Add Heat Pump Water Heater to Existing Indirect Oil Boiler Hot Water

MasterC

Hi! So, I have a Buderus G115/5 Oil Boiler that provides radiant heat to the house, as well as an integrated LT160/1 42 gal. indirect hot water storage heater. We run out of hot water on occasion from showers and etc., and we cannot fill up our big Jacuzzi tub without running out. We are also adding some renters to the basement, which will place additional demands on the hot water. I am considering adding a 50 gal. hybrid heat pump/electric hot water heater for extra capacity and perhaps savings on oil.

Will this work? Is it a good idea? What's the best way to plumb the two together?

I have posted previously with some heating questions about the boiler, and I have pictures of the boiler room here.

Thanks for your help!

HVACNUT

We can't really decipher if everything is sized and operating the way it should.
What you proposed can be done, but do you need it?
It looks like an L8124A aquastat on the boiler. What are the high, low and diff settings?
The Independent Energy ODR. Does a call for domestic hot water override ODR?
What temp is the indirect set to, and is there a thermostatic mixing valve?
Is the indirect circulator's GPM correct?
Without the jacuzzi, there should be sufficient hot water.
Whenever a HO has a jacuzzi, I recommend a reverse indirect water heater.

MasterC

Thanks for the reply! I'll see if I can address your questions.

What you proposed can be done, but do you need it?

Good question - I hope to find out! Should a boiler with indirect hot water setup like this should be able to keep up with most any household demand? Say, 6 people showering in a row? Or two showering at once? Or running a bath or two for the kids followed by someone taking a shower? Also, in any case, would it be more economical to get primary hot water from a heat pump water heater than the oil boiler? After rebates, I can get one for under $200.

It looks like an L8124A aquastat on the boiler. What are the high, low and diff settings?

Looks like it's actually an L8148A. I don't see high, low, and diff settings, just one setting dial. See new pics here, here, and here. I also noticed that the circulator terminals are not hooked up to anything, which seems odd. What tells the circulator to go then?

The Independent Energy ODR. Does a call for domestic hot water override ODR?

I could be wrong, but I don't believe there is an outdoor reset.

What temp is the indirect set to, and is there a thermostatic mixing valve?

I think it is set to just under 120 degrees. I do not think there is a thermostatic mixing valve.

Is the indirect circulator's GPM correct?

Hmm...how do I know? What should it be? I think this is the circulator.

HVACNUT

> @MasterC said:
> Thanks for the reply! I'll see if I can address your questions.
>
> What you proposed can be done, but do you need it?
>
>
> Good question - I hope to find out! Should a boiler with indirect hot water setup like this should be able to keep up with most any household demand? Say, 6 people showering in a row? Or two showering at once? Or running a bath or two for the kids followed by someone taking a shower? Also, in any case, would it be more economical to get primary hot water from a heat pump water heater than the oil boiler? After rebates, I can get one for under $200.
>
> It looks like an L8124A aquastat on the boiler. What are the high, low and diff settings?
>
>
> Looks like it's actually an L8148A. I don't see high, low, and diff settings, just one setting dial. See new pics here, here, and here. I also noticed that the circulator terminals are not hooked up to anything, which seems odd. What tells the circulator to go then?
>
>
>> Ok, an L8148A. High limit only. All your circs run from the switching relays on the wall. TT on the aquastat closes whenever there is a call from a relay and the burner starts as long as the temp is not above the limit setting.
What is the setting? Should be 180*.
>
> The Independent Energy ODR. Does a call for domestic hot water override ODR?
>
>
> I could be wrong, but I don't believe there is an outdoor reset.
>
>
>
>> Sorry, I assumed. I've never seen that brand. Do you have a manual for it? What is it? Maybe someone else can chime in on that one.
> What temp is the indirect set to, and is there a thermostatic mixing valve?
>
>
> I think it is set to just under 120 degrees. I do not think there is a thermostatic mixing valve.
>
>
>> Typical domestic hot water is set to 120*
A thermostatic mixing valve will allow you to set the indirect tank temp to 140-160 degrees, and the mixing valve will temper it down to 120* going out to the house. The higher temp, with the mixing valve will act as a larger tank because it's not depleting the tank as fast because it's mixing with cold. The higher temp will also kill bacteria in the tank (Legionella).
>
>
> Is the indirect circulator's GPM correct?
>
>
> Hmm...how do I know? What should it be? I think this is the circulator.
>
>
>> For that you'll have to check the manual for the indirect. It will give the required GPM and ft. of head.

