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Air to water heat pump to boiler...

IcarusIcarus Member Posts: 56
Short story, sorry if this is in the wrong forum. I am a retired GC who does energy/solar consulting. I hav e a client in a Victorian era house, heated with a cast iron boiler and cast iron radiators. (Hot water, Nat gas) Client is interested in changing to a enter system. His primarily interest is to use a heat pump, supplied by his roof top net metered solar system. I have suggested an air to water heat pump (we are in the Pacific N.W where a “coldest” design temp is ~25f, but 35f is a more normal overnight low).

As I said, I am not an HVAC professional, but I understand the technology, just not the fine design details. My plan would be to mate a heat pump to a Mod/con boiler, running the heat pump into a heat exchanger and a buffering tank, controlled with a outdoor reset. What I don’t understand how if you are running water into the buffer tank at an ideal Delta of ~120f, how do you use this to add heat to a more usable 140-160f for the radiators to be effective. The client wants to retain the radiators. (If this we’re not the case, I would suggest a minisplit system.

What I don’t have is a good contractor/design engineer who can do a custom design in what in this market is a cutting edge installation. The local boiler/HVAC contractor came in, never having heard of such a system, and simply suggested installing a new Buderus or Weil/McLean old fashioned cast iron boiler, of a size that is roughly 3 times as big as my load calcs would suggest. (Not going to even consider that contractor!). Why would anyone suggest a cast iron boiler for a moderate climate, when a mod/con with out door reset is so very much more efficient?

So, does anyone have a resource to find a good design engineer to work through the design/efficiency curves for me? In principle it should be pretty simple, but one needs a good baseline expertise.

Thanks in advance for any advice,

Icarus

Comments

  • IronmanIronman Member Posts: 5,302
    I can't see how a heat pump would be viable for that system. They max out at 120* SWT; a cast iron radiator system needs about 160*+ at design temp. Unless you can show from a load calc and radiation survey (very doubtful) that 120* SWT is sufficient at design temp, then this is not an option. ATWHPs are designed for low temp emitters, such as a radiant floor.

    Furthermore, even if the ATWHP could supply sufficient SWT, would it be less expensive to operate than a properly sized mod/con with ODR? Again, I doubt it.

    We do jobs where we marry mod/cons to old CI radiator systems all the time and that's the most efficient way to go. Solar can be integrated into these systems, but it's expensive and the ROI is usually very long. Even with a large array of panels, a 30% contribution is about the best you're gonna see.
    Bob Boan


    You can choose to do what you want, but you cannot choose the consequences.
  • Jolly BodgerJolly Bodger Member Posts: 132
    @Ironman PNW electric prices are low, and if the house has solar then it gets even more viable.

    @Icarus , I agree with Ironman with the general issues with this idea. you have a load calc already. Do the radiation survey and find the required water temp. Plot a few points. Find where the break point of 120SWT is and what OAT that corresponds to. You might find it is viable above a certain temp. The trick is to see if that is enough of the time to make all the trouble, cost and, and complex controls worth it. Remember to include the curve for the air source heat pump. They lose capacity as the temperature drops.

    I love it when people think out of the box. I'm doing a similar setup with geothermal heat pumps. But all my distribution is designed for 120SWT max.
  • SuperJSuperJ Member Posts: 557
    edited November 29
    With an aggressive ODR, and good flow control so you make sure your RWT comes back cold you could certainly use the AWHP much of the season with the right piping and control strategy.

    Caleffi has some documentation out on modifying existing systems to make them run on lower temperature.
    This idronics issue will be pure gold for you:
    https://www.caleffi.com/sites/default/files/file/idronics_25_na.pdf


    Ideally you would home run your zones so you can aggressively balance them since high RWTs are the enemy of efficiency for AWHP, even more so than modcons.
  • mikeg2015mikeg2015 Member Posts: 1,041
    Heat pump would work ok in mild weather above 40F, but would be prohibitively large to keep up below that, efficiency drops fast and it can’t deliver water over 130F, efeceincy is still crap at even 120F. So it’s practical balance point is probably around 40-45F.

