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Help designing a radiant / DHW system for a small home

doughpat
doughpat Member Posts: 36
Hi all -

I'm in over my head here and I don't have the time/energy to really do the research I need to do in order to be confident in my decisions.

I'm halfway through new construction (walls are open) and I've installed the tubing in the insulated slab, and the staple-up tubing into the subfloors. At this point I've been working with Radiantec, but honestly have my reservations the more I read.

The major problems I'm experiencing are:
1.) This is a small, well-insulated house that will probably have a small BTU heat load. I am concerned about short-cycling, which I think would be a real concern given relatively low ground-water temperatures (and the desire to be able to run a bath) but a relatively small heat load.
2.) I have already purchased the pumps, manifolds, controllers etc. from Radiantec and trying to return any of these items are 'sunk costs'.
3.) I don't trust the one local company in town. They did a shoddy job and drastically oversized a forced air furnace (separate project -- different house), and I haven't been impressed with what I've heard from them.

I'd love to talk to someone (or two) about the best option(s) for my project. I understand there is significant time in doing so and could pay a reasonable amount for someone's time in this. I realize you all help people design their systems via these forums, but I just don't think I have the time right now to invest in this process (normally I have childcare for X hours per day, but with this covid stuff....I'm full-time dad for the unforseeable future, and this project has stalled because of this radiant stuff).

If anyone is willing/able, perhaps we could exchange contact info and have a phone call? I have plans and could email what I've got from Radiantec as well.

Thank you thank you thank you. - Ryan in Bend, Oregon
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Comments

  • I would recommend a boiler sized for your heat load and an indirect water heater or on-demand water heater for your domestic hot water (DHW). Hopefully, you have a good size mechanical room. If not, you can install the on-demand outside.

    Our house is small (1,300 []) and we have a 50,000 BTU Munchkin boiler with a 50 gallon indirect. Plenty of DHW for 2 or 3 people as long as you are mindful.

    Have you calculated the heat load?

    Call me at (510)773-9870 if you have any questions.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour
  • Larry Weingarten
    Larry Weingarten Member Posts: 2,777
    Hello, Perhaps getting a discussion going here could help get the ball rolling. So, a few questions and thoughts. I like simple, so would see if it made sense to separate domestic hot water from space heating. If you did that and if the space heating load was small enough, might a tank-type heater work for it? What is the calculated load?

    Yours, Larry
  • GroundUp
    GroundUp Member Posts: 1,495
    If they didn't sell you a water heater yet, you may still be in luck. What size and brand (O2 barrier or not?) tubing did you install and what is the spacing? I would strongly suggest forgetting most everything you learned from them, honestly can't believe they're still in business. There are some great minds here that may very well be able to get you pointed in the right direction. Some photos and description of what you have so far would be a great start!
    Rich_49
  • Paul Pollets
    Paul Pollets Member Posts: 3,567
    Isn't Bobcat & Sun in Bend, OR?
  • doughpat
    doughpat Member Posts: 36
    They are. Unfortunately when I contacted them they were too busy to take on this job. Maybe its worth reaching to them again.
    kcopp
  • Paul Pollets
    Paul Pollets Member Posts: 3,567
    Some companies wouldn't want to connect to a RT system. Or simply exclude any liability with the product.
    Rich_49
  • GroundUp
    GroundUp Member Posts: 1,495
    We can't discuss pricing here, so somebody may reprimand you for posting the numbers from Radiantec- just FYI. Aside from that, they seem to have begun doing their job instead of what they used to do so you've got a pretty decent setup there actually. A heat loss calc is going to be the first order of business here, but given the R values you've listed I wouldn't bet your current assessment is too far off. Is this going to be a gas fired unit or are you leaning toward electric?
  • doughpat
    doughpat Member Posts: 36
    Oops. I shouldn’t have posted the prices. I will delete!