Page 49 will give the specs on Grundfos circs.

hot_rod

First you need to define your DHW needs or wants. Two showers running at the same time could be 3 GPM or more depending on shower heads, water pressure, etc. The boiler you have may not be able to keep up with a load like that once the tank temperature is depleted.

One option is to use a 5 gallon bucket and determine what flow rate of the showers actually is.

For large loads either you store a lot of DHW for "dump" periods, or install a boiler and tank to generate continuous DHW. It is not uncommon in large households to see larger DHW loads than heating loads. Boilers typically size to heat loads, not DHW loads.

Once you confirm the tank is producing what it is capable of, then try to pin down the actual loads. Also if you are on a public water system possibly incoming water temperatures in the mid 30° range in winter months, that will make a big difference on DHW production.

So either limit the shower flow or time, or add equipment and components to match your desired loads.

MasterC

@HVACNUT

Ok, an L8148A. ... What is the setting? Should be 180*.

I'm not sure how to read where the setting is, but I took a picture of it. It appears to stop heating at about 185 or so - like shown in this video.

A thermostatic mixing valve will allow you to set the indirect tank temp to 140-160 degrees, and the mixing valve will temper it down to 120* going out to the house....

Sounds like a good idea. Does it have an impact on fuel use of the boiler? Is it much more costly to maintain the hotter water?

...you'll have to check the manual for the indirect. It will give the required GPM and ft. of head. Page 49 will give the specs on Grundfos circs

OK, it looks like the water flow rate is 15.4GPM, and the coil pressure drop is 3.1 feet of head. My Grundfos UPS15-58 (I think I'm looking at the right circulator) is specced for 0-18GPM. Referencing the chart on page 57, it looks like at 3.1 feet of head it's more like 7-16GPM. So it's in the right ballpark if it's on the highest setting (it's multi-speed). How do I check the speed setting?


@hot rod

Thanks. I am definitely interested in figuring out if the boiler is working like it should, but I feel like we have strayed a bit from my original question.

Basically I'm trying to figure out if I should take advantage of large $800 rebates available on heat pump water heaters.

• Obviously doing so would add some amount of capacity.
  
• Would it also save money to get hot water from the heat pump WH?
  
• Is there some reason doing that is a bad idea?
  
• What's the best way to plumb the two together?
  

Thanks!

MasterC

Also, @HVACNUT .. what "Independent Energy" device are you referring to? I've looked through my pics and all over in the room, and I cannot find anything that says that brand. Thanks!

HVACNUT

The UPS15-58 at high speed for the LT160 tank will deliver 15 GPM (with check valve) at 2.1 ft. head, so its right there.
Sorry about getting off topic a little. I'm not too familiar with heat pump water heaters as far as max output, recovery rate @ GPM and so forth. Maybe others can give you better info.
You don't have an electric heater on the jacuzzi?
Again, I'm not sure, but I would think you'd get better recovery from the LT tank, so I would pipe the domestic cold into the heat pump, out hot, into the cold on the indirect, out hot on the indirect with the thermostatic mixing valve.

HVACNUT

> @MasterC said:
> Also, @HVACNUT .. what "Independent Energy" device are you referring to? I've looked through my pics and all over in the room, and I cannot find anything that says that brand. Thanks!
>
>
>
>> 7th. pic down on the left. Model SP-30. Looks like the switching relays. I've never seen that brand.

hot_rod

Some good advise on HPWHers here.

http://www.greenbuildingadvisor.com/blogs/dept/musings/heat-pump-water-heaters-come-age

Know that the heat energy that is exchanged needs to come from somewhere.