    Can it be used to supplent? Yes.

    Also keep in mind that 120F radiators don’t feel all that hot.

    But lets say 130F is adequate and you get delata T up to 25F so it can delivery is with OK economy. In cold weather you might need 15 Tons of total capacity for a larger Victorian to deliver 90,000 BTU poutput at 130F at 10F OAT. Keep in mind you have ot factor in defrost losses.

    Geothermal would work OK as capacity drops off less, but the field would be really, really big because in many cold climates, in large high mass homes, the heating load is much greater than cooling load, so the ground tends to get too cold. You would need a solar system for to push heat into the ground when possible or add supplemental heat after the water to water unit when possible. You still want a boiler for the 1-2%, really, really cold days.

    THere are high temp water to water geo that use r134a I think. You could run that in series with a standard unit and lower the flow rate a little to get delta T closer to 30F.

    A good way to do this is adding a air handler in series with a very low airflow as an “economizer”. Run air flow very low and send it only 110F water to deliver 105F air at a very low airflow (requires x13 motor).

    Just a few ideas.

    As I type this, I’m getting nicely cooked by a steam radiator 5’ away from me. Comfort beats economy when you live in a big old Victorian. If economy mattered, I’d live in a modern cookie cuter, eclectic, slapped up new construction box. NO way to economically fully retrofit a all brick and stone home with 50+ windows and over 30% window area. You can add some attic insulation, a steel roof (if you like the looks of that), put on storm windows, seal up doors, and add solar it thats economical, but thats about it. But the R2-3ish walls you are stuck with. you can seal up the sill plate but now you need to add a large radon mitigation system and supplemental ventilation, and your going backwards at that point.
  • IcarusIcarus Member Posts: 56
    Thanks folks for the start of the conversation. I do understand that simply doing a mod con boiler would be the “cheapest” alternative, but that is not the design criterion, heating the house efficiently with electricity is, ergo the heat pump idea. (And the desire to maintain the CI radiators) Second I do also understand that CI emitters need higher temps than the AWHP can ideally deliver. MY thought was...that by using a properly designed buffer tank to act as a “pre-heat” tank, the mod con would only have to raise the circ temp from ~120f to ~160-180f as needed.

    What I don’t get empirically, is if the return temp from the CI loop(s) is say 150, and you dump that into a buffer tank of ~140f water, how it can pick up any heat while circulating. The buffer tank needs to be big enough so that the water going to the heat pump is cool enough so that it doesn’t over heat the HP, AND large enough so that the water from the buffer to the mod con is ideal for the mod con. For example, if you are taking 140f water from the buffer into the mod con (an ideal temp) and have to fire it to 180f to meat the heating load, you want the buffer tank big enough to absorb hot return water so that the boiler doesn’t short cycle, and gets to pick up heat from the buffer.

    Once again, I know that there are folks doing this but finding design engineers/consultants/contractors locally, who think out of the box is the trouble. If you design the system wrong, then the performance is likely to be so poor as to be counter productive.

    Currently we pay ~$.11 kwh for electrcity and ~$1.30/therm for nat gas. All things being equal, at the temps required, a reasonably high COP hp will be ~20% cheaper to run. Add in the AWHP and the use of Nat. Gas for the coldest parts of the heating season (rare) and the equation is probably a wash. On the other hand, if one has “free” electricity from ones own roof top, the financial equation gets a lot easier...the heating cost approach near zero annually.

    Once again, keep the ideas coming,

    With kindest regards,

    Icaruis
  • SuperJSuperJ Member Posts: 557
    edited November 29
    You're correct the heat pump won't be able to handle a 150F return temp, but there are strategies to modify your system and control strategy to bring down the RWTs much of heating season. If it's truly cold you may be running of the modcon exclusively, but the ASHP will be able to handle.

    Here is an example (your numbers will be different but the curves will be a similar shape):






    You can probably reset the supply temperature between 110F and 160F depending on outdoor temperature and demand. There are a lot more hours per year in the milder range of the temperature curve so it's not necessarily crazy to run a low temp source.

