Heat pump water heater.

Icarus

Jamie, once again you have it in the nutshell.

I do think however we should be able to get “close enough” with off the shelf consumer hardware. The problem is, the set points are consumer driven, via smart phone apps, rather than mechanical set systems. As I think I mentioned, my frustration is, there should be no reason technically that we can’t set the first HPWH to say 90-100F instead of the lowest set point being 110f. We also as a matter of choice should be able to “tell” the HPWH to only operate in HP mode only...never clicking into resistance. If the cold water presents a problem for HP system, then we should be able to accommodate that another way. We could always give it a circ system to change the effective incoming water temp.

Not being a HP expert, I can’t imagine why a HPWH could be damaged with 50-60f incoming water. As you said, this is a learning exercise to some extent, but it shouldn’t be so hard.

Thanks for the ideas, and keep them coming. T

Zman

I can't see an advantage of the series installation. Sure the tanks in parallel would have a slightly higher heat loss than the series option. However, having both tanks at 120 degrees when a big demand period begins is a huge advantage over the series setup. There would be abundant buffer to satisfy the big demand surges,.

Icarus

Zman, you are missing the point..we are not looking for volume, or fast recovery but for ultimate efficiency. Icarus

ethicalpaul

With your admirable goal of ultimate efficiency, why not just use a single unit in heat pump only mode?

I saw your claim that they will go to resistive heating even in that mode, but mine definitely doesn't. My wife fills a soaking tub with hot water, taking basically every drop of hot water and it never goes to resistive heat. That's the last time I'll say it though because I don't want to be argumentative

Good luck!

psb75

It seems like you are trying to do an "end run" around all of the engineering and design that the heat-pump experts have done.
Get an 80 gal. set it on "heat pump only" and call it "good."

Icarus

The idea of using a 80 gallon makes sense in most cases. We are after all, trying to “do an end run” coming up (as an intellectual exercise if nothing else) with a scheme that will produce the required hot water with the least energy (net) input. This started as a result of getting a 60 gallon cheap, installing it and realizing that it was not big enough, which stimulated the exercise. We KNOW we can run them both in parallel (or even in series) to achieve the demand goal, but what we want to know is if we can actually run them (in the net) more efficiently.

The Rheem engineers have designed for most trouble free operation including calls from frustrated users for “too little hot water”. As such, they are not engineered for maximum efficiency. We are trying to do both.

Icarus

Icarus

Ethical Paul, Not thinking you are argumentative. I will to confirm that ours is indeed going into resistive mode. Question for you might be, what is your incoming water temp average?

Thnx for you thoughts

Icarus

ethicalpaul

I haven't measured it but it's typical cold city water

Can you share a screenshot of your phone app that interfaces with it? Mine shows the mode it's in. We can see how our units compare.

When I see you say this:

> As such, they are not engineered for maximum efficiency

Those words make me nervous, because in my other post I talked about how on their phone interface you get a nagging warning that tells you to choose "maximum efficiency" but it really isn't. You have to choose "HEAT PUMP ONLY"

Believe only the bottom mode indicator on this screen:

Solid_Fuel_Man

I wonder if it's a different generation, still allowing resistance heat even when in heat pump only mode. 

I know the Bradfors White units have EEV's (electronic expansion valves) which would be the best job of keeping the refrigerant circuit at the most efficient pressures for a very wide range of conditions. 

I assume the Rheem has an EEV as well. If it's a typical capillary tube system then its just decent at best. 

I see what you are trying to do with what you have. 

However, a single larger unit will certainly be the most efficient from a refrigeration standpoint. A larger tank would have the condenser at the coolest point longer than two tanks in series. When I say efficient, I'm taking watts in to hot water out. 

ethicalpaul

Yeah I wondered about a possible generational difference too. You can see on my screenshot I have a Gen 4. How about you, @Icarus ?

Icarus

I’;ll have my partner confirm a screen shot....

Tony

Icarus

EPaul.

We did this as test. We turned of the first tank before the evening load last night, then left it off over night to let the new water equilibrate. Then we turned it on this morning, in “heat pump only mode” and this is what we saw.


We can confirm when it is running on HP or resistance by looking at our single circuit emporia meter. I am wondering if yours is actually slipping into resistance with cold water and because you can’t monitor current draw in real time, you are unaware? Or maybe you have a different version, or perhaps we are doing something wrong?

