On Demand DHW Solar-ASHP Hybrid

Xringer · 01-06-12, 09:58 AM

We know On-Demand DHW heaters save money over continuously heated tanks.

I've been thinking about an On-Demand DHW Solar-ASHP Hybrid.
Here's the idea. A small (20 to 80 gallon) DHW tank that's pre-heated by a solar collector,
and can also be heated by an 18k to 25k BTUh ASHP.
(See crude block diagram).

Cycling of the solar collector would be sun dependent.
But the ASHP would be controlled by a program that checked for hot water usage,
by monitoring water temps out of the storage tank.

The ASHP would turn on under two conditions,
A. Hotwater was being used, and the tank's output temp was < 120F
B. Homeowner call for pre-heat.

The call for pre-heat could be a remote button that would start the ASHP.
The ASHP would stop, once the tank reached it's max set-point.
The controller could beep once to indicate your late night shower water was ready..

I think this type of system might have some advantages over other hybrid systems
that run a small ASHP (6k BTU?) to continually provide a heated storage tank.

Because this system uses a larger capacity ASHP, it will heat up the water much faster.
But, only when hot water is needed.

If the water in the tank was still warm from solar input, the ASHP could
provide a semi-continuous source of warm shower water..
Until, the solar pre-heated water was all gone and the ASHP was going it alone..
Then it might not be able to keep up, unless the flow rate was extra slow.

Since the ASHP isn't running very often, the air temperature in your
basement has time to recover it's normal temperature.
That allows the next ASHP run to harvest more heat, faster when it re-starts.

If the ASHP doesn't run all day, the basement isn't going to be as cold.
Which is nice when you want to do the wash etc..

With a system like this, I think you would have to program in a
bottom-end set-point for the storage tank.
It would be needed if there was no sun for a few days, and the tank temp
was getting too low.
So, at a minimum temperature set-point, the ASHP would run a while
to keep the tank from chilling out.. Maybe around 80F would work ??
It would depend on how fast of a recovery you wanted the system to have.

I think the sizing of the water tank is going to depend greatly on
the amount of solar collector and BTUhs from the ASHP.
Seems like this scheme maybe scalable, if you happened to find an
extra large ASHP..

This seems like it could be a DIY project, using some off-the-shelf hardware. (and some homemade gear).
Maybe a good app for the CAI controller..

Comments?

Student 07 · 01-06-12, 12:48 PM

Hi Xringer,
I like your idea of solar heat, ASHP, on demand heaters. The solar is pretty much a no brainer since there is no fuel cost (except for the pump) it makes sense to use it as much as possible.

The on demands have improved by leaps and bounds. Navien makes one that can take care of all your space heating and DHW needs. It puts out between 13,000 and 199,000 BTUs, and it is 98% efficient. This would make a great backup to the Solar and ASHP when needed.

An ASHP is another great idea, but I am not sure of the application of the ASHP. To use it in a manner like you are describing, it would have to be huge. Like the on demands, in order to heat quickly, they have to be over sized. The other problem, as far as I understand things, is that heat pumps work slowly. They don't have a fast recovery time, and they are most efficient when they run for a longer time. That is my understanding, but I could be wrong, I learn something new every day.

I still like your idea and would like to do something similar for myself. The way I would do it is to use solar with it's own controller for the circulation pump. Then when the solar isn't enough the aquastat at the top of the tank would be connected to a controller which would turn on the ASHP to get the tank up to temperature. The water then flows into the on demand. The on demand has a built in aquastat and flow valve that will turn on the heater if it needs to increase the heat. It would only need to turn on when the solar and ASHP isn't enough.

Maybe I misunderstood things but it sounded like you were trying to use the ASHP in a way that it really doesn't "need" a tank. I think it is better to use a small tank, basically as a buffer to store some energy (heat) and let the ASHP run longer and more efficiently. Using a tank will also mean you can use a smaller ASHP (BTUs). It will have a longer recovery time than a large ASHP, but with a properly sized tank, it will provide enough hot water for you.
Just my $.02

MN Renovator · 01-06-12, 12:57 PM

I'm nervous to add my input here but I will anyway and try to keep it brief.