    Give that Caleffi link a read, it will set you in the right direction.
    It covers a lot of the relevant issues that you will encounter.
    And provides some example systems:

  • Jolly BodgerJolly Bodger Member Posts: 132
    The biggest problem you have this this particular setup is the CI radiators. In order for a AWPH to be efficient you need a low RWT. If you have to supply 160 to the CI getting the RWT down to 100ish is going to cut the heating capacity of the CI. To get lower RWT means reducing flow to the Rads, increasing the Delta-t. When you do this the output capacity of the rad decreases as a lower portion of it is at the SWT. Then to combat this you need a higher SWT. It's a catch 22.

    The system you describe works well with systems designed for low SWT and thus low RWT. (radiant floor systems, or forced air)

    I don't think this can be overcome with any size buffer tank. Bottom line is if the RWT from the system is higher that the AWPH can deliver or deal with as RWT (think high head pressure) then the AWHP will never contribute to the system.
  • IronmanIronman Member Posts: 5,302
    edited November 29
    The radiation survey and load calc are gonna be what decides if an ATWHP is in any way suitable for this job.
    As you obviously know, heat moves toward cool, so dumping 140* RW into a 120* buffer tank will accomplish nothing to integrate an ATWHP with the mod/con.
    If the numbers don't look feasible, you may consider adding a radiant floor connected to the ATWHP with the mod/con and rad's as the second stage. Or, more rad's as shown above.
    Bob Boan


    You can choose to do what you want, but you cannot choose the consequences.
  • Larry WeingartenLarry Weingarten Member Posts: 1,608
    edited November 29
    Hi, It sounds like if you had low temp emitters to hook up to the heat pump, that could let the heat pump work in its optimal range. The boiler could then be there for really cold days. Is there any way to fit low temp emitters into the house?
    Yours, Larry
  • IcarusIcarus Member Posts: 56
    SuperJ said:

    With an aggressive ODR, and good flow control so you make sure your RWT comes back cold you could certainly use the AWHP much of the season with the right piping and control strategy.

    Caleffi has some documentation out on modifying existing systems to make them run on lower temperature.
    This idronics issue will be pure gold for you:
    https://www.caleffi.com/sites/default/files/file/idronics_25_na.pdf


    Ideally you would home run your zones so you can aggressively balance them since high RWTs are the enemy of efficiency for AWHP, even more so than modcons.

    This is very interesting, haven’t read it all, but it is very informative. Thanks,

    Icarus
  • IcarusIcarus Member Posts: 56
    So I met with the client (actually this is pro-bono work, as we are both interested in the technology) this morning, and we ran a few beta tests.

    We just so happen to be having unseasonably cold temps right now, perfect for design tests, ~25f over night, which is about the lowest expected temp.

    We let the house cool down over night to under 60f, followed by a full fired, all radiators on session. In less than 1 hour the set temp of 70F was achieved, with radiator temps at ~140f, and the boiler shut down. Of course, this is not a good situation for a conventional cast iron boiler, return temps being too cold.

    The Delta was ~25-35F between outlet and return water temp. A survey of the house reveals two or three rooms where flat panel, low temp (designed) emitters could be added or replaced conventional CI radiators. This would serve to drop the net return temp even further.

    So, knowing what I know, a design temp of ~140F for normal output would be plenty to maintain the temp, and to to bring up the temps under “normal” operation. With a 30F Delta, that would leave a return temp of ~110F. Adding emitters might drop that to ~100F. (What this boiler doesn’t have is an over running pump. When the Tstat stops calling for heat, the circ pump stops . Seems rather inefficient?)

    So, back to my original quest. If I heated water with an ATWHP into said buffer tank at ~120F, that would be above the normal return temps from the radiant loop(s). The HP would then be providing a significant % of the heat load. With a properly siz3ed and configured boiler, you could always configure the boiler to actually step up the the output temp as needed of the 140 was not sufficient to heat the house.

    Am I on the right track, and once again, how to find a competent professional to work on this design.