    This will be gas fired.
  • doughpat
    doughpat Member Posts: 36
    I tried to delete just the picture that showed pricing, but it deleted the whole dang comment. Fortunately I had copy/pasted it for my own reference, so here it is again (minus the pricing-- sorry about that!)
    I appreciate everyone's help as I am really kicking myself for even trying this. My DIY-tendencies are biting me in the butt here. I

    Disheartening to hear I may have "fell for" a bunk company. Anyway, onwards and upwards.

    The heat load is obviously important and I suppose that I should buckle down and do it myself, if Radiantec can't be trusted. They did provide some calculations though, and maybe they are better than what I would be able to do as a lay-person? I've attached their calculations below.


    If I wanted to try and do the Manual J calculation, would I just go to loadcalc.net and use that? I did it once before and felt pretty unconfident in the results. Its not terribly intuitive. I am actually also sourcing a mini split heat pump (as a backup heat source, as well as cooling) from eComfort.com and they supposedly also do a heat calc (you pay $50 which is refunded when you buy the mini split from them). So I suppose I'll be getting a second heat load calculation from them.

    I used 1/2" O2 barrier PEX, as called for (and sold by Radiantec). It is spaced as spec'd (6" in the slab, 8" in the joisted floor). The concrete slab has 3" of foam below the entire thing, and 3" around the interior perimeter.

    Big picture: this is a 2 story small home, with a "posh garage" (fully insulated slab-on grade garage, which is zone 1). Zone 2 is the main living area, which is a 1 bed, 1 bath (+ office, which might also be a sleeping room). Insulation and air sealing is very good (R39ceiling/floor, R23 walls, lots of attention to sealing).

    Zone 1 has 3 circuits, each of which are ~270 ft. They are all in the slab.
    Zone 2 has 4 circuits, each of which are also roughly 270 ft -- these are a bit more variable. Half of one of the circuits is in the lower floor (laundry room), while the other half is in the upper floor (office). I realize this may not be ideal, but "whats done is done".









  • doughpat
    doughpat Member Posts: 36
    I suppose the first thing I should start with is figuring out how many separate "systems" I want to have. It seems like the most common options are:

    1.) Combi-boiler. I spoke to a local radiant heat person and they suggested that 110k BTU would be sufficient, and 150K would be more than enough. 10:1 turndown.
    2.) Two completely separate systems (each could be tanked/tankless, but both would be water heaters)
    3.) A tanked heater with side-taps (I'm not sure how these work, but I like the sound of it!) Are heat exchangers required?
    4.) There are other ways, I'm sure!
  • Erin Holohan Haskell
    Erin Holohan Haskell Member, Moderator, Administrator Posts: 1,982
    doughpat said:

    I tried to delete just the picture that showed pricing, but it deleted the whole dang comment. Fortunately I had copy/pasted it for my own reference, so here it is again (minus the pricing-- sorry about that!)

    @doughpat, your post wasn't deleted, but it was held for moderator approval. We've sorted it out. Thanks!
    President
    HeatingHelp.com
  • GroundUp
    GroundUp Member Posts: 1,495
    doughpat said:

    I suppose the first thing I should start with is figuring out how many separate "systems" I want to have. It seems like the most common options are:

    1.) Combi-boiler. I spoke to a local radiant heat person and they suggested that 110k BTU would be sufficient, and 150K would be more than enough. 10:1 turndown.
    2.) Two completely separate systems (each could be tanked/tankless, but both would be water heaters)
    3.) A tanked heater with side-taps (I'm not sure how these work, but I like the sound of it!) Are heat exchangers required?
    4.) There are other ways, I'm sure!