Icarus

Jamie Hall

Perverse gadget.

ethicalpaul

Were you able to see what generation it is?

what temperature were your top and bottom sensors reading, did you happen to see?

the reason I think mine stays in HP mode is because of that time I pressed that “enable” button and I saw my daily KWh go way up

finally, did you turn it off at the breaker, or via the control panel? I’d like to match the conditions 

Icarus

Ep, I sent your question to the man who controls the unit. I’m not sure he can see the bottom or top temp, something we commented and complained about. Possibly he hasn’t delved deep enough into the set up, but that would surprise me. (I know I haven’t not being on site) Thanks for the ideas...keep them coming, I’ll keep you posted. Icarus

jumper

KISS. Pipe in series. Low demand or high ambient downstream heater stays off. Insulate and then insulate some more.

Icarus

Jumper...interesting idea, and quite simple. Still won’t solve the incoming cold water move to resistance, and...still can’t lower the set point below 110f.

Icaru

ethicalpaul

I just don’t get the lower set point desire on the first heater. Who cares which heater heats the water from 110 to 130 or whatever? They both use the same mechanism and energy cost, right?

Gsmith

Seems like the big problem is that the OP cannot get the units to run in heat pump only mode. I’d focus on that. Maybe the mfgr can confirm or not if these particular units can be set to heat pump only. If so just set both to heat pump only with the desired water temp the same and pipe them in series. The first one may do most of the work most of the time.

Icarus

The reason we “care” is that there is considerably more energy used to heat water to a higher temp. In other words it takes more energy to heat the 30F delta from 60-90f than from 90-120f. (Using a heat pump, using resistance, aside from losses it doesn’t matter) The other reason is stand by loses. The concept is to heat just the right amount of water at just the right time using heat pump only.

I may indeed be making this too complicate, but as Gary suggests, if I could simply force the water heater to run in HP mode, regardless of condition, and or drop the set point of the first heater I win.

Icarus

ethicalpaul

I love this discussion so don’t think I’m being picky, but how can it matter efficiency-wise which one does the heating to get the water to its final temp?

I can see a very marginal benefit from reduced standby loss in unit #1 but that could be eliminated by having one unit. 

If I were in your place I would shut off the first one, let it be a “room temperature buffer” (albeit not a very good one due to the insulation) and have it be a spare. If #2 fails you can plumb around it with pex and have a nice backup. Expensive buffer tank though!

Icarus

Paul, I don’t think you are being picky, but the reason it “matter” is the COP curve of the HP. It is fundamentally better the lower the Delta T. Unlike a conventional heater, it is not a simple 1/1 BTU/kwh ratio.

The gist here is to prevent either (or both) tanks from reverting to resistance every, and by getting the bulk of the btus in the bulk of the water at a lower Delta, one only has to raise some of it to say 130, instead of all of it.

The reason we have chosen (for now) to raise the final tank to 130 for the demand hours is to give a bit more capacity. If we could assure that neither would go into resistance, the issue becomes clearer. And yes, in that case a single, larger tank would have been an easier install, and might have been cheaper. That said, the current set up is what we have, and it affords some more flexibility than a simple one tank set temp fits all...if we can do so.

The really frustrating thing (as I have said) is that we can’t find a way to assure that they won’t go into resistance mode, and we can’t lower the set point below 110. I can see all kinds of scenarios where having a set point below 110 is useful, but I suspect that Rheem might be concerned with legionella?

Thanks for the thoughts, I hope I don’t sound crazy...Icarus

Jamie Hall

A few ifs here for you,, @Icarus (after all, you're doing the work, not me, so I can be a pest, no?). If you can persuade those things to work without the resistance elements coming on (hmm... I wonder... since you're playing with them anyway, any warranty is history... I wonder if you could alter the electrical circuitry to substitute a much small wattage load for the resistance elements -- like maybe a 40 watt bulb -- and let the silly thing think its running the resistance element? If it runs the heat pump at the same time, that is?) I think I would add some piping and valves so that you could run them either in series or in parallel, and compare the energy use. Wouldn't be that hard... in the spirit of experimentation.

psb75

These units have an initial "big heat-up" program esp. designed for the installation and filling. At that time...they "lock" into hybrid mode using the compressor and the resistance elements as well, in order to make a lot of hot water fast --going from a full tank of ~40F degree ground-water temp up to lots of "user- friendly" hot water to satisfy a waiting homeowner who just had a new tank installed. What I'm getting at is that I think the engineer/designers have given those resistance elements some kind of "priority" to kick-in if the tank gets too cold--even if set in "heat pump only" mode. It may default to the "start-up program." Think about...power outages. What happens when there is a 4 day power outage? I suspect the tank would go into start-up mode when powered-up again.
My bigger point is: we DON'T KNOW how they're programmed!