    Thanks for all the suggestions so far,

    Icarus
  • IcarusIcarus Member Posts: 56

    The biggest problem you have this this particular setup is the CI radiators. In order for a AWPH to be efficient you need a low RWT. If you have to supply 160 to the CI getting the RWT down to 100ish is going to cut the heating capacity of the CI. To get lower RWT means reducing flow to the Rads, increasing the Delta-t. When you do this the output capacity of the rad decreases as a lower portion of it is at the SWT. Then to combat this you need a higher SWT. It's a catch 22.

    The system you describe works well with systems designed for low SWT and thus low RWT. (radiant floor systems, or forced air)

    I don't think this can be overcome with any size buffer tank. Bottom line is if the RWT from the system is higher that the AWPH can deliver or deal with as RWT (think high head pressure) then the AWHP will never contribute to the system.


    Sorry to be a bit obtuse, but can you please clarify:

    RWT=Return Water Temp
    AWHP= Air to water heat pump
    SWT=??? (I assume that is supply water temp?)


    Icarus
  • IcarusIcarus Member Posts: 56
    No, getting caught up a bit reading all your thoughtful comments. Since I can add a couple of low temp emitters, it would make sense (critical actually) in this incarnation to make sure that any low temp emitters are “last in line” so that their SWT was as low as possible, and ergo their RWT contribution was as low as possible.

    Intuitively, one significant problem I can see with this strategy is that the owner is likely to turn off many if not most of the second floor radiators. In that case, there may not be enough radiation to lower the RWT enough. That said, if they were only using a small portion of the radiator capacity, and we added (in this case) low temp emitters downstairs, the boiler might be perfectly happy to run at ~120f any way?

    I confess I do these intellectual exercises on design schemes for lots of energy issues, and solutions often create new problems, so using a community such as this with people who have been there, done that reduces the chances of costly mistakes.

    Thanks for all the input,

    Icarus
  • SuperJSuperJ Member Posts: 557
    Icarus said:


    So, back to my original quest. If I heated water with an ATWHP into said buffer tank at ~120F, that would be above the normal return temps from the radiant loop(s). The HP would then be providing a significant % of the heat load. With a properly siz3ed and configured boiler, you could always configure the boiler to actually step up the the output temp as needed of the 140 was not sufficient to heat the house.

    Am I on the right track, and once again, how to find a competent professional to work on this design.

    Thanks for all the suggestions so far,

    Icarus

    You likely want to add heat in series, not in parallel. A buffer tank may or may not be part of the design, but won't be the silver bullet on its own. If the flow rates and thermal mass of the system are high enough a buffer tank may not be required.

    Ideally, the heat pump needs to receive the coldest return water from the system first, and be given an opportunity to heat it (if within acceptable parameters). After the heat pump, the boiler could finish raising it to a usable temperature. But you don't want to blend the hot boiler supply in a way that could raise the heat pump entering water temperature.
    -Envelope improvements could made selectively if some rooms are outliers with requiring hotter water
    -Extra radiation or piping changes could be made to reduce temperatures.

    Another approach would be a simple changeover that occurs at a predetermined outdoor air temperature.
    -This would leave some efficiency on the table,
    -May not meet the clients objectives of maximizing the electric versus gas contribution.
    -would be cheaper to install, and may require less rework of the system both in the mech room and house.
    -Perhaps above 25F the heat pump could carry the house, and colder.
    -Heat pump would probably be sized for 30-60% of the max heating load.

  • IcarusIcarus Member Posts: 56
    edited November 30
    To put a few more details in play. The house is only ~2000 sq ft plus finished basement. It has been extensively energy upgraded, with insulation, air sealing, full insulated glass etc. the heating load is actually comparatively small. The current boiler is 90k btu net.

    The owner told me that a couple of winters ago, during an energy/insulation refit, they had to vacuum out the attic spaces to remove vermiculite and there was no insulation in the ceiling. With daytime temps in the high teens, the house stayed warm just fine in spite of no insulation. Now it heats very easily. I would probably (off the cuff) figure a boiler of 50k or smaller for future. I live in a similar house, and heat easily with a E50 Rinnai combi mod con. It runs @25% of capacity at most, most of the time.

