Preheat DHW with heat exchanger off hydronic baseboard zone

apsinnott

Hello,

Let’s start off with what I’ve done to my 3k sq ft split home on Long Island, NY. It originally had a oil burner heating a 2 zone hydronic baseboard system with a gas hot water for DHW.

Over the last 16 years I have re-zoned my baseboard hydronic heating system into 4 zones, added a zone controller with an outdoor reset control, added 2 multi-zone heat pumps with 8 inside head units that provide the majority of my heating and all of my cooling. The heat pumps ability to provide heat slows down in temps below 40F which trigger the thermostats for the hydronic system. I removed my oil boiler and replaced it with a 55kw electric boiler which is sized to be capable of acting as a redundant secondary heat system but with the heat pumps on it was only activating during the coldest periods. Believe it or not from one year that I ran entirely on the mini splits to the next where I had both systems I actually lowered my electric bill using both. Part of my re-zoning the hydronic heating zones was to insulate and move all the pipes inside insulated walls so I can let the hydronic system basically be shut down most of the year. I plumbed the electric boiler with a primary loop and in the zones I updated my circulators to delta T ECM units and wired their sensors for each zone and the boiler.

For DHW my gas hot water heater was 15 years old. I finished my basement and along the way heavily insulated the space and built my boiler room into a confined space. I eventually decided not to install an intake vent for the gas hot water heater and removed it. I installed a 50 gal indirect hot water heater as the 5th zone of the electric boiler. I have it plumbed in 1” pipe off the heating manifold and I’m using 1” pipe for the mixing valve loop. I have the indirect aquastat set to 145 and my mixing valve set to 130. I also insulated all of the hot water pipes in the boiler room.

Now that the electric boiler is used more often my thoughts turned to pre-heating the domestic hot water from the street. I’m open to suggestions, but I have zeroed in on:

-Adding a 5x12 20 plate heat exchanger with 1” connections off the intake line of the indirect
-Plumbing the other side of the heat exchanger to a parallel loop off the longest hydronic zone

In this way when the priority zone for DHW calls it will pump the boiler water through the indirect, but also pull through the baseboard zone to preheat the DHW using the heat exchanger. I was planning on using flow check valves off of the baseboard hydronic zone to add in this parallel loop. Most of the year these baseboards will likely be cool and I’m aware I won’t see a huge rise in temperature then.

Thoughts?

Thank you

Adrian

GGross

Why? Isn't the same appliance that heats the baseboard, and will be used to pre-heat already heating the DHW by means of an indirect water heater?

apsinnott

The baseboard zones are home runs, each with their own circulator and flow check valves. They are rarely used now and my thought was if one side of the heat exchanger was 50 degrees from the street and the other was at least 70 from an idle zone there would be some benefit in further conditioning the potable water before it hit the indirect. 

Yes, this is a sanity check. I’m open to other suggestions to preheat the DHW. 

Hot_water_fan

I see no reason to do this. Get a Heat pump water heater if you want to save electricity.

Believe it or not from one year that I ran entirely on the mini splits to the next where I had both systems I actually lowered my electric bill using both.

I can believe this. But how did HDD and $/kwh compare between the two years?

Yes, this is a sanity check. I’m open to other suggestions to preheat the DHW.

Something like this would work too.

https://www.homedepot.com/p/Power-Pipe-3-in-x-36-in-Drain-Water-Heat-Recovery-Unit-R3-36/203455990

apsinnott

I looked at heat pump water heaters first, but the boiler room is a confined space next to a now furnished living area. Noise and airflow were a concern. 

I really like the concept of the drain heat recovery but my main line is on the complete opposite side of the house. 

As far as $/kwh as you can imagine not all of this was done at once and getting a stable read of efficiency is only just being possible. But the mini splits have been very good and the electric boiler was great as standby. I just want to eek every bit of efficiency I can find for DHW until I can get to doing a solar/battery setup for the whole house and eliminate my utility bills altogether. 

Hot_water_fan

 I really like the concept of the drain heat recovery but my main line is on the complete opposite side of the house. 