    The actual loads are going to determine your needs, but given the size of the home and approximate heat load per zone it may be worth looking at something like the Laars Combi-Heat. It's a tank style powervent water heater with an internal heat exchanger to be used for the space heating portion. You do NOT want to mix your space heating and domestic water. Personally I am not a fan of combi boilers due to the maintenance requirements and "cold sandwich" tendencies. The combi-heat tank style unit (Bradford White has a "Combi 2" tank as well, but much harder to source around here) is more simplified and DIY friendly, at a lower cost, but at a considerably lower efficiency than a condensing boiler. If it were mine, I would lean toward a heat-only condensing boiler and a slightly oversized indirect water heater tank. With all your low temp radiant, this maximizes thermal efficiency to 90+% for both heating and domestic water and leaves you with only one burner to service, one gas line, one exhaust pipe through the wall, etc. A powervent water heater tank is going to be closer to 70% overall efficiency, so the above route would save a considerable amount of fuel.
    Zman
  • doughpat
    doughpat Member Posts: 36


    The actual loads are going to determine your needs, but given the size of the home and approximate heat load per zone it may be worth looking at something like the Laars Combi-Heat. It's a tank style powervent water heater with an internal heat exchanger to be used for the space heating portion. You do NOT want to mix your space heating and domestic water. Personally I am not a fan of combi boilers due to the maintenance requirements and "cold sandwich" tendencies. The combi-heat tank style unit (Bradford White has a "Combi 2" tank as well, but much harder to source around here) is more simplified and DIY friendly, at a lower cost, but at a considerably lower efficiency than a condensing boiler. If it were mine, I would lean toward a heat-only condensing boiler and a slightly oversized indirect water heater tank. With all your low temp radiant, this maximizes thermal efficiency to 90+% for both heating and domestic water and leaves you with only one burner to service, one gas line, one exhaust pipe through the wall, etc. A powervent water heater tank is going to be closer to 70% overall efficiency, so the above route would save a considerable amount of fuel.


    Can I start with a couple of basic questions regarding your reply? You are proposing two separate approaches: 1.) A "Combi-Tank" or 2.) A "(Radiant) Heat only condensind boiler with indirect water heater tank", correct?

    I'm working on finishing the manual J calculations today and hopefully will have some specific numbers to work with.

    For now, if we assume a total of 40k for radiant needs (20k per zone), can you point me towards a specific "heat-only condensing boiler and slightly oversized indirect water heater tank"? I'd like to start wrapping my head around the actual products available, including their sizing. Once I get the sizing nailed down, I can tweak the heater/tank accordingly.

    Regarding the combi-tank idea; I like it....I also don't like the cold-sandwich effect, nor do I like the "intensity" of on-demand style heaters. It just seems like a better idea to heat water slowly and store the heat in a tank rather than these ferocious, noisy, lots-of-moving-parts on-demand setups. My plumber had, since the start, recommended the AO Smith Vertex as a potential option. Is that similar to the Laars Combi-heat model you mentioned? It says it has "side taps for simple installation in combined heating applicatons".

    If I'm understanding you right, the tradeoff is one of "cost/simplicity" vs. "efficiency".
  • GroundUp
    GroundUp Member Posts: 1,495
    edited April 2020
    Yes, those are two separate proposals. The tank option would be cheaper and easier but less efficient.

    With a 20k per zone heating load, pretty much any <100k boiler is going to suffice. How big the indirect tank should be is dependent on your DHW needs. A typical 2 bathroom household can get away with a 30 gallon indirect tank if operated at a higher temp with a mixing valve (which also curbs Legionella potential) but at a tradeoff of some efficiency as the boiler needs to run at a higher temp to heat the domestic water and will not usually condense. We're talking upper 80's percent efficient versus low 90's with a larger, lower temp tank. A larger tank would give more usable water at a lower temperature, which raises efficiency a bit but also allows Legionella growth. Not that Legionella is a common thing, there are hundreds of thousands of WH tanks run well below the 140 degree cutoff for Legionella growth and we never hear about anybody catching it so it's a moot point for a lot of people. I'm weird about it and I prefer a mixing valve anyway for consistent water temps so my vote would be the smaller tank with a higher temp.