  • Jolly BodgerJolly Bodger Member Posts: 132
    @Icarus you got the abbreviations right.

    If the numbers work out, and the empirical evidence seems to lean that way, then the proposal is to preheat the returning water with the heat pump and then bump that up with a MODCON boiler as needed as it goes out to the house? Sounds like it is viable. Buffer tank sizing would probably be more about satisfying the GPM demands of the house and the AWHP separately. More of a large hydronic isolator. if the GPMs line up then probably not even needed.

    Controls probably not even that demanding. Use a low temp cutout on the AWHP with a RWT interlock. Set the outdoor rest on the MODCON and let it go.
  • IcarusIcarus Member Posts: 56
    edited November 30
    Doing some more “research”. (The internet is a wonderful/dangerous/crazy resource). Including much of the advise gleaned here, along with a few tidbits picked up elsewhere, I am coming to a couple of options.

    #1 simply install a good Mod/con combi boiler.
    Pros. Cheapest to install net/net. Simplest to operate and service. Pretty efficient in todays terms.

    Cons. Most expensive to run net/net, both in terms of $$ per year/btu, but most carbon intense.

    #2 Install a “hybrid system” including a new mod/con boiler, low temp conventional ATWHP, buffer tank, heat exchanger etc to produce both DHW as well as some preheat.

    Pros. Would use heat pump capacity to it’s most effective if not it’s most efficient. Would most likely be the least carbon intense. (Using our utility grid with heavy reliance on wind and hydro, any heat pump system is less carbon intense than the best mod/con system).

    Cons. Far and away the most expensive to install, most hardware. Not the cheapest $$ net/net to fuel. Performance curves suggest best efficient on shoulder seasons, less efficient during coldest days. Would probably require some additional low temp emitters to reduce RWTs, a bonus for the overall comfort heating of the home net/net, and fairly easy to do. Would be the most complicated to operate and service, with the most moving parts, and complicated plumbing.

    #2B. As suggested design the system to rely on the HP when it is most efficient and the boiler at a temp certain point. This would allow the HP to operate at it’s most efficient COP.

    #3 I discovered “Arctic” makes a two stage ATWHP with a ~130f top temp, with a fairly high COP at the temps I am working with.

    https://www.arcticheatpumps.com/cold-climate-heat-pump-overview.html

    Pros. With the addition of some additional low temp emitters as above, the single, stand alone system is likely to have the capacity of covering the entire heating and DHW load across the seasons. Simple install, not as simple as mod/con alone but pretty close. Least carbon intense installation, cheapest $$ to I operate over the heating season, albeit not on every day.

    Cons. More expensive to install than mod/con alone, but less than hybrid system. Heat pump will not always operate at it’s most efficient COP. Biggest con...it is right on the edge of being 100% feasible and currently has no provision to add additional heat in the event it falls short. With out an extremely accurate analysis of the building, the emitter systems and the hardware there is a significant risk of it not performing well enough. The client currently heats ~60% with wood with a modern stove, so there would in the case of this owner be no issue, as one could simply fire the stove. Good design requires a system that will perform as needed however. The final potential “con” would be the unknown quantity of the current state of the technology as well as that of the manufacturer. I don’t have great faith that this (or any other) relatively new manufacturer is going to be around in future years. Name brand HP manufacturers like Daiken/Mitsubishi/Fujitsu etc are mature and have been around long enough, doing the technology to be comfortable.

    As far as I know, none of the name brand guys (above) currently import lines of ATWHP or dual phase, high temp heat pumps. If I am wrong please advise.

    Thanks once again for the help, and keep those ideas coming,

    Icarus

    PS I have found that Daikin makes a high temp ATWHP that heats to 80C or about 175F. Will post when I research it. Don’t know if it is available in N. America though

  • SuperJSuperJ Member Posts: 557
    It's fairly straight forward to add electric heat capacity as a backup.
  • IcarusIcarus Member Posts: 56
    ^ Understood... that said using resistance electric heat is a pretty expensive alternative for operating fuel. I agree however, that adding resistance heat is probably net/net cheaper if you (I) only have to rely on it at the very fringes of performance. I know that Arctic offers a buffer tank with an resistance electric element.