That’s fine! It doesn’t have to preheat the water entering the indirect - it can also preheat the cold side of the shower (with proper scalding protections of course). Right now, your DHW unit costs are high because you’re using an electric boiler, about the least efficient way to heat water. Almost anything pays back in this scenario ha. 

hot_rod

Are you trying to get the incoming water to 70 with just a single pass through a plate HX?

I think, depending on the flow rate, you will need a much larger delta T

Use one if the online plate hx sizing programs to enter different variables

https://www.caleffi.com/sites/default/files/media/external-file/Idronics_29_NA_Heat%20exchangers%20in%20hydronic%20and%20plumbing%20systems.pdf

apsinnott

hot_rod said:

Are you trying to get the incoming water to 70 with just a single pass through a plate HX?

I think, depending on the flow rate, you will need a much larger delta T

I realize it’s not the design purpose of your typical heat exchanger. I chose the plate heat exchanger as it supposedly can transfer a difference as small as 1 degree. I know flow rate will change dependent on usage. The reality is I have a 3’ section of the 1” return pipe on the side of the indirect and an empty corner of the room next to it which is right below where I could T into a zone so I started wondering if this was feasible. I would never do this, but I replaced all the copper pipe and baseboards in my house so they are technically lead free so I briefly considered what it would mean to run the DHW through the heating zone! I must get some sort of temp rise following a hundred or so feet of copper and probably 30’ of baseboard! And a case of legionnaires! But no, that’s kind of crazy to consider how to do properly, the answer is a heat exchanger if at all, and I just thought there must be some way to take the edge off the typical low street temp. It would be a cool experiment.
Ultimately solar and battery is my answer to reduce costs, which are higher now, but not as much as you would think since I have everything well insulated and staged for heating. My boiler room is so well insulated it holds the heat by itself! I wondered if just running a coil of copper, or more likely a water to air heat exchanger with an efficient fan would also help take the edge off the DHW.

Jamie Hall

About all I can say is that you'll use the same total energy heating your cold water to make hot water no matter how fancy -- or complex -- a setup you use. In fact, you may use more, due to inefficiencies in more complex arrangements, which have this dismaying tendency to add up.

On the ultimate dream of solar PV and battery to take you off grid. It's a lovely dream. I've had it, and so have many other people. However... if you are assuming that somehow you will heat your house and hot water that way, first, you need a very large PV array. Depending, of course, on where you are located. But anything in the northeast quarter of the country has only three usable hours of sunlight, on average, per day. Some days a lot more, of course. Some das... a lot less. You don't mention what your heat loss is -- that's a key number, of course, and will be the major determinant in how big an array you need, but both it and the batteries will be substantial. Just to give you something to chew on -- and you can verify the numbers yourself -- a building with a 50,000 BTUh heat loss will require around 300 KWh of stored power per day for electric resistance, or around 150 KWh if using a heat pump. Or either a 100 or a 50 KW solar PV. But then... you will need to be able to run that when the sun doesn't shine. At night, or -- again in the northeast -- during a more or less normal three day storm. So you are looking at somewhere around 500 KWh stored power at a minimum. A Tesla power wall has about 13.5 KWh capacity, so you're looking at about 40 of them. Minimum.

Think about it.

hot_rod

I have a 6200w grid tied PV array, facing mostly east, about 4800 W realistically is the output I see on the Sunny Boy
5 ton AC electric stove and dryer. My highest bill during  almost non stop AC was 20 bucks

Hardly worth looking at batteries if you can grid tie. I missed the great feed in rebate at par by a year. I think I get $.03 for feed in .13 to purchase

A single thermal panel  and 60 gallon solar  tank cover over 50% of my dhw
Ill check my gas bill

Jamie Hall

We're fortunate -- if all goes well (yeah, right) we will be putting in a 15KW grid tied array next spring -- and in our state the net metering is set up to pay the panel owner the same per KW as the panel owner pays the utility (in our area, $.31 per KWh). This will give the panel a decent payback period -- probably shorter than the life of the panels (we hope anyway...). The panel was sized so that, in theory we should generate just about as much power over a year as we currently use.