1. I don't think we can make this generalization "We know On-Demand DHW heaters save money over continuously heated tanks." This might make more sense for electric but NG heaters, even condensing ones seem to be worse than what a condensing storage tank is capable of. On-demand inefficiencies have to do with the wasteful slug effect that also wastes even more water than just waiting for the temperature to rise in a storage tank.

2. I think you'll want a 120 gallon tank to make the best use of solar DHW. There are plenty of Home Power articles that show how they are incorporated and the heat strips installed are placed at the top near the water outlet so if there is a sag in the temperature, it will warm the top of the tank and apparently there isn't enough convection in the fluid to circulate the heat down fast enough under high demand.

3. It would take 60k or more to keep up with even a 1.5gpm showerhead. Mine is rated for 1.5gpm and pulls 1.8gpm and my water heater is 40k BTU and the tank is 40 gallons and I can still run out of hot water in about 10 minutes if my setpoint is to the within 5 degrees point where I need the hot cranked fully open within the first few minutes of the shower to get the desired temperature. Granted input isn't the same as output for a NG burner but I think it would take about 60k to hold an indefinite level of heat.

4. With a bigger tank, the more time you can allow yourself to recover unless a ton of water is being used. This is where a smaller and usually more efficient ASHP works out, you've got enough water to be slow at recovery. I'm thinking a 12k would heat 40 gallons to 130 degrees in under 3 hours. That's a 16 minute shower at 2.5gpm. With a 120 gallon tank there will be plenty of heat left over for a few showers even more with a lower flowing head.

5. Pre-heat mode - I love the idea. Wish I could do that with a gas heater. I'd actually like an option to manually cut it off completely for a specific period of time that
I know I won't need the water. I don't think I'd save much though based on my experience.

6. Solar!

7. This might spark some ideas, I'm not sure if Rinnai produces an electric version but this would allow for the use of a large solar plus ASHP setup and be able to cover the excess needs too. Seems to be one of the only units around that can handle adjusting the output for a variable input temperature similar to what a solar tank would provide. Of course, high output should rarely be needed if everything is setup right. Guy Marsden - Sustainable Living - tankless water heater Just an FYI, the guy who runs this arttec site is on various social networking sites but he doesn't respond to any replies to posts he has places, even on the most social of them, which I find odd since he sells things to off-grid households so you would think there would be some response.

AC_Hacker · 01-06-12, 01:01 PM

Quote:

Originally Posted by Xringer

I've been thinking about an On-Demand DHW Solar-ASHP Hybrid.

The idea of dual mode heating sounds good, but your conceived implementation sounds overly complex, and somewhat mis-targeted.

Why not a fairly large, well-insulated tank, a small heat pump DHW (using outside air) and a solar axillary panel.

Your idea to have a HP boost is leveraging low-temperature heating at it's very weakest point.

If you want a quick boost, use fossil fuel, that's what it is best at.

If you want an economical source of heat and can stand a longer warm-up time, go low-temp heating, like a HP, if you can tolerate variable supply, add solar.

Dedicating a 18,000 BTU HP to a quick-boost function doesn't make much sense, especially considering that if you insulated your house with fiendish diligence, and used properly-sized low-temp radiators, an 18,000 BTU unit could probably heat the whole place, with enough excess capacity to also provide DHW, most of the time.

Nat Gas, Propane or Electric Resistance (in that order) could fill in the gaps.

-AC_Hacker

Xringer · 01-06-12, 01:05 PM

Rely to Student 07..

An ASHP can get your HX up to 120F pretty quick, depending on the system,
it could be within 2 or 3 minutes.

A really large ASHP might be able to run 'tankless' heating a few quarts in the HX.
Maybe with good flow, if the input water wasn't ice cold..

