Replace DHW Storage Tank with HeatPump Electric Water Heater

rheckbert3

I currently have a new Weil McLain SGO4 oil fired boiler with a tankless water heater coil insert and 40 gallon storage tank. I replaced the boiler myself with a new burner, tankless coil, and modified the plumbing as needed in September of 2020. In the year prior after buying the house I replaced the leaking wet return buried beneath basement. See post linked for more information.

https://forum.heatinghelp.com/discussion/177943/just-bought-house-leaking-wet-return

With the skyrocketing cost of home heating fuel seemingly overnight I am thinking about replacing my 40 gallon glass lined storage tank with a heatpump type electric water heater. I wanted to get your input on if this is a wise decision and if you have any suggests if I were to do so.

The storage tank has a 120v aquastat set for 145° that drives a taco circulator pump while the boiler has an aquastat set for 160° to fire the boiler. The boiler is also controlled by a Nest Thermostat for the steam house heat with a probe type LWCO, pressuretrol and automatic feed. 

With the cost of oil suddenly at 5.30$/gallon and rising I was thinking of installing a heat pump type water heater. A Rheem 40 gallon heat pump electric water heater would cost around between 1600 and 1900$ minus a 750$ rebate from eversource. Bringing the cost down to 900-1200$ after tax. 

I was thinking that there could be two options for doing this.

1. Plumb this in series with the tankless DHW coil insert replacing the 40 gallon storage tank. I could also add a tankless coil bypass with ball valves to bypass the coil in the winter when the boiler is running  

2. Plumb the heatpump water heater in parallel with the storage tank with ball valves for isolation to use one or the other.

Would there be any advantages or disadvantages for doing one or the other? Currently my cold water feed comes in and tees into the boiler tankless coil. The water then runs through the coil in the boiler and up to the top of the tank with a circulator pump. The bottom of the storage tank then tees into the cold water supply and back into cold side of the coil with a check valve in the middle. The DHW for the house then comes off the storage tank into a taco thermostatic mixing valve and out to the house. 

Could I replace the storage tank with the heat pump water heater? The cold water would feed through the boiler coil and into the cold side of the heat pump water heater and then out of the hot side to the thermostatic mixing valve and into the whole house? This way in the summer the boiler could be shut off with the water just running through the coil. And in the winter the coil will preheat the water before the water enters the heatpump water heater. Or I could plumb the water with ball valves to bypass the coil and not pull BTUs out of the boiler for the house heat  

Or should I abandon the tankless hot water coil all together and just plumb the heatpump hot water completely separate from the boiler for DHW production?

What else could you suggest? Just trying to get the best setup and ideas. These kind of projects are really fun and interesting to me. 

I have plenty of ampacity and space for a 240v breaker in the electrical panel. So this is not an issue. 

Thank you!

Jamie Hall

Step 1 is to look at your electric power rate, and compare the actual energy cost -- per gallon of hot water -- of the two approaches to make sure you'd save money. This is a little tricky with heat pump water heaters, as most of them have a habit of switching to resistance heat if the demand is greater than a certain amount.

OK. Having said that, the next thing to do is to find out how the heat pump water heater would deal with already hot water. That's not obvious, and you may have difficulty in actually finding out.

Then... if things still look good, evaluate just how much water you need how fast. As I noted above, heat pump hot water heaters are a good choice -- if you stay within the limits of what they will recover without going into resistance heating.

rheckbert3

So my electric rate delivered to my house is about 25¢/kWh. The Rheem 40gallon heatpump water heater has a 60gallon first hour delivery and a 27gph recovery rate thereafter for a 90° rise in temperature. Like you stated I think this 27gph recovery rate is with the 4500w resistive heater and the heat pump in full swing. 

I have a 2 bath house but currently live by my self. I usually take 1 shower per day, dishwasher 2 times per week and laundry once per week. The second bathroom is used in the summer as this addition is electric heat and gets shut down in the winter. So that would add another shower per day maximum. 

I think the rheem has 5 mode settings. Electric, heatpump, high demand, vacation and. I’m wondering if the the water heater were to be set to heatpump if this would prevent the electric resistive backup heater from activating. I think with my water usage this would be fine and provide ample time for the water heater to reheat the water. As Jamie wrote it seems difficult to find at which point the water heater would switch over to resorbier heating verse heatpump. 

Oil is about 5$/gallon right now and provides about 138,500btu of thermal energy but of course not all of this is converted to heating the water. I think the Weil McLain SGO4 is about 83% efficient meaning each gallon provides only about 115k btu at most. (Not Accounting for other losses). In the summer I use about 0.8gal per day meaning I need 92k of btu to heat the water. Or about 27,000 watts of power. This would cost me about 4$ per day to heat the water with oil.

With complete restive heating at 0.25 cents per kwh at 27kw. This would cost 6.75$ per day. 

Like Jamie said above I’m not really sure how to calculate the cost required to run the heatpump and at which point the water heater would switchover to restive heating back up. 

I guess my other questions are should I plumb the cold side of the heat pump water heater from the outlet of the tankless coil? I could also add a bypass to the tankless coil with a few ball valves. Or just abandon the tankless coil all together and just heat with the heat pump water heater and use the boiler for house heat. 

hot_rod

I have an 80 gallon and it does have the HP only mode. You should be able to find the manual online for the models you are considering.  If the slow recovery is not an issue for your use patterns and you have a space that meets the requirements, give it a try

ethicalpaul

Get the 50, you might have a partner some day. Put it on heat pump only and you’re golden. Separate from your boiler for simplicity. Have you or a friend install it. It’s so simple a computer programmer can install one. Turn off your boiler in the summer, imagine that!

