Energy Inefficiency of Water Heater with Recirculating Pump
So I found several related articles in the archive, but I haven’t been able to fully understand how it applies to me. But they were very useful in helping me to ask the right question.
My house currently has a gas tank water heater using a grundfos recirculating pump. The pump is on a timer, but the timer doesn’t really do anything, as the water is still recirculating 24/7. I assume this is a constant thermosiphon (if my terminology from the other articles is correct).
I estimate the annual cost of running the water heater with the recirculation at ~$425, which is pretty ridiculous. About half of this is due to heat loss from the recirculator.
I’m looking at energy efficiency improvements to my house in general, and I had a contractor recommend a Rinnai tankless heater with a built in demand based recirculating pump. I see this as a solution, but an expensive one.
Three questions:
1. Are there simple ways to solve this thermosiphon without a new water heater? If so, would a new water heater even be worth it?
2. Are there other solutions for this situation I’m not thinking of?
3. I have a general desire to shift from gas to electric for environmental reasons, but I wouldn’t spend huge dollars to make the switch. I understand that hybrid water heaters don’t work well with recirculation. Are there reasonable electric options out there?
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Replies
I have a recirculating system in my house which I stopped using and have save significantly on my electric. I use more water, but not much. I heat my water with oil.
You can heat your water with gas or someone else can burn gas to make electricity to heat your water. There is no significant difference to the environment. If the rates make sense, consider a heat pump water heater.
Tie your recirculator into the light switches at locations hot water is needed. ie - when the bath light goes on, recirc pump goes on for x minutes.
This is far easier for a homeowner to do than it ever used to be. You can do it with smarthome stuff - ideally you would install zwave switches at the baths, and put the recirc pump on a zwave outlet, get a good hub like the Hubitat to write tight commands and you are done. Each switch is about $30-$40, and a Hubitat hub is about $130, so if you have two baths and a kitchen that need hot water, you can do this for about $250. If you want to include the kitchen sink, but don't want your recirc pump to go on every time your kitchen light goes on, you can get a dumb zwave "trigger" switch for the kitchen, and tap it prior to wanting hot water.
That's a good idea. I think I have a slight preference for something with the on-demand capability built in, but that's exactly what I'm trying to accomplish from a functionality standpoint.
I found this one https://chilipeppersales.com/. I'll keep looking for ones from more better known manufacturers.
I also need to figure out how to solve the problem of water automatically circulating when the pump isn't on. I would assume there's some type of valve to prevent that.
I don't think you need a new pump. Just new light switches, and a zwave receiver at the pump that controls pump power.
Doesn't the pump have an internal valve so that when it is not on, the water does not recirc? Just a guess on my part, I don't really know.
The pump doesn't have any type of internal valve to stop the recirc. This is over 50% of my energy loss problem. I think this should be an easy fix for a professional based on the other comments here.
You're right that I don't need a new pump. I currently have a lot of my house wired up with Kasa smart plugs, including the recirculator. I'll see what I can find that's compatible with these plugs, as I'd hope to not add a new technology.
What temperature is the water heater…can you turn it down?
Are the pipes insulated? Can you insulate the pipes, do you have access to the pipes? Heat loss is heat loss, gas or electric is immaterial. Water pipes should always be insulated. Have you looked at crosstalk between the heating and cooling pipes? The pipes should not be touching, there should be an air gap or insulation between cold and hot. Otherwise you will get heat exchange between them.
Instead of thinking at an appliance level, look at the overall heating/cooling/DHW system. A heat pump can provide all three.
The water heater temperature is in the mid-low range. I don't know the exact temperature, but I'd guess it's about 120 degrees. It's at the lowest temperature we can use and still get comfortable showers.
I've insulated the pipes I have access to, which is our utility room, and a small crawlspace near the washing machine. Unfortunately, this is only a small part of the pipes in the house. I would need to take down some walls to get at the rest of them.