I understand that they are slow recovery units once drained.

You have several piping options, in series, probably HP first, indirect second.

Parallel, with exact balanced piping so the draw the same amount.

With manual balance valves to adjust exactly how much flow each tank receives.

If in fact you are looking to cover a large dump type load, and have recovery time available, you might just add an additional storage tank and recirc pump, if the system doesn't have one already.

Look at the HPWH spec and notice recovery times, crunch the numbers. The article has some data to help with that. The boiler may be able to recover an additional tank faster, and less $$ than the HP installation?

Knowing that most WH tanks fail in 7 years or less, especially high demand applications, consider a two piece HPWH, where a basis tank can be swapped without replacing the entire unit.

Tom up at American Solartechnics.com has some refurbished HPWH listed at $375.00 if you want an inexpensive way to try one. I'm sure he would give you the skinny on how well they work in your climate and application. best to get advice from someone with hands on experience.

Zman

A few things to keep in mind:

Fuel oil at $4/Gal is equal to electricity at $0.12/KW.
The heat pump will run 1/3 more efficiently than electric heater only under low load conditions.

Approx Capacities (DHW produced at 70 rise):
Existing setup running with DHW priority 3 GPM
Hybid heater in heat pump mode 0.16 GPM
Hybrid heater in electric mode 0.43 GPM

I would recommend first getting your system setup to run in priority mode. Radiant systems can put a big load on a boiler when zones first call for heat, priority will take that issue out of the equation and give all the boiler capacity to DHW. This is a simple control fix in most cases.

It sounds like you would be well served to have more storage to buffer you through high load periods. The suggestion of a thermostatic mixing valve was very good.

My suggestion is a big NO on the hybrid heater unless you plan to use it for storage only. They are not a bad idea when replacing electric heaters, just not in your case.

jumper

It's considered a no-no but a heat pump water heater in series makes sense to me. Under heavy demand it won't keep up and then your indirect will provide the extra energy you need. Under low demand -most of the time- your boiler won't need to fire up.

heathead

Adding a heat pump for $200. after rebates. That seems like a can't lose. Will be great energy saving in the summer. Keep the boiler off and turn it on when using the big tub or if you run out of water. The heat pump will cool the space, extra bonus in the summer and also dehumidify the space somewhat. .

MasterC

Thanks for all the help and info! I've received a variety of good advice and learned some key information through this process. I'll try to catch up on replies. First I'll repeat a couple questions I didn't see answers for (any help on those would be much appreciated!) - and then later I'll try to follow up to discuss some other ancillary information of interest from some of the posts.

Unanswered questions below. Some of them I have developed answers for myself, but would love to be sanity checked.

1. Would it be more economical to get at least some hot water from a heat pump water heater rather than the oil boiler? A few of you have chimed in on this (thanks!), but no clear definitive answer that I understand yet.

• edit: I started drafting the rest of this earlier and got pulled away; now I see @jumper and @heathead answered with opinions suggesting that it probably would be more economical
  
• @hot rod said:
  "The boiler may be able to recover an additional tank faster, and less $$ than the HP installation."
  I get the faster part; I'm not sure about the less $$ part...what's the reasoning behind that? If you presume the HPWH is in normal electric mode during recovery, I can see the oil boiler being cheaper...but probably not if the HPWH is in HP mode.
  
• @Zman said:
  Fuel oil at $4/Gal is equal to electricity at $0.12/KW. The heat pump will run 1/3 more efficiently than electric heater only under low load conditions.
  Thanks - this sparked me to do a bit of research and calcs on my own. It looks to me like that ratio ($4 to $0.12, or ~33:1) is based on assumed energy factors of approximately 0.81 for the boiler and 1.0 for electric. According to other info I found, including this handy comparison calculator, more typical EF assumptions might be 0.80 for the boiler and 0.92 for electric, which changes the ratio to more like (35.7:1, or $4 to $0.11). Now, for the heat pump, if we plug in an EF of 3 (lower than the 3.2+ typically claimed for HPWH), the ratio changes dramatically (10.9:1, or $4 to $0.37, or $1.31 to $0.12).
  

