Heat Pump water Heaters

[Zwy]

Do any of these run off of external air? It doesn't make sense to use heated indoor air here most of the year, since we use the mini split in AC mode for a week or two, tops.

I should really stick a power monitor on my electric water heater circuit, see how much it uses to heat vs stay hot.

The water heater will actually cool the area and your mini split will use less power when running in AC mode.

You can run vents to anywhere you want. Personally, I don't worry about it. I have mine in the un-insulated basement and it keeps the basement cool in summer with low humidity and no need to run a dehumidifier. The shoulder seasons the basement will be around ground temp at a minimum. In winter, I have an electric radiant heater on a Shelly smart plug and if I will hit close to full charge for the day it is turned on during the day.

If you are running the mini split without using grid power and the heat pump water heater is using some of that heat energy from the mini split, what difference does it make as the heat energy doesn't cost anything?

 

[BerndL]

The water heater will actually cool the area and your mini split will use less power when running in AC mode.

Like I said, I hardly use AC. It's 95% used for heating.

 

[Zwy]

Like I said, I hardly use AC. It's 95% used for heating.

Are you completely off grid? If the energy for the mini split is 100% from PV power, it doesn't matter. Or add more PV to cover the extra power needed.

A hybrid heat pump water heater like mine also has a heating element and that can be used during extremely cold weather.

What are your seasonal temps?

 

[SunDave]

Like I said, I hardly use AC. It's 95% used for heating.

If you are 95% heating dominated about the only thing you could benefit from a heat pump water heater is if you wanted to cheat your propane heating usage (if you have it) in order to use less kWh from your solar. You would be robbing from your furnace to pay what would otherwise be a higher electric bill from your water heater.

If you already heat with electric, it could be a net negative. Look at the bright side. 5% of the year you will benefit...

Not everything fits every climate. Great for warm climates for sure, but I hope the regular heaters are not legislated away for those of us in the frozen north.

 

[skyking1]

I'm putting one in the new house in the equipment room. I won't have many air conditioning days either, but I do have a water to water heat pump in there and all the associated piping for hydronic heating. Even with good pipe insulation, I think that room is going to be warm.
I'm hoping it will be a wash.
The other thing to consider is noise. These units make more noise than advertised, in many instances.
I will drop and suspend the ceiling sheet rock in the equipment room for all the potential noises down there.

 

[Zwy]

I use my heat pump water heater in winter and it gets cold here, -20°F every winter.

My basement contains my charge controllers, inverters, batteries, furnace, one freezer and water heaters. It is basically non insulated. Sunny days in winter, it does raise the temp down there. I do get some heat from the earth itself but it is also a large heat sink. Basically, it holds between 54°F and 58°F all winter, I can see the temp by looking at the Batrium. Range shown on the Batrium is in C, low never below 12°C and never above 15°C.

I don't worry about bottling the heat energy and whether it came from the charge controllers, inverters, batteries, furnace or freezer. It's just part of the house energy load. On days of excess PV and house is already close to 80°F (my wife hates it but it allows for heat coasting thru the evening) using the heat pumps, I turn on an electric radiant heater in the basement that is controlled by a Shelly smart plug. I can set how many hours I want it to run, I just set it for just before sunset to turn off. That heat can go into the masonry, the earth, or the heat pump water heater but also heats the floor above the basement. It's a dump load for excess PV and I will take advantage of it when I can.

I am quite certain @BerndL doesn't live in the frozen tundra above the Arctic Circle and would only benefit 5%.

 

[Madcodger]

I use my heat pump water heater in winter and it gets cold here, -20°F every winter.

Yup. Same here, in Maine (although we only got to -14F this past winter).

Our heat pump water heater sits in the mechanical room with the inverters. As they give off heat the water heater transfers it into the tank, so it's not wasted. Yes, the room also sits on the heated slab so in the dead of winter some of that heat ultimately comes from the propane boiler, but that's hardly a net negative, as some suggest. It's just a transfer of heat to where it's needed. And when we can finally install an air-water heat pump for that slab heat (when prices moderate in the US) we'll use almost no propane at all for the entire building. Pretty darn hard to beat a heat pump water heater!

 

[SunDave]

Yup. Same here, in Maine (although we only got to -14F this past winter).

Our heat pump water heater sits in the mechanical room with the inverters. As they give off heat the water heater transfers it into the tank, so it's not wasted. Yes, the room also sits on the heated slab so in the dead of winter some of that heat ultimately comes from the propane boiler, but that's hardly a net negative, as some suggest. It's just a transfer of heat to where it's needed. And when we can finally install an air-water heat pump for that slab heat (when prices moderate in the US) we'll use almost no propane at all for the entire building. Pretty darn hard to beat a heat pump water heater!