Needless to say, I think, we are not even trying to use it to heat the place!

Ahomeowner

I second the idea of a heat pump water heater. You seem to be very interested in energy efficiency, as am I. You said that your boiler room is well insulated and very warm with waste heat from the boiler. That heat could be going into your DHW!  At a very low cost!  The insulation will also reduce the noise that escapes into the living space.  The heat pump water heater will also provide a small amount of dehumidification and cooling to the basement, which is advantageous in the summer. I don’t know what your basement floor plan looks like, but as I am picturing my basement, I would bump the wall out a little bit to make the mechanical room a little bigger. It’s common to make mechanical rooms as small as possible so there is more room left for living space, but a large mechanical room had its advantages.  It can also provide extra storage space if it’s a little larger.

Congratulations on getting your pipes into the conditioned space. I did this too and it made a big difference for many reasons!

apsinnott

I appreciate the level setting from amongst you. Especially about what to expect from solar and the various realities. I have been waiting for products to mature in that arena. Check out some of the server rack batteries out there now- you can store a lot of energy to capture your peak production as opposed to tying it directly to use. Storage much less expensive than powerwall and twice the capacity. Also, bidirectional charging is here/coming to electric cars which means your car can become a house battery too. Or supplemental a house battery. All very interesting.  

In terms of heat loss, it’s not a large home. I replaced every window with low e models, every door, insulated every 2x4 wall with R-15, sealed the stud bay edges with spray foam and covered with 5/8 Sheetrock. It’s a split with a hard to insulate attic so I have 2” foam board against the interior walls in the attic and likely r-60 covering the attic floor from blown-in. My basement is also 2” foam board with studded insulated 2x4 walls. Every floor has its rim joists insulated against exterior with 2” and spray foam and insulation in the joist space. The house also has a real brick veneer over 80% of the siding and the rest is insulated vinyl. It’s not perfect or net zero but it’s sealed tight and well insulated. 

I’ll see how I far with my electric boiler for DHW, I used a 50 gallon stainless tank with 2” insulation and used k flex insulation on all the boiler hot water pipes. The rest of the pipes throughout the house have standard foam insulation. Watching it over the last week it seems to only have to cycle about 3 times with daily family of 4 usage. Between cycles the boiler is at about 120F so it doesn’t lose a lot of heat either. The boiler room is insulated and I had built it as fire resistant with 5/8” walls and a steel door because I didn’t trust the old oil boiler. 

So perhaps there’s nothing else to do for preheating the DHW in my config but I’m still open to any concepts you think might work in my unique configuration

GGross

Personally I do not think you would gain in the efficiency department from the proposed pre-heat, but you would potentially lower the effectiveness of your heating circuits. Your current setup will probably provide you with more hot water than you would ever need, that boiler can provide a ton of BTU/hr. heat pump water heaters are really great when they work, lots of good experiences with them, but it would require a whole new tank, and I do think your recovery would either be slower or it would kick on resistance heating elements which would be equal to the cost of running the boiler anyway. Your current boiler is equal to a standard 199MBH gas boiler which is a ton of energy available to heat hot water, for reference standard water heaters are 40-50MBH. I think in your situation if I were you I would keep the setup you have currently, and in the future I would consider changing the electric boiler to an A2W heat pump unit if electric costs are out of control.

apsinnott

I should add that part of how I transition from mini split to the the hydronic system powered by the electric boiler is by using the warm weather shut down feature of my outdoor reset control. I have it lowered to 40 degrees which keeps it off most of the time. Below 40 the mini splits start to slow and I rely on the thermostats to call for heat from the electric boiler as the temp lowers further. 
The priority zoning of my zone controller is now used for the DHW, which overrides the WWSD to heat up the indirect occasionally. 

This was why I had the thought of using the mostly unused hydronic baseboards as a heat exchanger. I realize the baseboard elements don’t work great with minor temp differences, but I thought since plate heat exchangers do, maybe there was a chance of making a difference to the cold street temps.

apsinnott

GGross said:

in the future I would consider changing the electric boiler to an A2W heat pump unit if electric costs are out of control.