One idea was to take a solar DHW system and add-on a Jumbo ASHP with it's
HX inline with the output of the solar tank.
The big ASHP would only come on when solar hot water got down to luke warm.
So, the ASHP would never be used to heat the tank, but just what came into it's HX.
Like a regular on-demand heater, but with a bit slower reaction time..

AC_Hacker · 01-06-12, 01:07 PM

Quote:

Originally Posted by Xringer

An ASHP can get your HX up to 120F pretty quick, depending on the system,
it could be within 2 or 3 minutes.

Sounds like a great hack. You should build it.

Take lots of photos, inquiring eyes want to see.

-AC_Hacker

Xringer · 01-06-12, 02:40 PM

Okay MN Renovator,

1. That's for electrically heated. If someone has gas, they are home free..

2. I think amount of water in the tank will depend on how much collector you have to play with.. Size accordingly.

3. "60k or more to keep up with even a 1.5gpm showerhead"..
But, maybe not if your solar provided a good long slug of pre-warmed water to start with.
Yes, there will be limitations. The system you have to be designed to
meet the needs, or the users would need to learn new use habits..

Check this out..
Sanyo mini split AC (inverter/variable speed) - Page 31

And this one is good too..
New GE Hybrid Hot Water Heater in Series - Page 2 - Energy, Coal News and Discussion

4. Bigger tank is good if there is a big collector on the roof..
But, can this scheme be scaled down? Like for two retirees?

One 4x8 panel, 50 gallon tank with an 18k ASHP?
Plus, it's gonna depend on where you live too..

5. Pre-heat is a must with this scheme. Plus it needs to be done cheaply.

6. Solar.. I've already considered using a small ASHP (6k) to add some heat to my DHW storage (76 gal),
by using PV to drive the ASHP..

(Using 800w of PV in direct DC heating mode isn't anywhere near what 800w could do, when driving an ASHP).

Xringer · 01-06-12, 02:54 PM

Quote:

Originally Posted by AC_Hacker

The idea of dual mode heating sounds good, but your conceived implementation sounds overly complex, and somewhat mis-targeted.

Why not a fairly large, well-insulated tank, a small heat pump DHW (using outside air) and a solar axillary panel.

Your idea to have a HP boost is leveraging low-temperature heating at it's very weakest point.

If you want a quick boost, use fossil fuel, that's what it is best at.

If you want an economical source of heat and can stand a longer warm-up time, go low-temp heating, like a HP, if you can tolerate variable supply, add solar.

Dedicating a 18,000 BTU HP to a quick-boost function doesn't make much sense, especially considering that if you insulated your house with fiendish diligence, and used properly-sized low-temp radiators, an 18,000 BTU unit could probably heat the whole place, with enough excess capacity to also provide DHW, most of the time.

Nat Gas, Propane or Electric Resistance (in that order) could fill in the gaps.

-AC_Hacker

I'm not saying that I would go out and buy a new 18,000 BTU HP for this project..
But, if the price was right? (like very close to free), would it be wasteful
to crank up that 18k HP and run it in a semi-on-demand system?

I admit, I'm looking at this from my own constraints. (No gas)..
Maybe my old 18k AC could be hacked to heat my old oil boiler.
How would that work, compared to buying a Nyle Geyser??

I'm just thinking about how tie two good ideas together. Solar+ASHP to get DHW..

Might be possible..

Xringer · 01-06-12, 03:03 PM

Quote:

Originally Posted by AC_Hacker

Sounds like a great hack. You should build it.

Take lots of photos, inquiring eyes want to see.

-AC_Hacker

I've seen a few car AC systems that worked almost instantly.
I know my Sanyos can start making serious heat within about 90 seconds,

It's kinda a slow-mo on-demand, and wouldn't be good for people in a hurry..

I would love to have an old inverter Sanyo 36k outdoor unit to try out,
just making some hot water..

If it weren't for our chilly den, I would have tried it with Sanyo#2..

Student 07 · 01-06-12, 03:42 PM

Hi,
I think this idea interests a lot of people. It is a great idea.