    Again this is merely a personal choice, but I've been very pleased with the performance and pricing of the Laars Mascot FT boilers as well as their Laars-Stor indirect tanks. Bradford White Brute and PowerStor tank is pretty much identical to the Laars. Triangle Tube Smart, Lochinvar Squire, and HTP SSU tanks have also treated me very well but I like the boiler and indirect to match.

    Budget minded, I'd quote it for the Mascot MFTHW-80 or MFTHW-100 and 40 or 50 gallon Laars-Stor indirect.

    For an added cost with a slight quality upgrade, the Lochinvar Noble NKB080 or NKB110 and Squire 40 or 50 gallon indirect would be my choice.

    Tank and boiler sizing really depend on DHW usage. Personally I'd go with the 80k and 40 gallon if it were my own home.

    The Vertex is domestic only and while it has side taps, you'd need to add an external heat exchanger to appropriately separate the two systems. For the cost of the Vertex and added piping/headache to make that work, a boiler and indirect would be a much better option IMO.
  • doughpat
    doughpat Member Posts: 36
    *Thank you* so much for these specifics. Now I can really sink my teeth into how this all ties together.

    It is only a 1 bathroom home (it is a bath/shower) (which would push towards a smaller 80k burner) but we do have relatively cold groundwater (which would push towards the 110K burner).

    I'd lean towards efficiency vs. the remote chance of legionella, so the larger tank at a lower temp would probably be my choice.

    Newbie question here: The indirect tank is where the DHW is drawn from, correct? While the modulating condensing boiler is what provides heat for the radiant?
  • pecmsg
    pecmsg Member Posts: 3,526
    edited April 2020
    For control of the legionella bacteria if hot water is to be stored it needs to be kept at 140°f for at least 1 hour. Point of use should not be over 120°F the tank will need a mixing valve.

    I see a chart above (post 10) showing 36,000BTU's so why are you considering 100K BTU heater?
  • doughpat
    doughpat Member Posts: 36
    Thank you. I had only considered the 100K boiler because it was suggested by GroundUp? I'm sure he has a reason for this suggestion. Obviously I'm learning as I go here. I'm guessing the reason for the 'oversize' (110K or 150K) is so that for DHW, the burner can "keep up" with a bath?

    I suppose a different approach is to use a smaller boiler (something closer to the 36K BTU needed for the radiant) and use a larger indirect tank. That way there is a larger reserve for a bath?
  • doughpat
    doughpat Member Posts: 36
    Alright -- got a proper Manual J calc done by the sizing specialist at eComfort. I'm buying a mini split from them so they provide that service ($50, but credited back if you buy the mini split).

    I'll post the whole thing here, but will interpret it as best as possible.

    The entire house heating BTU load is 28,655. Broken down into the two zones, that is 11,752 BTU (zone 1) and 16,903 BTU (zone 2).