If anyone is interested, I made myself this spreadsheet to come up with the figures above, and to compute the expected operating costs if all my water were heated by oil vs HPWH. My conclusion: although the HPWH can't keep up with my demand in all cases, every gallon I do get heated from the heat pump costs me roughly half as much as gallons heated by the oil boiler. Please do chime in if I'm out to lunch on this.

2. Does a hotter water storage temperature have an impact on fuel use of the boiler? Is it much more costly to maintain the hotter water?

• I don't think anyone addressed that, but I think the answer is yes - it increases capacity, but at the expense of higher operating cost. Increasing the water temp from 120 F to 150 F requires more BTUs to raise the temperature by ~100 F instead of ~70F (a ~43% increase in temperature rise), but only increases effective capacity by 25% (150 F is 125% of 120 F). The result, if I didn't mess up when plugging the scenario into my spreadsheet, is an increase of operating costs by ~14%.
  
• So, if I have enough capacity without increasing temperature, it's cheaper not to. If I need some more capacity, it's not a bad way to squeeze out some more.
  
• Any other considerations here? How big is the Legionella concern? I don't know anyone who runs their heater that hot around here.
  

3. How do I check the speed setting on my Grundfos UPS15-58 circulator? It's in a tough place to get to, but I don't see any dials, knobs, or switches, and I haven't located that info in the documentation I've seen.

4. Is there some other reason plumbing in the HPWH is a bad idea?

5. What's the best way to plumb the two together?

• It sounds like in series, with the HPWH first, then the indirect?
  

MasterC

Zman said:

A few things to keep in mind:

I would recommend first getting your system setup to run in priority mode. Radiant systems can put a big load on a boiler when zones first call for heat, priority will take that issue out of the equation and give all the boiler capacity to DHW. This is a simple control fix in most cases.
...
My suggestion is a big NO on the hybrid heater unless you plan to use it for storage only. They are not a bad idea when replacing electric heaters, just not in your case.

Thanks, @Zman ! I am interested in this priority mode suggestion. Come to think of it, I think the majority of times that we have run out of hot water has been in the winter when the radiant heat is on. I presume priority mode basically means the space heating zones take a break whenever DHW calls for heat? Is that a significant setback on the space heating? I'm also guessing it's not something I can set up with my existing equipment? I would like to figure out what I need to add to do this.

I appreciate your weighing on on the hybrid heater. After reading the replies, researching more, and doing my own figuring...I have developed an opinion on this. But maybe I'm wrong, and if I am, I want you all to please tell me so.

If you look at my previous analysis in my last post, and/or my spreadsheet, can you suggest why I'm wrong in thinking that adding the HPWH can't hurt, as the boiler would do the heavy lifting during high demand (higher than 2 tanks of water!), but the HPWH would produce gallons of hot water more efficiently than the boiler during periods when HP mode can keep up.

One other benefit, as it seems to me, is this: the boiler room is very hot - usually the hottest room in the house, especially in the winter months. I think the HPWH would make a good pairing, because it can harvest this wasted heat and put it back into making hot water. That would cool the room down, which I suppose would have some impact on the stand-by losses of the hot water tanks, and losses in the radiant heat piping heading out of the room. Ultimately though, seems like a net gain?

Harvey Ramer

As others have said, definitely get your DHW in priority. Install a mixer and keep the tank at a higher temp.

At $200 I wouldn't think twice about installing a HPWH. It a dehumidifier that heats water! Use it in efficiency mode and preheat the water before it enters your indirect.

You might also want to consider recovering some of the waste heat that goes down the drain. Something to the order of 80% - 90% of the btu's are still left in the water as it's going down the drain. Take a look at these http://www.gfxtechnology.com
This product will also help increase DHW production in shower scenarios. Not so much when you are filling the Jacuzzi though.

Zman

Lot's of smart folks on here with good ideas. You are wise to gather all the info and make your best choice.