Not to nitpick, but heat is heat. So the inverter heat in the winter would just offset your furnace. Instead some could be used by the heat-pump, which is then replaced by the furnace. No free lunch.

They are absolutely great for cooling dominated climates. And likely decent for others, but we have a ways to go on the whole logic of it. Eventually they will not source the heat from in the house. Right now my water heater is "typical" but gets some of it's energy from a "desuperheater" (I know it sounds fake ). Basically my ground source heat pump extracts the heat from a closed loop in the earth, then some of the compressor heat goes into the domestic hot water, or the in-floor heat, or vent heat. As all these techs get more efficient, we will see that there is plenty of heat to be extracted, even from -40 degree air. It's not extreme cold compared to how little heat it can get to (absence of heat all together).

 

[Supervstech]

Not to nitpick, but heat is heat. So the inverter heat in the winter would just offset your furnace. Instead some could be used by the heat-pump, which is then replaced by the furnace. No free lunch.

They are absolutely great for cooling dominated climates. And likely decent for others, but we have a ways to go on the whole logic of it. Eventually they will not source the heat from in the house. Right now my water heater is "typical" but gets some of it's energy from a "desuperheater" (I know it sounds fake ). Basically my ground source heat pump extracts the heat from a closed loop in the earth, then some of the compressor heat goes into the domestic hot water, or the in-floor heat, or vent heat. As all these techs get more efficient, we will see that there is plenty of heat to be extracted, even from -40 degree air. It's not extreme cold compared to how little heat it can get to (absence of heat all together).

Now for me to nitpick...

Technically, a desuperheater benefits from additional cooling of the refrigerant, by heating another medium. They work well for tempering ground water temps from feeding the hot water device.

When the refrigerant is compressed, it gets superheated beyond the vapor pressure of the refrigerant, and a desuperheater aids efficiency of the condenser system. Most geothermal systems have very small condenser coils, so desuperheaters aid cooling function, but are a negative for heat pump heating.

 

[SunDave]

Most geothermal systems have very small condenser coils, so desuperheaters aid cooling function, but are a negative for heat pump heating.

Agreed. Because just like heat pump water heaters, you are robbing Peter to pay Paul. Works great in the summer, not as great in the winter.

(To clarify for non Geothermal Heat pump users, we are talking about a VERY small part of the system, not the whole thing)

 

[skyking1]

The new inverter heat pump has no desuperheater.
It does have better COP than most, so I think I will put a little heat exchanger on the service water side in front of the water heater.
The system will have an outdoor reset curve and will run constantly during heating seasons. Service Water Temperature (SWT) will vary from a high of about 105 F to 85, so it would preheat the cold water coming into the heat pump water heater a significant amount during heating seasons.
That would speed up recovery.
One bad thing about inverter water heaters is they do not play well with recirculation in the traditional sense. I will use a smart recirculator control that minimizes the pump time as much as practical.
I may dump that return in before the heat exchanger so at least I am not dosing the water heater with cold directly.

 

[onokai]

From what I have read each hybrid heater can be different noise wise-some more quiet than others

 

[onokai]

From what I have read each hybrid heater can be different noise wise-some motre quiet than others

On another note I am using a grunfus smart pump to recirculate hot water return thru the drain valve (I took that out and out in a brass nipple)
Now if I could only recalculate my solar hot water 80 gallon into that hybrid tank instead of a static only which only goes into hybrid when hot water tap is on.

 

[time2roll]

One bad thing about inverter water heaters is they do not play well with recirculation in the traditional sense. I will use a smart recirculator control that minimizes the pump time as much as practical.

My circulation pump works fine to provide hot water to the taps. Or is the issue with hydronic heating?

 

[skyking1]

Mine circulation pump works fine to provide hot water to the taps. Or is the issue with hydronic heating?

A great portion of the power savings is how little it takes to keep a tank of water warm. When you tap some hot it ramps up, then back down. A circ pump keeps it in a higher gear most of the time, which defeats some of that economy.
Leridian makes a nifty smart control.
Leridian Smart 32 control

It uses a flow meter and temperature sensor with a microprocessor to control the pump. Bump the hot water on then off in the bathroom. That will trigger the flow. If the return water is cold, it will run the pump. If it is not, it will not waste energy circulating an already hot return.

They make another unit that works well with tankless.
This unit can do up to 10 timer settings if you want a timed system too, and through a phone app.

 

[BerndL]

Now for me to nitpick...

Technically, a desuperheater benefits from additional cooling of the refrigerant, by heating another medium. They work well for tempering ground water temps from feeding the hot water device.