Thank you for the insight. I sized the boiler to act as a complete standby unit in the event the mini splits did not work well enough or broke in the winter. Even though I have 2 for each side of house. I guess I like redundancy! 

I looked at A2W when I bought the mini splits, must have been 5+ years ago, and they were rare. I’m glad they are now more available. I think heat pump including A2W plus electric element backup is the likely future. Probably a tech company will figure it out and then offer it with in app subscriptions!

Oil will become unsustainable, gas will pop in pricing and electric will continued to be offset by innovations in solar/battery in my opinion.

Hot_water_fan

Probably a tech company will figure it out and then offer it with in app subscriptions!

Ha! This is not a software problem, I hope they leave it to the experts. 

This was why I had the thought of using the mostly unused hydronic baseboards as a heat exchanger. I realize the baseboard elements don’t work great with minor temp differences, but I thought since plate heat exchangers do, maybe there was a chance of making a difference to the cold street temps.

The other issue besides the inefficiency and cost is that you’d be running cold water through your baseboards, effectively turning them into radiant cooling systems without a system for handling condensation. It could very effectively grow some mold!

apsinnott

Hot_water_fan said:

The other issue besides the inefficiency and cost is that you’d be running cold water through your baseboards, effectively turning them into radiant cooling systems without a system for handling condensation. It could very effectively grow some mold!

Yeah no bueno! That would likely happen if it was the actual street water flowing through the baseboards. But with the heat exchanger I thought it might be unlikely since I'm only circulating a closed loop inside the home and only when I'm calling for DHW.

Since I have 1" pipe at the indirect I did do some math to see if I used bare 4x4 1" finned baseboard elements and It came out to something like 60' to get a 10 degree increase assuming ambient of 75 and input temp of 50. Which is where the idea of what if I used a plate heat exchanger and the 150' of 3/4" elements I already had sitting idle inside the house.

I also looked at air to water heat exchangers, again 1" piped, likely 20"x20" to get the flow rate and either use it passively or with a blower fan (yes I know moving electric use from one device to another). Concerns would be effectiveness and letting the potable water sit in a radiator, etc, etc.

I suppose there comes a point when no one is doing something it's probably for good reason, I'm just surprised I can't find an easier solution to temper the street water by 10 or 20 degrees as it seems that would be worthwhile, especially if it was something passive.

Damn laws of thermodynamics.

EdTheHeaterMan

All that I could think of while reading thru the discussion was: The First Law of Thermodynamics is that "energy can not be created or destroyed ..." and a "BTU is a BTU is a BTU."

You have electric resistance heat that will generate 3412 BTUh per kWh. And how you put that energy in your system to heat your house ain't gonna matter much. As you take BTU energy from one place and put it in another place, the place you took it from will require those BTUs to be replaced by the kWs from somewhere.

Since heat pumps do not use resistance heat to convert electric energy to heat energy by way of resistance elements, they are more efficient when it comes to transferring heat. The heat in the source atmosphere turns into heat energy for the indoor environment by using the refrigerant cycle.

That said, why would you NOT consider an air to water heat pump as the source of DHW. You have chosen it for space heating already. Why not both space heat and DHW?

EdTheHeaterMan

Another thought about swapping out a Gas Automatic water heater for an electric sourced water heater...

Hypothetical numbers based on my gas bill and my electric bill
If it takes $400.00 per year to make hot water with a gas water heater, and it takes $1100.00 to make the same amount of hot water with an electric water heater, but the gas heater causes infiltration to increase my heating needed, then will I save any $$$ by closing off my gas heater vent completely?

1. How much will lowering the infiltration save on my heating usage?
2. How much will the new equipment cost to heat water with only electricity?
3. Is the operating cost difference enough to offset the cost of operation of the replacement water heater?
4. If it is enough to offset the cost or operation, then is there any extra available to offset the cost of the equipment?