I attached a spreadsheet that let's you compare all this easily.
I think 80% on the oil boiler is about right. The hybrid can be tricky to figure as it varies with usage habits It will run very efficiently under light loads. It will behave exactly like and electric water heater if you are taking long showers or filling tubs.

Can you post a diagram of how your controls are wired? That will help with the DHW priority.

HVACNUT

> @Zman said:

> Can you post a diagram of how your controls are wired? That will help with the DHW priority.
>
>
>> From the pics in his first post, it looks like seperate Independent Energy (never heard) switching relays with dry contacts in parallel closing TT on the L8148A. A RIB relay could isolate the SH relays for priority.

hot_rod

You have a few different vintages of Grundfos pumps. The newer ones have a black lever to change speed. the design sheet shows the speed each should be set at.

The older version needs a screwdriver in that small round plastic post you see to change speed. Looks like covers missing, so be careful sticking a tool or finger inside this electrical junction boxes.

The IE are setpoint temperature controls. Somewhere in each room or zone is a sensor, you can see one hanging under the basement zone control. be good idea to find where all the sensors are located.

The system also has a 4 way tekmar mixing valve. It could, should have a motor operator so the valve can adjust water temperature based on outdoor or building loads. Right now you would need to manually adjust it to get optimum operating temperatures.

In it's day it was a nice design, it needs some TLC and possible upgrades.

I agree with 140F setting for the DHW with a listed mix valve. also find someone with control experience to wire the DHW call for priority. A RIB (really in a box) would be simple, inexpensive way to do that with proper wiring.

mp_e46

Hello MasterC. Checking to see what you ended up with. Which route did you go. I am in somewhat similar situation as you. Here is a link I posted.

https://forum.heatinghelp.com/discussion/170047/rheem-hybrid-water-heater-with-35-or-55gal-indirect-water-heater#latest

Thanks

lightingguy

Hello All, I'm considering adding a heat pump hwh to the existing indirect hwh (oil, Buderus 215/5, Smart III 60 gal., all excellent operating condition). Keeping this existing boiler and multi-zoned hydro air and radiant baseboard backup heat for a planned retrofit of the building with 3 new ASHPs and 3 replacement VS air handlers.

Looks like a series install would work best (HP then indirect). For the HP to do the low-demand work, could a recirculating pump be used? Otherwise the indirect tank would cycle the boiler when it needs heat.

Controls/Operation: If the HP HWH is set at 150F, the indirect tank is set for 130F (so below that calls for heat and cycles the boiler), and there's a second stat set for 140F on the indirect tank for the recirculator, would that work? Any problems you can see? Thanks.

hot_rod

you could pipe and valve it to use it several ways. In the summer months do you want to run the boiler for DHW? If not pipe it so you can switch back and forth, or in series.

I would think run in series, let the HPWH do the first lift, then the indirect to 140F. If you run the HP at 140 does it kick in the resistance to get to that temp? Different brands have different control logic. I think the AO Smith allows HP and resistance to run together for better recovery, but COP drops.

Do you have a DHW recirc now?

lightingguy

Thanks. We do have DHW recirc now (the recirc pump is remote from the HWH though). Only reason we'd run boiler in the summer would be if the HPHWH wasn't meeting demand. Adding 60+ gal indirect tank as a buffer might be enough. I'm trying to avoid using the HPHWH resistance heaters since here in RI the electric rates are high.

hot_rod

You probably do not want to recirculate thru both tanks when one is off? If so then off tank becomes a cooling tower:) A small amount of jacket loss from two tanks.