Of course each one of the three elements: on demand, ASHP and solar, would have to be optimized.

Xringer brought up a very good point.
"I'm just thinking about how tie two good ideas together. Solar+ASHP to get DHW.. "

A HVAC guy in our area told me that Solar thermal isn't very productive in our area, and I would have to agree that it would provide limited benefit with our weather. What he recommended was Photovoltaics to run electric heat elements. Photovoltaics work (at a lesser output) in semi cloudy conditions.

So, going back to Xringer's comment, would it be better to use a solar thermal system to heat water directly or use photovoltaics to run a heat pump that heats the water?

I have always been told that you lose a lot of energy when you convert it, so it should be better to heat the water directly with solar. But, the photovoltaics work (with less output) in more adverse weather conditions, and the ASHP is more than 100% efficient: does the rule of energy loss still apply?

Of course if you have the space, sun and money to buy the equipment it would be better to use both forms of solar energy.

Xringer, I would be interested to hear your ideas about using an ASHP to produce hot water, ie.. heat exchanger and controls.

01-06-12, 09:58 AM	#1
Xringer Lex Parsimoniae Join Date: Feb 2009 Location: Woburn, MA Posts: 4,918 Thanks: 114 Thanked 250 Times in 230 Posts	On Demand DHW Solar-ASHP Hybrid We know On-Demand DHW heaters save money over continuously heated tanks. I've been thinking about an On-Demand DHW Solar-ASHP Hybrid. Here's the idea. A small (20 to 80 gallon) DHW tank that's pre-heated by a solar collector, and can also be heated by an 18k to 25k BTUh ASHP. (See crude block diagram). Cycling of the solar collector would be sun dependent. But the ASHP would be controlled by a program that checked for hot water usage, by monitoring water temps out of the storage tank. The ASHP would turn on under two conditions, A. Hotwater was being used, and the tank's output temp was < 120F B. Homeowner call for pre-heat. The call for pre-heat could be a remote button that would start the ASHP. The ASHP would stop, once the tank reached it's max set-point. The controller could beep once to indicate your late night shower water was ready.. I think this type of system might have some advantages over other hybrid systems that run a small ASHP (6k BTU?) to continually provide a heated storage tank. Because this system uses a larger capacity ASHP, it will heat up the water much faster. But, only when hot water is needed. If the water in the tank was still warm from solar input, the ASHP could provide a semi-continuous source of warm shower water.. Until, the solar pre-heated water was all gone and the ASHP was going it alone.. Then it might not be able to keep up, unless the flow rate was extra slow. Since the ASHP isn't running very often, the air temperature in your basement has time to recover it's normal temperature. That allows the next ASHP run to harvest more heat, faster when it re-starts. If the ASHP doesn't run all day, the basement isn't going to be as cold. Which is nice when you want to do the wash etc.. With a system like this, I think you would have to program in a bottom-end set-point for the storage tank. It would be needed if there was no sun for a few days, and the tank temp was getting too low. So, at a minimum temperature set-point, the ASHP would run a while to keep the tank from chilling out.. Maybe around 80F would work ?? It would depend on how fast of a recovery you wanted the system to have. I think the sizing of the water tank is going to depend greatly on the amount of solar collector and BTUhs from the ASHP. Seems like this scheme maybe scalable, if you happened to find an extra large ASHP.. This seems like it could be a DIY project, using some off-the-shelf hardware. (and some homemade gear). Maybe a good app for the CAI controller.. Comments? Attached Thumbnails __________________ My hobby is installing & trying to repair mini-splits EPA 608 Type 1 Technician Certification ~ 5 lbs or less.. Last edited by Xringer; 01-06-12 at 10:40 AM.. Reason: typos galor