25 cents per day, that’s what mine costs.

EdTheHeaterMan

You are not the first one here with this question.

Don't make a decision based on one month of price spike. the Electric and LP and Natural Gas prices will follow in short order. History tells us so. There are no free rides when it comes to energy consumption.

This is how I answered that at least 3 times already. If you are going to get a new water heater don't throw the perfectly good, old one away. Do the smart thing if you have room. Pipe both tanks so you can use either one as the primary
https://forum.heatinghelp.com/discussion/comment/1693154#Comment_1693154

Larry Weingarten

Hi, I think I can throw a few comments in. HPs like to be fed with cold water as it helps keep them working efficiently. I would not preheat water to the HP. Recent research indicate you should get a COP of roughly three. This assumes the HP is not drawing on "very cold" (subjective, I know) air and the air filter is kept clean to allow good airflow. Also make certain that outlet air from the unit cannot get back to the inlet side, as it will hurt performance. Install a bigger tank than you think you need, as it will help to boost the COP and insure you don't ever get a cold shower. And, there have been problems getting HPs to stay out of resistance mode. For whatever model you get, talk with the manufacturer and installer beforehand to nail down that the unit can be run strictly in HP mode. Lastly, it would be good to monitor the energy input to the heater so you know what it's actually doing.

Yours, Larry

JakeCK

Larry Weingarten said:

Hi, I think I can throw a few comments in. HPs like to be fed with cold water as it helps keep them working efficiently. I would not preheat water to the HP. Recent research indicate you should get a COP of roughly three. This assumes the HP is not drawing on "very cold" (subjective, I know) air and the air filter is kept clean to allow good airflow. Also make certain that outlet air from the unit cannot get back to the inlet side, as it will hurt performance. Install a bigger tank than you think you need, as it will help to boost the COP and insure you don't ever get a cold shower. And, there have been problems getting HPs to stay out of resistance mode. For whatever model you get, talk with the manufacturer and installer beforehand to nail down that the unit can be run strictly in HP mode. Lastly, it would be good to monitor the energy input to the heater so you know what it's actually doing. Yours, Larry

The issue of preheated water affecting the cop can be mitigated by switching entirely over to using the boiler during colder months right? Even if that preheated water is going to the tank it would just be a storage tank at that point. I can't imagine it being fed 120-140f water would actually damage anything if it's actually turned off would it?

Jamie Hall

@Larry Weingarten has an excellent analysis up there. The only thing I might add is due your math. With a COP of three, at the oil and gas and electricity prices current where I am a heat pump is only just barely cheaper to run than an oil fired hot water heater --and more expensive than gas.

mrhemi

A wise old friend of mine from my racing days rebuffed my comment about preferring manual transmissions to automatic transmissions. He replied:

"There's nothing wrong with auto transmissions, just the people who work on them".

I have found the same to be true of heat pump systems.

Larry Weingarten

The issue of preheated water affecting the cop can be mitigated by switching entirely over to using the boiler during colder months right? Even if that preheated water is going to the tank it would just be a storage tank at that point. I can't imagine it being fed 120-140f water would actually damage anything if it's actually turned off would it?

Hi @JakeCK ,I think you have it right. As long as it's not trying to do any heating, feeding hot water into it is not a problem, and I'm not aware of any damage that could be done to a HP when it's off, by feeding it hot water, so it's used as just a buffer tank. (I don't think feeding it hot water will hurt it when it's on, just kill the COP.) To add a bit to what @hot_rod said, having a big tank, heated slowly by the HP promotes stratification in the tank, which helps maintain the high COP. The slow recovery plays nicely with a big tank, both for the user and the grid.

Yours, Larry

TAG

With your current usage a HPWH will have no problems keeping up in heat pump only. We were with friends for a week last month and they have a 50g Rheem and it only went into resistance a few times over the week with the four of us .... he told me they typically never have it go to resistance w/o guests. He is a bit of a nerd like me a checks. That's an all electric house in a warm climate. The unit makes some noise ... be aware of that

How many times does the current tank have to load in a day? How long does the boiler have to fire to finish a cycle in the summer ... figure out the oil use per cycle.

When I did my first Buderus boiler/ indirect 30 years ago the controller had a neat little timing feature built into the Eccomatic boiler controller. The default setting would stop the tank from reheating at 8pm. At 5am the next morning the timer allowed it to fire again. The weekend default was a bit later on both settings. At first I was a bit worried about this .... but the only time I had an issue was with a houseful of people a couple years later when I had forgotten all about the timer and we had a lot of late night showers -- a simple button on the controller overrides the timer.

When I would get up and take my shower at 6am ... most mornings the tank had just fired. So in the summer the boiler was nice and hot for the tank to reheat when others showered just after. After the morning use the system shut down for the day ... depending on what was used the next evening it may or may not reheat again.

Years later when I bought a weekend house with oil hot water I put in a simple timer to keep the unit from heating the tank at the same times ... since that tank was not as well insulated as my Buderus indirect it could not go the whole day .... but, it kept the tank from firing at least once at night and in the winter it was twice as the basement was colder.

it's a neat little trick and in the summer when the boiler has to heat from cold keeps the cycles down