    If I understand correctly, I should size the heat source for the combined zones (~29K). I would imagine adding some amount of "fudge factor" might be wise here, though I am quite confident that we are pretty close with this number. The technician from eComfort was asking a lot of specific questions regarding insulation, window/door U factors, orientation, etc. Their number is reasonably close (29K BTU from eComfort vs 36K from Radiantec). I suppose I could design using an average value of the two (33K). I'll start digging around for boilers in that range.
  • GroundUp
    GroundUp Member Posts: 1,495
    Boilers have a "turndown ratio" which means they can reduce the firing rate to a certain point. For example the Laars 100k has a 5:1 TDR which means the low firing rate is 20,000 BTU. Any load less than 20k would cause that boiler to short cycle without a buffer tank. The indirect tank is little more than just a storage tank with an internal heat exchanger which acts as another zone from the boiler. When the domestic water temperature in the tank falls below X, the thermostat in the tank (just like your wall thermostat) tells the boiler to fire and send heated water through that exchanger which heats the stored domestic water by conduction. The only reason to go with a larger boiler would be for recovery purposes of the indirect, as domestic water is always priority. This means you would not be able to get any space heating from the boiler until the indirect tank is satisfied so the faster the tank is satisfied, the sooner you can get space heating again. Not a big deal for the high mass slab, but potentially a problem for the low mass upper level. If your small zone is only 11k, you're going to want your boiler to be able turn down to 11k or less in an ideal scenario. The Laars will not do the 11k, but the Noble has a 10:1 TDR. As far as I know, 8,000 BTU is the lowest firing rate on the market at this time and can be accomplished with the 80k Noble or something a little simpler like the HTP UFT-80W. With only a single bathroom, domestic water may not be as large of a priority for you and you could afford to wait for the tank to charge which would allow a smaller boiler if you wish. There are some tradeoffs with this type of thing, you can't really have your cake and eat it too. Larger tank equals higher efficiency but longer recovery times. Smaller boiler equals lower firing rate but longer recovery time. If it were mine, I'd go with the smaller boiler and smaller tank at a higher temp but if you'd rather have the added efficiency of a larger tank, a larger boiler might be worth considering to maintain setpoint in your low mass upper level during heavy DHW usage. 40 minutes of consecutive 2 GPM shower would require a 27% longer boiler cycle with an 80k boiler to catch up, versus a 110k. You might lose a degree or two in ambient air temp upstairs between priority cycles waiting for the tank to catch up. What's your risk tolerance on setpoint?
  • Larry Weingarten
    Larry Weingarten Member Posts: 2,777
    Hello, It seems to me that you're down in the BTU range where in addition to looking at boilers, you might compare tank type water heaters, including heat pump units.

    How stable is the power supply in your area? If it's good, then going all electric could be a good and future-proof way to go, as gas is being phased out in different areas.

    Any chance of preheating with solar thermal?

    Yours, Larry
  • pecmsg
    pecmsg Member Posts: 3,526
    I would redo that calculation here
    https://www.slantfin.com/slantfin-heat-loss-calculator/
    I question eComfort for a legitimate Manual "J"
    No you don't need a Fudge Factor.

    I respectfully disagree with a 100K and turn down. Even with turn down you want it close to 100% for full efficiency.

    @Larry Weingarten brings up a good point. Air to water heat pump although pricy is something to look into.
    Larry Weingarten
  • doughpat
    doughpat Member Posts: 36
    Great points, everyone. Let me try to respond to each:

    @GroundUp I am not concerned about losing heating abilities in the recovery period. I believe the house will be well insulated enough that it won't be a major issue. Plus, there will be a mini-split which would be able to take over if it really did become an issue. I am definitely ok with 1 or 2 degree temp drop (or more, hoenstly) upstairs during heavy DHW usage. Efficiency of operation (and avoidance of short cycling) is a higher priority for me.

    If I understand you correctly, it sounds like I should go with a smaller boiler (one capable of turning down to 11k or lower), and a larger indirect tank. That would mean that my indirect tank would provide enough DHW for a long shower/bath, but it would take relatively long to recover. During that recovery period, there would be no radiant heat.

    @Larry Weingarten I am definitely open to a tank. Whether that tank is an indirect tank, or a "direct tank" (if that's a term) is beyond my knowledge. As stated earlier, I like the idea of a tank serving as a buffer. I suppose the benefit of the indirect tank is that it combines the heat exchange and the buffering capacity in one.

    I don't want to go with electric, as I've gone through considerable expense to connect to gas. And if I understand it right, heat pump water heaters seem relatively new to the scene (expensive and maybe a bit untested, and unfamiliar to local tradespeople for installation/maintenance). Also I'd have to have more refrigerant lines, and another outdoor compressor unit (exterior space is limited). Last but not least: I understand some big left-leaning cities are forbidding new gas hookups, but in my area, I don't see that happening for a very long time, let alone forcing people to give up existing gas appliances.