01-06-12, 01:05 PM	#5
Xringer Lex Parsimoniae Join Date: Feb 2009 Location: Woburn, MA Posts: 4,918 Thanks: 114 Thanked 250 Times in 230 Posts	Rely to Student 07.. An ASHP can get your HX up to 120F pretty quick, depending on the system, it could be within 2 or 3 minutes. A really large ASHP might be able to run 'tankless' heating a few quarts in the HX. Maybe with good flow, if the input water wasn't ice cold.. One idea was to take a solar DHW system and add-on a Jumbo ASHP with it's HX inline with the output of the solar tank. The big ASHP would only come on when solar hot water got down to luke warm. So, the ASHP would never be used to heat the tank, but just what came into it's HX. Like a regular on-demand heater, but with a bit slower reaction time.. __________________ My hobby is installing & trying to repair mini-splits EPA 608 Type 1 Technician Certification ~ 5 lbs or less.. Last edited by Xringer; 01-06-12 at 01:07 PM..

01-06-12, 02:40 PM	#7
Xringer Lex Parsimoniae Join Date: Feb 2009 Location: Woburn, MA Posts: 4,918 Thanks: 114 Thanked 250 Times in 230 Posts	Okay MN Renovator, 1. That's for electrically heated. If someone has gas, they are home free.. 2. I think amount of water in the tank will depend on how much collector you have to play with.. Size accordingly. 3. "60k or more to keep up with even a 1.5gpm showerhead".. But, maybe not if your solar provided a good long slug of pre-warmed water to start with. Yes, there will be limitations. The system you have to be designed to meet the needs, or the users would need to learn new use habits.. Check this out.. Sanyo mini split AC (inverter/variable speed) - Page 31 And this one is good too.. New GE Hybrid Hot Water Heater in Series - Page 2 - Energy, Coal News and Discussion 4. Bigger tank is good if there is a big collector on the roof.. But, can this scheme be scaled down? Like for two retirees? One 4x8 panel, 50 gallon tank with an 18k ASHP? Plus, it's gonna depend on where you live too.. 5. Pre-heat is a must with this scheme. Plus it needs to be done cheaply. 6. Solar.. I've already considered using a small ASHP (6k) to add some heat to my DHW storage (76 gal), by using PV to drive the ASHP.. (Using 800w of PV in direct DC heating mode isn't anywhere near what 800w could do, when driving an ASHP). __________________ My hobby is installing & trying to repair mini-splits EPA 608 Type 1 Technician Certification ~ 5 lbs or less..

01-06-12, 12:48 PM	#2
Student 07 Helper EcoRenovator Join Date: Dec 2011 Location: Gresham OR Posts: 35 Thanks: 0 Thanked 3 Times in 3 Posts	Hi Xringer, I like your idea of solar heat, ASHP, on demand heaters. The solar is pretty much a no brainer since there is no fuel cost (except for the pump) it makes sense to use it as much as possible. The on demands have improved by leaps and bounds. Navien makes one that can take care of all your space heating and DHW needs. It puts out between 13,000 and 199,000 BTUs, and it is 98% efficient. This would make a great backup to the Solar and ASHP when needed. An ASHP is another great idea, but I am not sure of the application of the ASHP. To use it in a manner like you are describing, it would have to be huge. Like the on demands, in order to heat quickly, they have to be over sized. The other problem, as far as I understand things, is that heat pumps work slowly. They don't have a fast recovery time, and they are most efficient when they run for a longer time. That is my understanding, but I could be wrong, I learn something new every day. I still like your idea and would like to do something similar for myself. The way I would do it is to use solar with it's own controller for the circulation pump. Then when the solar isn't enough the aquastat at the top of the tank would be connected to a controller which would turn on the ASHP to get the tank up to temperature. The water then flows into the on demand. The on demand has a built in aquastat and flow valve that will turn on the heater if it needs to increase the heat. It would only need to turn on when the solar and ASHP isn't enough. Maybe I misunderstood things but it sounded like you were trying to use the ASHP in a way that it really doesn't "need" a tank. I think it is better to use a small tank, basically as a buffer to store some energy (heat) and let the ASHP run longer and more efficiently. Using a tank will also mean you can use a smaller ASHP (BTUs). It will have a longer recovery time than a large ASHP, but with a properly sized tank, it will provide enough hot water for you. Just my $.02