Solid_Fuel_Man

@Jamie Halljust beat me to it! Resistor in place of the heating elements will fool the board into thinking there is an element. 

ethicalpaul

I like what @psb75 said about startup mode. This is why earlier I asked if you had powered off the unit in your test.

you may find in actual operation it stays in heat pump only mode.

and I know about the COP curve stuff, but you can’t avoid it. One of your heaters is going to be hitting every point of that curve, right?

Icarus

Will check what mode it is in, if indeed there is a start up mode.

Also, it would be interesting to wire a dummy load to see if it reacts. Have to find a 240 vac light bulb, or a small 240 vac heater. Seems kinda silly though, but worth a thought.

Icarus

PS to Paul. Gotta think and rethink the COP curves. What my intuition tells me and what actually happens in any real world might be very different. Thanks for the thoughts.

T

psb75

There is definitely a pre-programmed "start-up" mode. I have installed, filled and seen many dozen of them "start up."

ethicalpaul

I'm glad to find someone who has a lot of experience with these, @psb75. Not to hijack the thread, but how happy are you with these units? Mine has been flawless but I'm only a sample of one. There is a lot of fear, uncertainty, and doubt out there for sure but it doesn't seem to be based in actual experience.

Icarus

Psp75. I have had my partner in this, kill the unit at the breaker and re boot and restart it. I am awaiting an up date. He also mentioned he gets an occasional (once a week) error code “t 009” which translates “ defaulting to resistance, compressor wiring may be faulty.” That is why I suggested a reboot.

Paul, I think the owner is very happy, as he has cut his net energy consumption in raw btus by 75% which is pretty impressive given that we are not in the “best” time of the year for HPWH. I will keep you up to date. No clue yet as to long term reliability.

Interesting question is who will comber a warrantee repair if needed with a non dealer install?

Icarus

psb75

I've not had anyone be unhappy with the install of either Rheem or Brad White units of 50gal., 65gal. and 80gal. I have replaced 4 motherboards--2 Rheems and 2 Brad. Whites. This was after many dozen installs, for the past 10yrs. That is the only type of repair I have done. If the screen goes blank with power to the unit--the motherboard is "cooked." They are covered under warranty. But not labor.

Icarus

PSB, Thanks for that. I am concerned with the fault code t-009. But we will see if that recurs. Icarus

Zman

Please post the results of the study when you are done. To be fair, unless you are located in a warm climate where the heater is helping cool the building, the cost to replace the heat these units are stealing from the conditioned space needs to be part of the equation.
I am enjoying reading this thread, just not sure I "get it".

Icarus

Understand that the HPWH is simply stealing from the room. In this case the room is an unfinished basement where the temp is pretty much moderated by the slab and foundation wall. We introduce air when the Delta between indoor and out door is greater (ts tat controlled in the summer) and will vent as needed the same way.

I’m not sure I “get it” either. This started as a simple replacement water heater, and now we are testing different strategies to maximize efficiency.

We will keep in touch.

Icarus

ethicalpaul

The heat that mine takes from the basement was getting sucked away by the foundation and the poorly-insulated sill area anyway.

Insulating my steam mains took away way way more heat than my heat pump water heater. It's a non-issue.

Solid_Fuel_Man

They work fantastic in a 100 degree boiler rooms. 

I've replaced several in indirects in commercial buildings (just used for hand washing) with HPWH. 

TAG

Do they still make one with a damper and vent tube attachment? For a while they were doing a bunch of them at the beach -- I always thought that was a decent idea as you could pipe the hot summer air into the unit. I have NG .... most of the new houses are all electric w/ heat pumps

WMno57

Icarus said:

Also, it would be interesting to wire a dummy load to see if it reacts. Have to find a 240 vac light bulb, or a small 240 vac heater.

Wire up several 120v bulbs or heaters in series.



The above could be dangerous. Do not attempt this if you are a plumber or HVAC tech. This should only be done by qualified strivers, raycers, and mad scientists.

http://www.priups.com/misc/update-2005-09-2.htm

www.priups.com and the above photo are not mine. They belong to Richard Factor.

Icarus

240 ac bulbs actually are available quite readily.

Haven’t tried anything yet...Icarus

Macro

Are you not required to set your HWT at 140? We are code mandated here and have to mix down to 120 out of the tank. How do these HPWH get around that? I've actually never even heard of them before and I've gone down a bit of a rabbit hole now haha