    Icarus

    Still waiting on finding out about Daiken Altherma. It seems rare if not withdrawn.
  • hot_rodhot_rod Member Posts: 12,020
    I think the Altherma has been pulled. Tough finding parts for the unfortunate owners. There are more and more ccA2WHP coming on the market, the industry is growing quickly in other parts of the world, and the technology is making its way over here.

    Challenge is selecting a brand with staying power, something we heard Dakin had.

    I think that 120° number is where you want to be for efficient operation.

    https://www.pmengineer.com/articles/92581-cold-climate-air-to-water-heat-pumps
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • Jolly BodgerJolly Bodger Member Posts: 132
    @Icarus, just a question here. Is it an option to install a air to air heat pump? Then in stall a ModCon to handle load with existing hydronic system when the HP can't or is not efficient?
  • IcarusIcarus Member Posts: 56
    Jolly,

    All options are out there, but many would be rejected by the owner. I suggested to the owner that the simplest easiest, yet least elegant total solution would be to install a minisplit, with multiple zones, but for various reasons that is not what he wishes to do. A couple of things, there is no A/C (nor much need for A/C in the house, and no duct work, so a ducted heat pump is a non starter. Second, because both of us are “energy/technology” nerds we like thinking out of the box. Installing cutting edge technology, and demonstrating the efficacy of it to a larger comminuted is an effective way of pushing that technology. For example, the first Prius buyers did’nt have much effect on total emissions (and didn’t tree hugging “save the world”) but what they did do was prove to the wider audience that the technology was not only viable, but also cost effective. If we can change this one house from an inefficient energy hog with a out dated boiler, and run it with the least carbon emitted system we can currently install, and show to the tradesmen we use, that their future clients could benefit from such a scene...we all win.

    I am not a complete “tree hugger” but I am willing to embrace energy and potentially money saving technology when I can.

  • hot_rodhot_rod Member Posts: 12,020
    I think you get a buffer tank by default with an A2WHP, the manufacturer will give you sizing info for that.

    Go online and find this free series from SpacePak, published by waterworks. It has some of the most up to date info on sizing, piping, and determining the correct applications.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • Jolly BodgerJolly Bodger Member Posts: 132
    @Icarus , I to am an efficiency geek. That is why I asked about air to air. I believe that air to air HP options will be more efficient that Air to Water with your current radiators. I see that neither efficiency nor upfront cost are the all deciding factor.

    I don't mean to discourage experimentation. Can't wait to see how it turns out.

    Can you add staple up infloor heat? That would be efficient, and super comfy.

    Is the owner apposed to fans? Some fan powered cabinets could drive down the RWT to an efficient point for the AWHP.
  • IcarusIcarus Member Posts: 56
    ^we talked about fan forced hot water rads, like toe kicks or recessed wall precisely to drive down the RWT. A posibility.

    Staple up is probably not in the cards as the basement is fully finished including GWB ceiling.

    Thanks all for all the info, keep it coming and I will keep getting edjamacated!

    Icarus
  • Jolly BodgerJolly Bodger Member Posts: 132
    @Icarus , this is fun and ill be keeping up with the story.

    What was the heat loss calculated to be? If I was doing it I would run the numbers at the lowest effective outdoor temp for the heat pump. This would be my HP sizing. Then the ModCon sizing would be at design temp.

    I have been looking at some Air to Water heat pumps for domestic hot water that are rated to 140 degrees. But as far as I can tell they are only available in 1ton. Meaning several would be needed. But then again, that would allow scaling.
  • IcarusIcarus Member Posts: 56
    hot_rod said:

    I think you get a buffer tank by default with an A2WHP, the manufacturer will give you sizing info for that.

    Go online and find this free series from SpacePak, published by waterworks. It has some of the most up to date info on sizing, piping, and determining the correct applications.

    Thanks HR. One problem I have is that there are exactly ZERO installers anywhere near me. The nearest one that shows up on the Spacepack installer is 200miles away. Would then wonder about the competence of an install, as well as long term service.