    @pecmsg I'm not sure if my numbers would be any more reliable than the two calcs that were done (independently) by Radiantec and eComfort. Especially the guy at eComfort -- we had 3 conversations where he was asking lots of clarifying questions and made it clear to me that he was very qualified and putting personal attention on this calc. I explained to him that i'd be using it for radiant applications -- that is why he also did the first floor calcs, which won't even be served by the mini split that his company will provide.

    One more question: If the DHW is served by the indirect tank (which inherently serves as a buffer, and short cycling won't be an issue with DHW), then that means the radiant heat is served directly by the boiler. That means the radiant side has no buffering capacity (other than the mass of the floor). If someone really wanted to reduce on/off cycling, could they use another tank "in series" with the radiant heat in order to provide more mass to absorb/release heat energy over time?

    YOU ALL ARE AWESOME. Thank you from both my wife (who has to bear with my head-scratching and too-verbal-thinking) and myself.
  • hot_rod
    hot_rod Member Posts: 18,157
    All heat load numbers have a fudge factor, at least 5%. Also that load number is only on a design, coldest day. In many areas you are at design less than 10% of the heating season.

    A combi's boiler with a turn down to 8,000 would be my choice. A buffer could be added.

    A reverse indirect with a small boiler could be another good option. It would buffer the small loads and provide the DHW, since the plate system needs a higher temperature.

    Here is how I built my system with a 2 zone, load close to what you have. Insulated piping to the right is DHW, small 6 gallon HW tank as a buffer. Lochinvar Nobel Combi 120 on LP

    The tank has an electric element capable of 18,7000 BTU in event I run out of LP or the boiler goes down.

    I do have solar pre-heat and can get through 4 months usually with no LP consumption.

    If you can still order a Bradford White Combi 2 it is ideal for these small load, jobs. Probably the least expensive, simplest one tank solution, although only high 70's or low 80% efficiencies.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • GroundUp
    GroundUp Member Posts: 1,495
    pecmsg said:

    I would redo that calculation here
    https://www.slantfin.com/slantfin-heat-loss-calculator/
    I question eComfort for a legitimate Manual "J"
    No you don't need a Fudge Factor.

    I respectfully disagree with a 100K and turn down. Even with turn down you want it close to 100% for full efficiency.

    @Larry Weingarten brings up a good point. Air to water heat pump although pricy is something to look into.

    Where do you suggest finding a boiler with an <11k low fire but a high fire less than 80k? The 100k theory was only in light of the DHW requirements.
  • GroundUp
    GroundUp Member Posts: 1,495
    @doughpat yes, that is essentially the gist of what I'm after. The reverse indirect is certainly another option which would use the tank volume as a buffer, but pull DHW from the internal heat exchanger. Opposite of a standard indirect. Honestly I would not be comfortable with anything under 80k serving my DHW unless the volume was huge, and this is only my opinion but I'm not a fan of any of the boilers on the market with a high fire lower than 80k. Several ways to skin this cat!
  • doughpat
    doughpat Member Posts: 36
    edited April 2020
    Yes, you are right -- so many options. Unfortunately all of them looks like they are going to be $ plus substantial labor.

    I didn't want to derail this thread completely, so I posted a new thread that I'd ask if you all would mind reading?

    https://forum.heatinghelp.com/discussion/179602/dhw-now-radiant-later/p1?new=1
  • doughpat
    doughpat Member Posts: 36
    hot_rod said:


    A combi's boiler with a turn down to 8,000 would be my choice. A buffer could be added.

    A reverse indirect with a small boiler could be another good option. It would buffer the small loads and provide the DHW, since the plate system needs a higher temperature.

    Here is how I built my system with a 2 zone, load close to what you have. Insulated piping to the right is DHW, small 6 gallon HW tank as a buffer. Lochinvar Nobel Combi 120 on LP

    If you can still order a Bradford White Combi 2 it is ideal for these small load, jobs. Probably the least expensive, simplest one tank solution, although only high 70's or low 80% efficiencies.

    Hey @hot_rod --thanks for the pictures.