I have used this calculation to induce customers to get rid of electric heat and put gas furnaces and boilers in their homes for over 1/4 of a century. I find it hard to believe that you have not already shot yourself in the foot with the removal of any gas appliances.

Just some random thoughts from an old man.

Hot_water_fan

Who’s using $400/year of gas for DHW? That seems extreme! No doubt electric resistance is often more expensive for DHW than gas, but not hundreds more. 

EdTheHeaterMan

Hot_water_fan said:

Who’s using $400/year of gas for DHW? That seems extreme! No doubt electric resistance is often more expensive for DHW than gas, but not hundreds more. 

The same person who replaces the Gas with Electric and ends up paying triple using electric! I believe this is an example of missing the point

Hot water fan, is this more in line with your thinking?

If it takes $100.00 per year to make hot water with a gas water heater, and it takes $370.00 to make the same amount of hot water with an electric water heater,

Now I will wait for the next comment

Who’s using only $100/year of gas for DHW? That seems too low!

Hot_water_fan

$100 is more like it @EdTheHeaterMan! The point isn’t that electric resistance can be more expensive. It certainly can be! It’s whether or not it’s a meaningful difference - even if I spent $200/year less on hot water using gas, that doesn’t cover the meter fee so it’s a economic loser (for this scenario, not all the time). 

$400 was just eye popping. That’d be like seeing a $30 whopper 

EdTheHeaterMan

Hot_water_fan said:

Who’s using $400/year of gas for DHW? That seems extreme! No doubt electric resistance is often more expensive for DHW than gas, but not hundreds more. 

Did some homework on South Jersey Gas billing, (where I used to live) and from their website using their price/cost numbers and then using the average cost to operate a typical gas automatic water heater where they project 156 therms usage per year, I was a little light on the $400.00 guess.

But I have been retired for over 3 years and have not made a real time comparison for a customer for some time.
Turns out I would be closer to $500.00 in today's numbers

That is the bill if there was only a gas water heater, so the $14.00 is applied to only that appliance. If there were other appliances, that $14.00 would be pro rated to the other appliances. But I usually just did the total bill divided by the therms use thru the meter to get the customers real cost per therm. (or per kW on electric bills)

So there are some folks that may pay $400.00 for hot water in a year

Hot_water_fan

Ah including the fixed costs, okay. But then electricity doesn’t pay that. 

All in, close to $2/therm stinks! Especially when $1.17 is delivery. Seems like that’ll never go down, even if the gas itself does 

apsinnott

I’m not trying to start a thread war on fuel sources… I rarely post on forums as I have to establish all of the pre-work and conditions to get to a the “oh that makes sense moment” for the level I do my projects at. 

The reason I led with all the things I’ve done was to try to get ahead of some or the comments like “just put in a gas combi” and to try and establish my DIY chops before asking a specific question I was looking for perspective on. This has been over a decade of improvements stacked on one another, each being necessary at the time and ultimately supporting one another. 

In another forum a while ago I asked about using the electric boiler as a standby, not supplemental, heating system for my mini splits. Everyone told me to do something else, including getting rid of the hydronics altogether. Disregarding the re-zoning, control system, and detail work (like insulated in wall pipes) that had served very well and ultimately contributed to the success of the new decision where it lowered my electric bill. No one predicted that. 

In this case, yes I could have done a high efficiency gas hot water heater tankless with an indirect. But I would need to hire someone with permits and delays to get the quality and comfort I would want at three times if not four compared to what I spent to DIY. 

I’ll deal with the consequences of my decision regardless. I was just here to poke your minds if there was any supplemental option to preheat DHW and tossed my idea out to get started.

EdTheHeaterMan

You think that customers make that distinction? Since I'm only paying 100.00 for the gas then it is OK to forget the rest of the bill?

By the way, the Electric company does the same thing.
That comes to over $76.00/per month without even adding in the actual cost of generating electricity of $0.086 per kWh. But you can select other suppliers at different rates.