When the refrigerant is compressed, it gets superheated beyond the vapor pressure of the refrigerant, and a desuperheater aids efficiency of the condenser system. Most geothermal systems have very small condenser coils, so desuperheaters aid cooling function, but are a negative for heat pump heating.

Rewording for my retard brain, a desuperheater cools the superheated gas to just above the condensation point, so that a small condenser can spend all its capacity turning gas to liquid, rather than cooling liquid down?

Why does excessive superheat happen in the first place again? I thought it was to ensure refrigerant is 100% gas as it goes through the compressor.
Edit: ah, in a ground loop system you can end up with the water heating up the refrigerant in the evaporator way past its boiling point, because there's lots of heat from the ground loop?

And the desuperheater removes that excess heat after it goes through the compressor, right? Since there's no real harm in the gas in the compressor being a bit hotter than usual, you just don't want it too hot when it gets to the condenser?

Edit-edit: ah, think I get it. So the desup is working kind of like a counterflow heat exchanger. The condenser heats water up to moderate temperatures, and then that warm water is boosted to hot water by the desup.
So the lower heating temp means lower pressure going into the condenser, which means less work for the compressor.
And you could do the same thing by having a bigger condenser and counterflowing your water past it, but the desup is cheaper and easier.

 

[Supervstech]

Rewording for my retard brain, a desuperheater cools the superheated gas to just above the condensation point, so that a small condenser can spend all its capacity turning gas to liquid, rather than cooling liquid down?

Why does excessive superheat happen in the first place again? I thought it was to ensure refrigerant is 100% gas as it goes through the compressor.
Edit: ah, in a ground loop system you can end up with the water heating up the refrigerant in the evaporator way past its boiling point, because there's lots of heat from the ground loop?

And the desuperheater removes that excess heat after it goes through the compressor, right? Since there's no real harm in the gas in the compressor being a bit hotter than usual, you just don't want it too hot when it gets to the condenser?

Edit-edit: ah, think I get it. So the desup is working kind of like a counterflow heat exchanger. The condenser heats water up to moderate temperatures, and then that warm water is boosted to hot water by the desup.
So the lower heating temp means lower pressure going into the condenser, which means less work for the compressor.
And you could do the same thing by having a bigger condenser and counterflowing your water past it, but the desup is cheaper and easier.

When the vapor is compressed, the heat of compression will superheat the gas far above the vapor pressure temp of the gas, the desuperheater assists reduction of the compressor superheat.

This is NOT the same as evaporation superheat.

That is the vapor line leaving the evaporator, any heat gained into the vapor after it boils is evoporation superheat.
It needs to be above a certain point to insure the compressor does not see any liquid that could compromise the compressor components.

 

[Supervstech]

The term superheat simply means heating a gas after it has reached its boiling point from a liquid...

So, any type of heating of a boiled liquid is superheated above the boiling temp at a specific pressure.

Water cannot achieve a temp above 212F/100C as a liquid, but once it has boiled, the steam or water vapor can be heated massively, iirc, it can reach around 2000F...

 

[time2roll]

Like I said, I hardly use AC. It's 95% used for heating.

OK and do you have gas/propane water heater? I have been reading that burning gas will remove a lot of heat as the interior air goes up the flue. A person posted that his basement is actually warmer after replacing the gas heater with a heat pump.

 

[Madcodger]

I have been reading that burning gas will remove a lot of heat as the interior air goes up the flue.

If gas heating is needed, a good strategy for dealing with this is to use a heater and vent system that pulls outside air in for combustion rather than use indoor air. Rinnai and many others make systems that allow for this. Works great.

 

[onokai]

OK I started this thread a bit ago and now have been operating a 50 gallon Reem hybrid heat pumpo water heater for about a month now (turned on 5/5/25) . I have no cost figures yet on switching from gas to electric, wait for a whole utility bill of no gas use for water heating. I can address the noise factor as we are using heated house air from corner of kitchen to take the warmer air and exust to outside shed to. That noise is minimal . The air goes thru a 5 x 14 hole in sheetrock under a upper cabinet then thru a wood sealed periscope box (like you made as a kit to see above things). That box is connected to a flex 93 inchsand then to a 90 elbow on unit. all fully sound deadened and insulated. The noise is realy only noticed at start up (sometimes it a 2 second rattle) then is backgound noise like our refrigerator which is near that vent opening. So far its works very well with lots of hot water and our battery system still has lots of reserve power.

 

[efficientPV]

I only have gas for heat and stove, used to have a gas water heater. Many don't know it, but many utilities allow you can turn the gas off for the summer and save distribution charges. They do a soft close monitoring if you use any. I leave for the summer house now. Yearly distribution charges used to be more than for the gas itself. Factor that into costs.