Booster after hybrid water heater
jabloz
| Posted in General Questions on
I’m planning to install heat pump water heater (set it in heat pump only)and
Tankless electric water heater as a booster
Is it good idea?
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No. If the HPWH is properly sized, you shouldn't need a booster. The booster is going to be expensive and it will require a BIG electric supply. That's a lot of work for something that should never be really needed. There are very few conditions where a tankless electric is a good idea. If you have highly variable loads (like lots of company but not all the time), a better approach would probably to use a tank-style electric or gas WH in front of the HPWH. Leave it turned off except for when you need it. In fact, you could prep for it with the needed space and utilities and not even install one until, or if, you actually need it. I suppose one appropriate use would be if you need very high temperature water for a specific use, like for an industrial dishwasher. In that case, the HPWH could provide the bulk of the water heating demand, with the booster only being used for the specific application that needs it. You would then be able to use a smaller booster. Still a lot of effort for a single use.
Finally, understand that the HPWH already has an electric booster built-in and you are bypassing it by turning it off and running in heat pump only mode. You've already paid for that one, why not use it when loads are high?
Roughly speaking, the best efficiency would be a large enough tank that you can be happy with leaving it in heat-pump-only mode. Your setup would be second best. And allowing the heat pump water heater to use resistive heating when needed would be the lowest efficiency option among the three.
But refining that story a bit, if you make the tank so huge that you always have plenty of hot water available, you will have a little more standby loss than with a smaller tank, and there might be a situation where a moderate size tank with the booster after, with the booster rarely kicking in, that is more efficient than the giant tank option.
Efficient, possibly. Cost efficient almost never. It is unlikely that you will ever recover the additional cost of the booster, plumbing and giant electric service in terms of energy savings. Except possibly for standby losses, I'm not sure why you think the electric resistance heat in the HPWH is less efficient than the electric resistance heat in the booster.
Thank you Pete_Engle
I'm planing to that setup in summer house.
I might be renting it.
There might be periods of time (5 days a week)
I won't need hot water There is no need to keep it hot and I don't want to wait (hours) when needed
Also if there will be 6 people taking shower 1 after another. I would need huge tank
The HP H20 Water Heaters have a hybrid mode that will switch back and forth between heat pump and pure electric mode in cases where there is a spike in demand. Also, keep in mind that the HP H20 heaters dehumidify the air around them, especially when they are working a lot in heat pump mode, so in a summer house this could add to the benefit.
In a summer house you might also want to look at some of the other options available such as solar assisted systems. There are quite a few. A new type of solar assist by a UK company named SAHP looks interesting. I also recently stumbled upon something called the "Hot Spot" which takes heat from the return pipe on your A/C unit and runs it through a heat exchanger to assist the H20 heater (supposedly this provides free heat for water and better a/c efficiency)....I have not installed one of these but it looks interesting on paper.
In summer, people generally take cooler showers. Often shorter too. And, the ground water temperature is higher in summer too. And, with low-flow shower heads, you use less water anyhow. Consider water entering the house at 60F, and the water heater maintaining 120F. Mix the two 50-50 to get 90F water. Probably a bit cool for most people, but close enough for our purposes. With a 50-50 mix, you're using .75 gpm of hot water in a 1.5 gpm shower head. 40 gallons will last an hour, not considering any recovery at all. That's enough for 6 people to take 10 minute showers. Not bad.
One of the few good applications for tankless water heaters is in applications where there is no water use for days at a time and the standby losses add up. If using a tankless, it's best to have it as close as possible to the fixtures. A small tankless mounted in a closet between two bathrooms would provide the best performance. So yes, there would be some advantage to having a HPWH located in a basement or somewhere slightly farther away and serving the whole house and tankless boosters in one or more locations near the points of more frequent use. I still would not go with electric. Nat gas or propane would be better and cheaper in most markets. There are lots of bad things that electric tankless units do to the grid. Those giant power spikes are not grid-friendly.
We had a 60 gallon electric resistance WH for a long time in a previous house near the shore. We lived there full-time, but became Club Med for our family in the summer. We rarely ran out of hot water, even with a lot of serial showers when everyone came back from the beach. I think you might be surprised at how long a tankfull lasts. And even if you turn it off when you're not there, on pure resistance, the tank will get hot in a couple of hours and then you could switch back to HP mode, or let the WH do it automatically. And pure resistance heating in the tank will cost exactly the same in electric $$ as the tankless booster at no additional infrastructure costs. If the lag time really bother you, hook it to a web-enabled switch so you can turn on the tank remotely when you're headed to the house.
To answer your question, Peter, the electric resistance heat is of course just as efficient in the tank as in the booster. In terms of system efficiency, the advantage is that the electric booster would only use electric resistance heat the very minimum needed to complete the shower. At the end of the shower, the electric heating ends, whereas the controls in the tank system might keep it on a little longer, and even if they didn't, the electric heater in the tank will have already heated some of the tank with electric heat, heat the could have been provided by the heat pump before the next use happened.
And yes, I agree, cost effective never, even if it was only a low power unit to boost the water by 10 F when the temperature sagged. I'm not recommending it--I'm recommending a big enough heat pump tank.
Why would you choose to set the water heater to heat pump only?
It seem to me the manufacture spent a lot of time and money testing and writing the code for the hybrid mode to do exactly what you ware wanting your water heater to do. Use the HP as much as possible while trying not running out of hot water.
Seem to me the HP only setting is made for people that are more willing to run out of hot water in order to save some electricity.
Walta