SO, Like 3 times the amount that of Natural Gas cost (in my neighborhood)

Hot_water_fan

@EdTheHeaterMan you misunderstood me. I’m saying in a scenario where the options are: pay a fixed fee for gas and a variable fee for gas, if my only applicance is a gas water heater, then the fixed fee enters the equation. People are not considering ditching electricity, because it’s so intertwined in our lives, so the fixed part of the bill is irrelevant.

jumper

Drive yourself crazy guys. For example when using a heat pump for pool heater is ideal location for evaporator inside or outside? Inside helps with A/C but outside can be warmer so.....

EdTheHeaterMan

jumper said:

Drive yourself crazy guys. For example when using a heat pump for pool heater is ideal location for evaporator inside or outside? Inside helps with A/C but outside can be warmer so.....

YES, I agree with your selection of evaporator location!

EdTheHeaterMan

Hot_water_fan said:

@EdTheHeaterMan you misunderstood me. I’m saying in a scenario where the options are: pay a fixed fee for gas and a variable fee for gas, if my only applicance is a gas water heater, then the fixed fee enters the equation. People are not considering ditching electricity, because it’s so intertwined in our lives, so the fixed part of the bill is irrelevant.

What do you mean by the "Fixed"part of the bill? Is that the part that is not associates with the amount of energy used that is measured by the meter, or the part that is not the actual energy portion like the Generation Charge on the electric bill or the BGSS on the gas bill?

I believe that if you pay 100.00 for something then you pay 100.00 for it, regardless what you call the parts of the invoice. But that is just me. I grew up in the Fuel Oil business. We didn't charge $1.00 per gallon for fuel oil then add a delivery charge and a delivery tax and a energy tax and a loading rack fee, and a refinery surcharge and a overhead compensation fee. and a bulk plant operating cost. then give you a $400.00 bill for a 100 gallon delivery of fuel. Then try to convince you that you oil only cost $1.00 per gallon.

I believe that if I advertised

FUEL OIL FOR SALE $1.00 PER GALLON

plus shipping, handling and tax
and everyone else was selling Fuel Oil for $4.00 per gallon, that I might end up in court over this marketing practice.

Just Sayin'

Hot_water_fan

What do you mean by the "Fixed"part of the bill? Is that the part that is not associates with the amount of energy used that is measured by the meter,

Yes obviously! The part that is the same every month, aka fixed. It’s part of every bill until you decide to cut the cord and then gas is gone. 

I believe that if I advertised 
FUEL OIL FOR SALE $1.00 PER GALLON
plus shipping, handling and tax 
and everyone else was selling Fuel Oil for $4.00 per gallon, that I might end up in court over this marketing practice. 

If you sold gas or electricity you could get away with it . It’s easy to understand 

Jamie Hall

Our electricity bill is really three parts (people never read them anyway...). A fixed monthly fee, which is really quite reasonable, which is intended for normal low level maintenance of the local grid -- the lines in the highway, poles when hit by errant motorists, the odd transformer, that sort of thing -- then a generation charge which is what the supplier pays through directly to the generating company (and which is the subject of much gamesmanship and advertising and somewhat sneaky ploys on the part of the generating companies) and another fee to the distribution company, which pays for major grid upgrades and maintenance. Seems fair enough...

EdTheHeaterMan

The Fixed part of my gas bill is $14.00. The other two are determined by the gas meter. They are not fixed. The same goes for the electric bill. So anyones cost for gas or electric that is subject to the number of turns on the meter are not fixed. But the actual supplier of the product like the electric generator or the BGSS portion of the gas bill is not the only rate I considered when I said $400.00 as a hypothetical annual cost. If I yank the $14.00/mo. out of that equation, I'm not that far off like $350.00 annual.

Still does not explain why someone would change from a more expensive energy source to stop infiltration. especially when the EPA has determined that we need some ventilation and fresh air in order to keep our indoor air quality at a healthy level. Besides , we all found that out over 100 years ago when the Spanish Flu caused so many deaths due to "Bad Indoor Air". Have we learned nothing from the past.

And don't even try to tell me the Earth is Round. There are members of the Flat Earth Society all over the Globe that will fight you on that point!

apsinnott

Uh, so much for any discussion on preheating DHW?