I know there is crosstalk between the pipes in the walls, as there are several faucets where the cold water will briefly turn warm when turned on. I'm not to the point of taking down walls to get at it though.
A heat pump water heater is the most efficient option and it allows you a path to get off fossil fuels. If you replace your water heater, that's the way to go. You can get ones that only need a 15 A, 240 V circuit to avoid challenges of accommodating them within your available electrical capacity if that's an issue.
You've been given lots of options for how to stop the constant recirculation, and once you do that, there's no problem using a heat pump with on-demand recirculation.
To stop the thermal siphoning, you can use anti-siphon valves, which are built into many water heaters these days, or you can use S-shaped traps on the connections to the water heater.
Good to hear! I'll use this to talk to a contractor about the installation. It sounds like the anti-siphon piece should be easy for someone who knows what they're doing.
I think a big part of the problem with my conversation with the last contractor was not knowing the correct terminology to use, and not understanding how a re-circulation system can work. I generally knew what was going on, but I wasn't able to describe it in terms that were helpful to the conversation.
All of the advice here will help immensely in my next conversation.
I just hope there's capacity on sub-panel that's 5 feet away instead of needing to go to the main-panel that's half of a finished house away. That might be a stretch for the budget.
I've found the anti thermosyphon valves on most tanks to not seal well enough to stop thermosiphon especially if you have a more than one story of height. I've also opened up water heaters where the little valve flapper in the valve had gotten stuck fully open.
The best bet is to install a low operating pressure spring check valve. Some pumps come with this built in or it can be installed into the pump body, if not an in-line one works just as well.
As for controls with a system that is already installed, the simplest is one of the adaptive on-demand controls. You can look at replacing your current pump with a Grundfos UP10-16APMBU/LC (this has a check valve built in as well) or your current pump controlled by a Taco Smartplug.
All you need to stop the thermosiphon is a spring-type check valve. The thermosiphon action is VERY low pressure and won't be able to open a spring-type check valve. That's why people who are intentionally putting in thermosiphon recirculation systems have to use swing-type check valves instead.
You could also use a solenoid valve tied into the power source that runs the recirculation pump. The solenoid valve would be closed when the pump was off. That would also stop the thermosiphon, but the spring check valve will be cheaper, and is also very unlikely to ever fail on you. Either way, you don't need a new water heater to deal with this problem.
If you aren't going to use a heat pump hot water heater, and your goal is to be more green, then you do NOT want to shift away from a gas fired water heater. Heat pump water heaters are able to move heat from the room to the water, so they "make" (by moving) more BTUs than the input energy they use. Note that during the heating season, some of these BTUs are essentially robbed from the home, so they're ultimately coming from whatever heat source you use to heat your home. During the cooling season, this helps you -- the heat pump on the hot water heater is essentially cooling your home a little. During the heating season, it hurts you.
Your other electric option is electric resistance. That's not a more green option compared to natural gas in most regions in the US. The reason is that the primary sources of electricity in the US are natural gas and coal fired generation. Some areas have a bit more hydroelectric and/or nuclear power in the mix, but in all regions wind/solar actually contribute only a pretty small portion of the total capacity of the system. If you use an electric resistance water heater in an area where the grid is primarily sourced from natural gas (let's just pretend that's where you are, since it makes the example simpler), then you are heating your water with heat from electricity that was made from burning natural gas. The ultimate amount of natural gas you use will then be MORE than if you burned the natural gas in your water heater directly. The reason for this is that the conversion efficiency of the power plant is probably not as high as a good water heater (60% is usually considered "really good" for a power plant), and you also have losses in all the power lines and other wiring between you and the power plant. The end result is you consume less natural gas burning natural gas directly than you do if you use electricity that was made by burning natural gas. One of the pesky rules of physics is that every time you convert energy from one form to another, you lose some of it.
Basically if you really want to be green, you have to consider the entire system and not just any one isolated part of it. Sometimes the most green option isn't what it first appears if your end goal is to minimize your consumption of any one particular resource.
Bill