01-06-12, 12:57 PM	#3
MN Renovator Less usage=Cheaper bills Join Date: Nov 2010 Location: Minneapolis, MN Posts: 939 Thanks: 41 Thanked 116 Times in 90 Posts	I'm nervous to add my input here but I will anyway and try to keep it brief. 1. I don't think we can make this generalization "We know On-Demand DHW heaters save money over continuously heated tanks." This might make more sense for electric but NG heaters, even condensing ones seem to be worse than what a condensing storage tank is capable of. On-demand inefficiencies have to do with the wasteful slug effect that also wastes even more water than just waiting for the temperature to rise in a storage tank. 2. I think you'll want a 120 gallon tank to make the best use of solar DHW. There are plenty of Home Power articles that show how they are incorporated and the heat strips installed are placed at the top near the water outlet so if there is a sag in the temperature, it will warm the top of the tank and apparently there isn't enough convection in the fluid to circulate the heat down fast enough under high demand. 3. It would take 60k or more to keep up with even a 1.5gpm showerhead. Mine is rated for 1.5gpm and pulls 1.8gpm and my water heater is 40k BTU and the tank is 40 gallons and I can still run out of hot water in about 10 minutes if my setpoint is to the within 5 degrees point where I need the hot cranked fully open within the first few minutes of the shower to get the desired temperature. Granted input isn't the same as output for a NG burner but I think it would take about 60k to hold an indefinite level of heat. 4. With a bigger tank, the more time you can allow yourself to recover unless a ton of water is being used. This is where a smaller and usually more efficient ASHP works out, you've got enough water to be slow at recovery. I'm thinking a 12k would heat 40 gallons to 130 degrees in under 3 hours. That's a 16 minute shower at 2.5gpm. With a 120 gallon tank there will be plenty of heat left over for a few showers even more with a lower flowing head. 5. Pre-heat mode - I love the idea. Wish I could do that with a gas heater. I'd actually like an option to manually cut it off completely for a specific period of time that I know I won't need the water. I don't think I'd save much though based on my experience. 6. Solar! 7. This might spark some ideas, I'm not sure if Rinnai produces an electric version but this would allow for the use of a large solar plus ASHP setup and be able to cover the excess needs too. Seems to be one of the only units around that can handle adjusting the output for a variable input temperature similar to what a solar tank would provide. Of course, high output should rarely be needed if everything is setup right. Guy Marsden - Sustainable Living - tankless water heater Just an FYI, the guy who runs this arttec site is on various social networking sites but he doesn't respond to any replies to posts he has places, even on the most social of them, which I find odd since he sells things to off-grid households so you would think there would be some response.

01-06-12, 03:42 PM	#10
Student 07 Helper EcoRenovator Join Date: Dec 2011 Location: Gresham OR Posts: 35 Thanks: 0 Thanked 3 Times in 3 Posts	Hi, I think this idea interests a lot of people. It is a great idea. Of course each one of the three elements: on demand, ASHP and solar, would have to be optimized. Xringer brought up a very good point. "I'm just thinking about how tie two good ideas together. Solar+ASHP to get DHW.. " A HVAC guy in our area told me that Solar thermal isn't very productive in our area, and I would have to agree that it would provide limited benefit with our weather. What he recommended was Photovoltaics to run electric heat elements. Photovoltaics work (at a lesser output) in semi cloudy conditions. So, going back to Xringer's comment, would it be better to use a solar thermal system to heat water directly or use photovoltaics to run a heat pump that heats the water? I have always been told that you lose a lot of energy when you convert it, so it should be better to heat the water directly with solar. But, the photovoltaics work (with less output) in more adverse weather conditions, and the ASHP is more than 100% efficient: does the rule of energy loss still apply? Of course if you have the space, sun and money to buy the equipment it would be better to use both forms of solar energy. Xringer, I would be interested to hear your ideas about using an ASHP to produce hot water, ie.. heat exchanger and controls.