Are there tankless heat pump water heaters?
bluesolar
| Posted in Green Products and Materials on
Hi all — Are there tankless heat pump water heaters? If so, where can I find them? Web searches haven’t borne fruit.
If they don’t exist, is it something about the way heat pump water heaters work that makes tankless models infeasible?
Thanks.
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I can't say definitively that they don't exist, but they're not a good application. Domestic hot water consumes BTU's at a pretty hefty rate, but typically it is only used sporadically throughout the day. Just to throw out some numbers, let's say your shower uses two gallons per minute of water, which entered the house at 50F and comes out of the shower at 100F. That's an instantaneous rate of 48,000 BTU per hour or 14,000 watts.
However, let's say you only use 100 gallons of hot water per day. You would only be heating water at that rate for 50 minutes a day, the other 23 hours and 10 minutes your heater sits idle. With a tank you can heat that water slowly and save it up for when you need it, you'd only need a heater with a capacity of 1667 BTU/hr or slightly under 500 watts.
Heat pumps are better suited to continuous, constant loads.
Its a question of BTUs. It takes A LOT of btu to bring water from cold up to shower temperature at a reasonable flowrate, most tankless units are around 150k to 200k btu. In comparison your average heat pump water heater has a 4k btu heat pump (15 to 20 times smaller).
The physical size of the required heat pump would be about the size of a small room, never mind the cost.
Then there is the question of where this heat would be pumped from. Running a 200kbtu air source heat pump inside your basement would cool it down to freezing temperatures in seconds. You would need a fairly large thermal mass like a big water tank.
Now you have a room sized heat pump connected to a large closet sized water tank. Not the most compact water heater.
I think the existing tank heat pump water heaters are a pretty good compromise. It would be nice if they would put out a bit more BTU and handled lower temperatures so you could steal a bit of output for space heat.
I've heard nothing of tankless heat pump water heaters, and as the first two posts point out, they seem unlikely. Even tank-style heat pump water heaters are not gaining a whole lot of traction in the industry. One emerging technology is gas-powered heat pump water heaters, which are expected to be hitting the market in the next few years. Check out this article for more on the most efficient water heaters and what manufacturers are currently developing: Choosing an Efficient Water Heater
I was pondering this question from a purely theoretical point of view.
IMHO a tankless heat pump water heater might make sense as part of an integrated system where the heat pump capacity is also used for space heating, because as others have noted the energy demands for heating water are so huge.
But even here is isn't likely to be of use; a tankless for a single shower might need to be a 4 ton unit, and 4 tons is quite a lot for a home small enough to only have a single shower.
The other place a tankless heat pump system might make sense is in something like a hotel where you have _lots_ of water use going on all the time. Such a system might even be able to recover heat that is otherwise going down the drain.
But these are purely speculative musings, not a description of any products that I actually know about.
-Jon
4 tons is 48,000 BTU/hr. That's a decent amount of energy. A typical heat pump water heater scavenges heat from it's surroundings, and a 4 ton monster unit would do the same. The downside is that with a 4 ton monster, you'll be actively cooling the space around the water heater, exactly the same way a 4 ton air conditioner is capable of cooling a pretty good sized home. I don't think a 4 ton heat pump water heater is practical inside a home. If you were scavenging heat from the outdoors in a warm climate it might make sense, but it's still probably not practical.
A heat pump water heater works by scavenging a relatively small amount of heat over a relatively long period of time and storing that scavenged heat in the thermal battery formed by the water in the tank. With an on-demand system, you can't build up a reserve of hot water over time, so you have to scavenge ALL of the energy needed RIGHT NOW, which means a LOT of cold air is going to be produced in the process.
Bill
Interesting, thanks. I almost forgot about this question. It seems like heat pumps are also ideal for keeping large volumes of water warm or hot, kind of like a "trickle charge" for battery maintenance. One constraint I don't understand is why hot water tanks are so small when there's typically plenty of room for more hot water storage. I don't know what the heat loss math looks like given typical tank construction and insulation, but it seems like a good solar thermal system should be able to heat a lot of water during the day, and a heat pump should be able to keep it optimally hot. I'm not sure why we don't start with 100 gallons and go up from there.
One guy told me about his solar water heating setup where he had the kind of abundant hot water I expected was possible. He has four tanks, each at least 50 gallons, purely solar, at least up to the tank stage. I don't remember, but it seems like you'd want a tankless gas heater as a finisher for solar. Can solar water heater systems give you the arbitrary heat level on demand that normal gas and electric systems do? I don't understand how that works, since the tanked water will only cool over time, so at 6:00 am how do people get hot water?
Solar thermal is basically dead, I don't know if they even make the panels any more. It's more efficient to put solar electric panels on the roof and have them run a heat pump water heater, especially if you have net metering.
Large tanks take a long time to heat up if you use all the hot water in them. Homeowners tend to like compact equipment too. The manufacturers then end up doing a sort of balancing act trying to find the optimal place -- the "goldilocks zone" -- that is big enough but not too big. the common 40 gallon water heater seems to be a pretty popular "big enough" size.
There is no reason you couldn't use solar thermal to pre-heat the water going to a heat pump water heater (HPWH) to reduce the load on the HPWH, but I'm not aware of anyone actually doing that. DC may be right about the solar collectors no longer being manufactured too -- I rarely even see used ones anymore. You can easily use rooftop solar to run an electric resistance water heater as a preheater though, but if you aren't using hot water during the day you need to run a circulation pump too to circulate the water in the HPWH tank through the electric resistance tank. These tend to be functional ideas, it's just that the savings they offer tend to not be enough to justify the cost of putting the system together, which is unfortunate.
Bill
This is a graph of my tankless electric water heater power usage.
If you look at it is all spikes and is a poor fit for use with a heat pump that work bests with constant loads and long run times.
With My electric rates all the spikes add up to $0.85 for water heating.
Walta
Electric resistance heating is brutal for tankless water heaters. Gas is much more efficient for heating water or other materials. It seems like heat pumps would be ideal for keeping water heated in large tanks, maybe water originally heated with solar thermal.
Electricity is actually more effient than gas for heating water, and often other materials too (such as using induction furnaces to heat or melt metal). The reason is that electric heating is usually near 100% efficient, since you have no wasted heat going off in the exhaust gasses. The downside to electric resistance heating for water (or occupied buildings) isn't that it's inefficient, it's that it tends to be the most expensive option.
The one time electric resistance heating can be considered to be less efficient is when it's used to go "all electric" and replace a gas water heater (for example). In this case, your electric water heater, in most areas of the country, is primarily powered by gas anyway (and coal), and you are now less efficient compared to burning the gas directly due to all the conversions losses of converting the gas to heat, heat to electricity, and then transporting that electricity to your home. You're better off burning the gas directly in this case, which is more efficient overall and produces lower overall emissions compared with running an electric resistance heater off of a primarily gas fired electrical generator.
Note that HPWHs are very different from electric resistance water heaters since they are heat pumps, which have a COP number -- they basically move heat instead of making it -- so you get more BTUs out per unit energy in compared with making heat directly in a resistance heating element.
Bill