    The bigger problem is that hydronic heating in this neck of the wood is very rare, especially CI old school. Most houses here use. Forced air simply because it is cheap. Older (1890ish like mine and the clients) often used gravity hot air and have been converted to forced air. My 1890s house was converted to fin tube, which is rare.

    Tony
  • ZmanZman Member Posts: 5,334
    I would begin experimenting with what you have. A proper room by room heat loss and radiation survey would be the normal way to do it but you have an existing system to work with.

    I know that you are not supposed to run a conventional boiler at low temps but what the heck, it will only be for a few nights and you are replacing the boiler anyway, right?

    I person in your profession should have some gadgets, so pick up a bunch of these, https://www.onsetcomp.com/products/data-loggers/ux100-003, put them in every room and one outside.
    Then strap one of these,https://www.onsetcomp.com/products/data-loggers/ux120-014m to the supply and return pipes.
    Now, find a few days when the outdoor temps are going to be at the lower end of average and turn the boiler down to 120. Download the data and figure out just how low you can go and which rooms need additional radiation.
    You can dump all the data into Excel and create cool graphs.

    I am thinking that using the heat pump for DHW preheat and space heating on most days may work well. You could get a combi boiler to boost the DHW and take over for the heat pump when the temp drops. I think to try to boost the heat pump with boiler on cold days will give you serious brain damage (then not work) because of the variable delta t.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • IcarusIcarus Member Posts: 56
    ^This entire matter is made more complicated because of a couple of factors. The first is the current use of the wood stove, used for ~90% of the heating load.

    I have convinced him to do exactly what you suggest, but old school way. I don’t think that we can dial down the current boiler to run at 120f (maybe, will have to look into that). But what I am going to have him do is sit in his dining room with a IR thermometer (wood stove colds out side temp as cold as we can get given current weather) have the nest t-stat call for heat, IR the radiator to 120,,,,kill the boiler, fire the boiler when the rad temp drops to say 115, rinse and repeat and see how long it takes to get to se temp (or if it can maintain set temp) and figure it out from there. What I am also not sure of is if the pump over runs when the burner is off, or if I can make it over run.

    I’ll let you all know what I find in that beta.

    Icarus
  • Jolly BodgerJolly Bodger Member Posts: 132
    @Icarus I was just talking with on of my suppliers (Pacific Green Room) and they said that Bosch and LG are coming to market with Air to Water heat pumps designed for space heating next quarter. COP pushing 4-5. Hopefully they will be able to supply water in excess of 120 and deal with higher return temps. They also rep the Sanden SANCO2 water heater. It currently boasts water temp range 130-175 but it is only a 15,000 btu machine.
  • tim smithtim smith Member Posts: 2,292
    Icarus, I am in Seattle, I went through most of these discussions and research about 2 yrs ago for job I was going to do but ran out of time so I referred out to another contractor I know. They did what I was planning, air to water Spacepak Solstice extreme. The client had exactly the same goals, use the large PV array they put on the roof to supply their heat. They did not want any fossil fuel so they used an electric boiler as backup. It was finished last winter. So far no complaints that I heard, the general contractor is a good client of mine. Btw, I talked to John Siegenthaler about the unit prior to recommending as he was and I think is beta testing the unit for factory. The client knew they were a be a bit of guinea pigs as this model is newer higher temp version. They were willing to roll dice. House about 3500 sft. She works for a energy conservation foundation, she had motivation.
    Good luck
  • Jamie HallJamie Hall Member Posts: 11,432
    As you are well aware by now @Icarus ( >:) ), I like to rock boats. So, in that vein, has your client given thought to using solar thermal collectors instead of part of the PV grid? The conversion efficiency with decently designed collectors is much better -- and the result is already hot water. The biggest problem with them is storage -- always has been -- since when it's sunny it's warm, and when it's dark or cloudy it's cold. However, given that you folks seem interested in cutting edge technology, would you be at all interested in looking at using phase change storage to minimize the volume required and hence the insulation required? I know some research has been done on this, though I regret that I can't direct you to any of it (it's way out of my fields of competence).
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
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