallon of cool water before the cold water from the street hits it and it needs to increase heat output to compensate. That’s 313 BTU for the first 3/4 gallon and 11,243 BTU for the remaining 19.25 gallons for a total of 11,556 BTU, which is another 125 BTU lower than the main tank/booster scenario. Nevertheless, a 1.5 gallon-per-minute faucet (which is low for a kitchen sink) would still require a 240-volt 45-amp heater to provide the 11kW necessary for cool inlet water, whereas you need 65-amp 15.4kW for cold inlet water. Ouch. The total BTU’s don’t change, but those are some serious electrical circuits.
All that said, the booster (as a small tank rather than an instantaneous heater) is a winner in some scenarios, and a slight loser in others, but I think it’s the best solution under the circumstances.
We do not have the mini tank heater for energy savings. We have it for water savings (important here in the desert) and convenience. Our water from the street is never very cold (Mrs. Martian has to use ice cubes to get her film developer down to 68F), but our hot water pipes run through the slab so cool off pretty quickly. It takes quite a bit of water to get hot enough water at the tap.
We have a long run from our water heater to the kitchen as well. The worst case scenario is that you just need a bit of hot water - Maybe a cup for tea, or forma quick wash of a dish or something. You pour maybe half a gallon of cold water down the sink to get your cup of hot water, then all that hot water in the pipes cools back down again and you have to do it all over again.
If you are filling a bathtub or washing dishes once a day, the extra loss isn’t that great, but when using small amounts of hot water on a regular basis, it can make a big difference. Not just in energy, but now a cup of hot water causes you to lose 1/2 gallon extra.
I would assume a tankless under-sink heater would be connected to the cold line, not the hot line. Is that generally the case?
I was wondering the same thing. I assumed I would connect my hot water line to it so that if I need a lot of hot water I would pulling it mostly from my large tankless water heater rather than the small one under the sink, which may not be able to keep up after a while.
I figured it was connected to the cold line because generally you aren’t supposed to drink from the hot water line as hot water dissolves more contaminants from the lines and water tank. It’s not a huge deal, but enough that I wonder if industry practice is to connect consumable water devices to the cold line.
A related problem we have is that the plumbers routed many of our lines by pulling them alongside duct work. So in the winter our cold water is warm for a while, and in the summer the hot water is cold. So we have to run a lot of water in winter just to get a cold drink. Our solution is a pitcher of cold water in the fridge.
I fill empty gallon milk jugs under my kitchen sink when it’s heating up and then use those to fill the toilet tank after flushing. I hate waste.
Another good idea, but shouldn’t there be a way to address the problem that doesn’t involve collecting water and then doing something with it?
Yes there is. An expensive and energy wasting recirculation pump will provide you with instant hit water.
Or the cheap undersink hot water heater will do it as well.
In new construction you can add a drain loop so that when you use water it goes through a heat exchanger which warms the water coming into the house. So instead of heating water from 50 F, you heat it from maybe 70 F. This is very hard to install in existing housing, however.
I tried to quantify the loss when running the hot water tap until it’s hot. In our house it took almost exactly 2L of water before it came out hot. We pay about $2.22 per cubic meter for water, and 2L is .002 cubic meters. So the cost per use of the waste in water is about 0.44 cents. We also pay the same for water waste removal, so double it. Maybe .9 cents.
In terms of energy, heating 2L of water from 50 to 120 degrees is on the order of .1 kWh, So on average maybe 1.2 cents?
Overall, we are looking at roughly 2 cents in lost water and energy every time we run the sink until hot water flows in my house. Do that 5 times per day, and it’s maybe $3/mo. Breakeven for an under-sink heater that costs $500 installed is close to 15 years. Do it for the convenience, not to save money.
Thanks for all the advice and guidance. I am going to move in and see how much it bothers me not having quick hot water in the kitchen. If after a few weeks I decide it’s really a problem, I’ll probably go for the under-sink hot water heater solution since I can install it myself. The only question is do I have only cold water coming into to it, or should I hook up the hot water line instead?
I strongly recommend the hot water line based on my experience. 2.5 gallons is not much and you don’t want to run out of hot water at the kitchen sink.
Thanks… that was my thinking.
Doing some reading, it seems like the hot line is the way to go for a setup that provides temporary hot water until the hot water from the heater arrives. Hot line to under-sink heater, hot line from that to the faucet, and you’re good to go.
Make sure you have under-sink power, and if it’s a garburator plug make sure that both outlets aren’t switched or you’ll have to run the garburator to heat your water. (-:
Thanks for the tip. I wouldn’t have thought to check for that.
When I remodeled the house I was living in I put in a 2 pipe hot water system with a pump. Hot water flowed to the kitchen downstairs 1/2 bath and then back to the garage and hot water tank.
The house we moved into has a long way to the bathrooms and kitchen different sides of the house. In the master bathroom under the sink I put a hot to cold by pass witha pump at the hot water tank. HOt water flows to the bathroom sink and returns to water heater. When hot water gets to the sink the bypass is thermal and will shut off.
In my kitchen I installed a 2.5 gallon water heater. If I use enough water to the point that the water going into the tank is hot the water heater will shut off if the water temp is above seat point.
If you have lived in a house where you do not have to run water and wait for hot water, you will hate being in a house that you have to run water before you can do anything.