So the supply and recirculation piping becomes a bit messy if you want three different options
Indirect only
HP only
Both together
Add recirculation thru only the "online" tank

If you series them, recirc thru both, the HP basically heats both tanks, the indirect could come on if the temperature drops to a low temperature you decide, say 110°

If you have or add a 3 way thermostatic the recirc piping becomes a bit more complicated.

lightingguy

Thanks, yes there would be additional loss.
Your diagram: The recirc loop here doesn't pipe all the way back to the existing tank and due to finished space would be tough to change. Would a local recirc loop that was just the 2 tanks in series work? Like you said:
"series them, recirc thru both, the HP basically heats both tanks, the indirect could come on if the temperature drops to a low temperature you decide, say 110°"
So - keep HPHWH on eco only, recirc back from indirect tank through HPHWH, keep boiler as backup for DHW year round and for backup heat+DHW in winter.
This simplifies piping (add HPHWH into CW supply just before indirect tank; pipe in a recirc/return line + pump between indirect and HP tanks - also need to provide new, separate CW supply for existing DHW tempering valve).
It should be simple to pipe in a bypass that cuts the indirect tank out of line. If you shut off power to the HP then the indirect can operate alone. That gives 3 options, yes?

hot_rod

How does the recirc work if it doesn't return to the tank, draw a pic maybe. Use a "smart" recirc pump or a timer and temperature control to eliminate constantly circulating the loop.

Is the piping HW and recirc insulated? If not you will have some heatloss, so the timer and temperature control would be useful.

In series you are basically turning the system into one large vessel with two heat inputs.

Run the HP at desired temperature, set the indirect at lowest possible usable temperature, tight differential otherwise it will eliminate some of the HP contribution.

There are other piping methods that would involve motorized or thermostatic valves. All have pros and cons are far as cost, complexity, ongoing service, etc.

Here is an example of how 3- 3 way thermostatic valves could work, but they are maintenance prone in hard water and have some pressure drop involved.
This was assembled for a system that would pull from a solar HW tank until it dropped to say 110F, then flow diverts to and an instantaneous/ tankless backup. The 3 rd valve assures you never get above 120F to the home should, or when the solar tank, for example run to 160-180 or higher.

This is also do-able with motorized valves and a set point and ∆T controller.

lightingguy

Excellent picture (and concept). Please send a pic of the other side too.
Didn't realize that thermo valves could work in that configuration, but it makes sense and would work here if the tanks/heat sources were installed in parallel (we already have one Taco mixing valve piped after the existing indirect).

I can't figure out exactly how the HW recirc is plumbed in here, but this is what I know. Metlund D'mand S-02T pump is in the basement, approx. 35' from HWH tank, no visible return, finished space in between. One pipe into pump, one pipe out. 3 baths, all 2 floors above, each with a trigger for pump (wired in parallel).

So I'm guessing that the pump is piped from a common HW return line from the 2nd floor baths, into a CW line in the basement. So no direct return line to the HWH.

hot_rod

We build these solar assemblies in either a 2 or 3 valve configuration, these schematics better show how they flow.

Yes I believe that recirc uses the cold as the return path.

That may complicate thermostatic valve operation. What may happen is the mixer starts to see warm water at it's cold port and lose the ability to regulate accurately. The valve needs both cold and hot to blend to give accurate mixed output.

Mix valves can both drift or droop depend on temperature and flow conditions. Too little flow and the loop droops, too much and the 3 way loses cold input to mix.

When a thermostatic mixer is used on a system with recirculation it needs to be piped with a bypass and balance valve.

When set properly the balance allows just enough flow, and temperature, to maintain the temperature in the loop. Basically you are adjusting flow to match the heat loss of then piping in the building. Obviously with well insulated piping merely a trickle of recirc flow is needed.
With un insulated piping, more gpm is required.

With the cold "contaminated" with hot I think the valve starts to lose its mind. But I have not tried a mixer on a system with the cold being warmed.

Here is how the Caleffi 262, 2 valve and 263, 3 valve assemblies work.

And info on the proper piping of a recirc when used with a thermostatic valve. Many mis-behaving thermostatic mixing valve can be traced to improper piping and lack of balance function.

lightingguy

Excellent information again. Many thanks.

FWIW, the recirc system here operates by manual demand only (user-operated, with a temperature-sensitive pump), so there isn't any regular recirc from the HW piping into CW. We seem to have a properly functioning mixing valve, from what I can see (stable HW faucet temps).

