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one tankless 2 AH's and DHW

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morelli
morelli Member Posts: 4
Hi Folks,
I was referred by a member from a different forum, suggestion it's been covered here, but I can't find the post. 
I recently purchased a home with a hot water heater and recirc setup feeding two AH's, one up and one down. the WH is very old and I would like to replace with tankless. The AH's are old as well but in good shape, suntherms. I do already have an NPE 240a so would like to utilize it. my question is, (questions are), am I expecting too much out of the single tankless to do two AH's and DHW? would I be better off with the tankless, committing the 240a to DHW and a smaller unit to the AH's ? my concerns are not having either AH's called for a stop and having hot lines not flushed as a DHW call occurs, or have cold air pushing when DHW calls. Second concern is being on well water. Certainly will need  a pressure regulator and perhaps a larger buffer tank of some kind? Well is typically between 7 and 10gpm.  As I write I'm thinking two units, separating AH's from domestic might be the better option?. if that's the case, how do I control more than one AH on a tankless. Any comments, suggestions appreciated. home is 3k square two level, 2 full baths 2 half baths and temps rarely get below 30f.

thanks.

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  • Jamie Hall
    Jamie Hall Member Posts: 23,543
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    With a design temperature of 30 F or so -- relatively warm -- you may not need the capacity of a tankless. The first thing you need to do is figure out what the actual heat loss of your house is. It really isn't good to guess, but fortunately it also isn't that hard to figure out. There are a number of heat loss calculators available, but the one I prefer is this one: https://www.slantfin.com/slantfin-heat-loss-calculator/

    A bit of a learning curve, but nothing a homeowner can't manage pretty easily even with no experience!

    Once you have the heat loss, you can start looking for boilers or water heaters or what have you to power your air handlers. I -- and I think it would be fair to say "we" -- don't really like to use tankless water heaters as the heat source for heating. They aren't meant for it; not designed for that at all. I would expect that you would get better results (and better efficiency!) by looking at a wall hung mod/con boiler). However, that is not to say you couldn't use the Navien, since you already have it, but there are some potential problems -- which you seem to have thought of. The first is that when you are calling for domestic hot water, you will need all, or certainly most of, the output going to the hot water, not the air handlers. Second, you want the air handlers to run all the time, or almost all the time, so you will need an arrangement where they are controlled by their own pump with the Navien injecting heat into the loop from time to time as needed to keep the loop temperature where it should be. Not that this can't be done -- it's just a bit tricky. The third, consideration, however, is more serious: the domestic hot water must be separated by a heat exchanger from the heating circuit. Yes, I know that is not always done-- but even if code permits it in your jurisdiction, it just isn't healthy to use the water from your heating circuit as your domestic drinking water.

    So what we come to is this, if you want to use the Navien: you will have a heating loop, consisting of a pump, a heat exchanger, and the two air handlers. The heat exchanger will take domestic water on one side from the Navien and recirculate that (another pump) when it's called for by the heating loop. The air handler circulating temperature would, ideally, be controlled by an outdoor reset control, trimmed by indoor thermostats if needed.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • morelli
    morelli Member Posts: 4
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    I appreciate your input. it appears the existing HW tank is supplying to the AH's and the DHW. access is very tight in second story cripple wall/roof area. the existing HW tank is ancient. it has to go. rather than me trying to recreate the wheel, maybe just replacing that with a newer HW heater is the best route. would you have any recommendations based on the picture I am going to try to attach for  a good brand, efficient replacement? 
  • kcopp
    kcopp Member Posts: 4,446
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    Wrong application for the water heater. Especially a navien.
    You need a Boiler to run heat to the AHU.
    The tankless is meant to heat domestic hot water only.

    You are mixing your domestic hot water int the AHU... not a great set up.

    A boiler w/ an indirect water heater would be best... especially in a house of that size.


  • morelli
    morelli Member Posts: 4
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    so the tankless that are designed for AHU and DHW don't exist? I've seen several of them. the H2 air kits coupled with navien tankless provide just that.rinnia has some setups as well. is it that you are just opposed to such setups, or navien itself? not trying to be argumentative but no real explanation of why in your response.  Please correct me if I'm wrong, no ego here.  At this point my thought is to replace this HWH with something like a Bradford white that separates the AHU from DHW, rather than deal with tankless. looking for recommendations/ solutions, along those lines. On that note, what brand   "boiler with an indirect water heater" have you had good reliability experience with? 
    thanks-
  • kcopp
    kcopp Member Posts: 4,446
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    Both.
    I dont like navien and dont like using a water heater to heat a house with.
    I equate it w/ a car and a truck...
    Can you pick up 100 2x4 w/ a car and deliver them? Sure. is it the proper tool to do so... no. The car wont last long.
  • HVACNUT
    HVACNUT Member Posts: 5,870
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    You want a combi, not just a tankless water heater. 
    The fans on the air handlers can be controlled by an aquastat or temperature sensor. 
  • Jamie Hall
    Jamie Hall Member Posts: 23,543
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    morelli said:

    so the tankless that are designed for AHU and DHW don't exist? I've seen several of them. the H2 air kits coupled with navien tankless provide just that.rinnia has some setups as well. is it that you are just opposed to such setups, or navien itself? not trying to be argumentative but no real explanation of why in your response.  Please correct me if I'm wrong, no ego here.  At this point my thought is to replace this HWH with something like a Bradford white that separates the AHU from DHW, rather than deal with tankless. looking for recommendations/ solutions, along those lines. On that note, what brand   "boiler with an indirect water heater" have you had good reliability experience with? 
    thanks-

    No, a tankless water heater designed for AHU and DHW doesn't exist. There are, however, devices called "combi" boilers which do exist, and it is possible that this is what you have in mind. These are designed, in fact, quite specifically to do what you want to do. They are single units. One side, with one set of controls, is for heating. The other side, with another set of controls, is for domestic hot water. Properly sized, they work splendidly well, and it is possible that your application may be one for which they would work well. I would be concerned that the heating side would be too large -- although they do modulate -- or the domestic side too small, but once you know how much heating you need it would certainly be worth looking into. There are a number of manufacturers, many of whom are very good. The chief concern would be the local availability of parts and maintenance, as not all brands are available in all areas.

    As to why I am opposed to using a tankless heater for space heating -- and I am -- it is because tankless water heaters are designed and built to use relatively small amounts of water with a very large temperature difference. Heating boilers, on the other hand, are designed and built to use relatively large amounts of water, with low head loss, with relatively small temperature differences. The internal heat exchangers, as a result, are completely different in design and construction.

    As to why I am opposed to using the same water for space heating and for domestic water -- although I have seen this done -- is that space heating water is often in the 120 F range. This is about as ideal a temperature for growing nasty bugs as one can possibly have. Domestic hot water should be created and stored at a higher temperature -- 140 F -- and mixed down for use.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • morelli
    morelli Member Posts: 4
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    Great response thank you! yes I had the npe- 240a in mind as I have an extra with a brand new spare exchanger,( probably for sale shortly), but here as well as another forum discouraged just as you have here for the same reasons. My apologies, I had a combi in mind when I read your response and I thought, "whaddya ya mean they don't make tankless for this". All that aside- I think a Navien fire tube burner would be a good unit to look into, recommended from my other reference. I was looking at the NFC-H. of course I would have to get past the sticker shock, and get a load calc done first. They look like solid units with built in capacity for three AHU's. Would you have a recommendation outside of Navien that you have had good experience with that would be worth a look? I really like naviens support most of the time. Thanks so much for your response.