Robert_25

apsinnott said:

In another forum a while ago I asked about using the electric boiler as a standby, not supplemental, heating system for my mini splits. Everyone told me to do something else, including getting rid of the hydronics altogether.

I am not surprised. Most of the country doesn't use hydronic heating at all, and even in areas where it is common there are plenty of HVAC people that don't understand it. I think you made a smart choice.

Back to the plate HX - don't bother. Although it might scavenge some heat out of the living area for DHW, that heat still has to be replaced. A heat pump water heater would be a much more direct approach.

Hot_water_fan

Uh, so much for any discussion on preheating DHW?

It’s not really a good idea! Try the drain heat recovery, reduce flow, or get a heat pump

jumper

apsinnott said:

Uh, so much for any discussion on preheating DHW?

The OP is on to something that one can improve by heating at colder temperature. That is what preheating is about. But improving thermal performance goes to diminishing return. Even on millions btu s industrial systems.

apsinnott

Okay so here’s a few thoughts of mine to see if there’s still any curiosity left in this thread:

When I re-did my DHW from a standalone gas hot water heater to an indirect off the electric boiler I added a backflow preventer, a potable expansion tank, and then a mixing valve on a 1” circuit back to the indirect. I previously only had a backflow and expansion tank on the boiler feed. The area I’m focusing on to pre-heat the DHW is the 1” pipe inlet to the indirect which is also the return of the mixing valve (so there is already some heat added as aquastat is 145 and mixing valve is 130).

If adding a plate heat exchanger to that line and running the other side as a loop through the nearby mostly unused hydronic zone is nonsensical in terms of limited heat gain (likely) and potential condensation. Maybe that’s less likely, I guess it is possible if the zone had just been used for heating and then there’s a call for hot water, bit even then I would think that would actually make the heat exchanger work better because the delta T is higher. But I digress, since then my thoughts returned to creating a different passive solution.

Firstly, if a potable expansion tank is added on the cold side, could someone not create a “temper tank” for DHW by using a well storage tank sized to their typical peak usage instead of a 5 gallon standard expansion tank? My tank is on the 3/4’ feed after the backflow preventer so I probably wouldn’t replace it, but just add a 20 gallon well storage tank on the aforementioned 1” circuit. Maybe add a copper coil before to pickup as much of the higher ambient air temp from the boiler room.

So in closing, could I combine a well storage tank with a copper coil to create enough pipe length and storage to allow the ambient temperature in my boiler room to influence the cold water inlet by 10 maybe 20 degrees?

I’m really dabbling in the hypothetical now, but my curiosity was piqued and I’ve been thinking through passive designs in my head.

apsinnott

And yes I realize a common well tank has a single fitting so stratification is poor, it was more a question whether an oversized expansion tank has any value preheating DHW as it appears finding a 20 gallon potable water tank is not a readily available thing.
I assume in today’s age no company wants to be sued for allowing a consumer to create a potable water problem nor am I a typical consumer as I’ve been willing to spend more on residential plumbing like it’s a hobby.
Ideally a 20 gallon uninsulated stainless water tank would work, which don’t appear to readily exist. I see a lot of people use or suggest standard water heaters they switch off but that doesn’t appeal to me.
The thought on the coil was to allow for more ambient heat transfer, I can more readily get them, but it seems a waste to have just add the coil alone.

Hot_water_fan

@apsinnott you can preheat the DHW, but if the preheat is coming from a building that’s heated by a resistance water boiler, you’ve just spent a lot of time and effort to gain nothing. 

I think setting some benchmarks might help: 
1. The low end: you transfer electric boiler heat to water feeding an electric boiler powered indirect. 0 to negative savings. 

2. Same as 1 but you’ve taken the heat from the heat pumps. COP just above 1, slight savings. 

3.  The high end: a heat pump water heater removes heat during the summer and during the winter, removes heat generated by a heat pump. COP max of about 4, about 2 for the winter when it’s scavenging from the heat pumps. 

There’s so little to gain here.