A parallel piped, two-tank system with one of the Caleffi valve assemblies could be an alternative to the HP + indirect in series. But would this force the boiler to fire occasionally to maintain a minimum setpoint temp in the indirect tank? Maybe a better arrangement for an instant HW setup with solar?

Would a 2-tank in series layout (HP, then indirect, no tank-to-tank recirc) be decently efficient (the HP does most of the lifting, even for the indirect tank under light load) and also be good under high demand (backup by the indirect)? Simplest to add in, too. Does that make sense?

hot_rod

I think two in series, without recirc with the indirect first, set at a low temperature, say 80, and the HP second at the final temperature.

If the HP is first, you need to maintain the 2nd tank always at the lowest useable temperature. After periods of no use the second tank could drop to a temperature lower than useable, maybe?

I suppose it comes down to how much DHW is used, how often, etc. If summertime incoming temperatures rise, perhaps the HP is enough? Just add the indirect in the winter.o

For the indirect to be an available player instantly, it needs to be maintained at some temperature.

If it were a tankless like those diverter valve drawing show, it can ramp up in seconds, based just on flow, to be the backup.

You could run a simulation for your location and application and see what % solar would add, but it may not be a cheap install, unless you are a DIY and have access to components at a great price.

That is the only way solar thermal pays for me, I install it and got components at cost, some for free that I recycled

Or with the dirt cheap prices these days, add 4KW of PV module to run or offset the HP operating cost. If operating cost is the concern?

Actually you could connect the PV to the resistance element in the tank. Sunny days you have PV thermal hot water. Other days just HP. Solar resource maps would show you how many days a year you average in your area.

Many way to skin the DHW cat.

lightingguy

Thanks.
Last question - if I use the 2 tanks in series (HP, then indirect) I can use the existing recirc loop intermittently to partially refresh the indirect tank with higher temp HW from the (efficient) HP tank. The recirc pump has a thermal shutoff. The run time for refilling cold HW pipes is about 30 - 45 seconds. About 3 - 5 gallons or so of recirc.

I figure that if this recirc is set to run somewhere between 1x per hour and 1x every 6 hours (maybe longer...) it'd prevent the indirect from calling for heat. The HP tank would be set 10 - 15F above the indirect tank.

To do this I'd need a programmable timer that triggers a momentary relay contact, in order to mimic another pushbutton switch on the recirc pump. Got anything you'd suggest?

I realize that this guarantees a small periodic dump of heat from the HW system into the walls, but this may be better than having the indirect tank periodically call for heat from the cold start boiler. If this ends up being an energy hog then the timer can just be shut off.

hot_rod

We are kind of shooting at a moving target here. Ideally you could define or have good estimate of how much DHW you require. Then build the system around that. In some cases running a higher tank temperature may cover your demands.
The renters in the basement throw another unknown into the equation.

If you were to run the tank at 140- 150° mix to 115 or 120 possibly you cover the loads without adding additional capacity?

The tub dump loads are the hardest to cover. Get rid of the large Jacuzzi Either you store that much capacity of have an instantaneous type DHW appliance that can generate that as it fills. If someone drains the tank to fill the tub, you have a bit of time to recover for the next shower draw.

Look at the spec on the HPWH, as I recall they are not very fast to recover, unless the resistance element kicks in. But operating costs may spike if that element runs frequently. So the HP tank alone may give you less not more DHW if one goal is to not fire the boiler?

What about upsizing or adding and additional indirect, since you already have the boiler. Or add an electric 50 or plain insulated storage tank and circulate thru the current indirect.

Funny thing about DHW, you give someone 120 gallons and they figure out how to use it all.

If the renters are paying electric, add a 40 gallon tank just for the basement apt?

Sure if you have the time and patience you can try different loading techniques as you mentioned, since you are not sure what you actually will need to keep everyone happy.

Zman

To me, the main driver for this would be to not have to run the boiler in the summer. The heat pump would also give you some space cooling in the summer.
If you pipe is in series, as suggested, be sure to included a bypass for the smart tank. Other wise the boiler will fire to take care of the inevitable heat loss in that tank.