    -I think the problem with a combi-boiler than can turn down to 8,000 BTU's would only be 80,000 BTU under full power. I am almost certain that isn't enough power to give a 70F temp rise for DHW.

    -WIth your system, is the DHW served directly from the Lochinvar combi boiler? I wonder if that would be sufficient for our groundwater. My plumber seems to think that 180K is minimum and 199K is worth it.

    And just so I understand your setup fully: the boiler probably turns down to ~12K for your radiant system (assuminig it is a 10:1 turndown), and the 6 gallon buffer tank helps to reduce short cycling?

    What is your groundwater temp? And you said the heating loads are 'similar'....can I ask exactly what the heat load is of each zone? If you can't tell, I like your setup....it looks reasonably simple, and a local company that does radiant heat has steered me towards the Lochinvar Nobles. I might use a larger buffer tank, but that seems pretty trivial.
  • doughpat
    doughpat Member Posts: 36

    Hello, It seems to me that you're down in the BTU range where in addition to looking at boilers, you might compare tank type water heaters, including heat pump units.
    Yours, Larry

    Hi Larry -
    Can you describe how I'd use a tank-type (gas-fired) water heater for my setup? I still think the tank's buffering would really be the answer to a lot of my issues (small heat loads, but cold groundwater+bath).

    (also, keep in mind I have to vent horizontally through a sidewall....I believe that means I have to use a power-direct vent model??)
  • hot_rod
    hot_rod Member Posts: 18,157
    doughpat said:

    hot_rod said:


    A combi's boiler with a turn down to 8,000 would be my choice. A buffer could be added.

    A reverse indirect with a small boiler could be another good option. It would buffer the small loads and provide the DHW, since the plate system needs a higher temperature.

    Here is how I built my system with a 2 zone, load close to what you have. Insulated piping to the right is DHW, small 6 gallon HW tank as a buffer. Lochinvar Nobel Combi 120 on LP

    If you can still order a Bradford White Combi 2 it is ideal for these small load, jobs. Probably the least expensive, simplest one tank solution, although only high 70's or low 80% efficiencies.

    Hey @hot_rod --thanks for the pictures.

    -I think the problem with a combi-boiler than can turn down to 8,000 BTU's would only be 80,000 BTU under full power. I am almost certain that isn't enough power to give a 70F temp rise for DHW.

    -WIth your system, is the DHW served directly from the Lochinvar combi boiler? I wonder if that would be sufficient for our groundwater. My plumber seems to think that 180K is minimum and 199K is worth it.

    And just so I understand your setup fully: the boiler probably turns down to ~12K for your radiant system (assuminig it is a 10:1 turndown), and the 6 gallon buffer tank helps to reduce short cycling?

    What is your groundwater temp? And you said the heating loads are 'similar'....can I ask exactly what the heat load is of each zone? If you can't tell, I like your setup....it looks reasonably simple, and a local company that does radiant heat has steered me towards the Lochinvar Nobles. I might use a larger buffer tank, but that seems pretty trivial.
    By the numbers
    500 X flow X ∆T
    500 X 2 gpm X 110-70 = 70,000 BTU required

    A 110 Combi should get you around 2.5b gpm at a 77° rise
    The smallest combi with a 10-1 I have found is around a 110- 120K. An 80K imput with 10-1 would be a nice size if you can find one and if 2 gpm is adequate.

    With the Nobel I locked the heat output to 30% it never fires above that, which also helps short cycling.
    6- 20 gallon tanks are fairly inexpensive and give you just enough to buffer low loads.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • doughpat
    doughpat Member Posts: 36
    Here's a strange question: It seems that a lot of the issues I am having are because there is a bath in the bathroom, which "requires" a high flow rate. Could I restrict the flow rate to the bath, and accept that the bath will take a relatively long time to fill? That way it would allow me to use a much smaller combi boiler that will be able to down modulate to a BTU output more appropriate to the radiant?
  • hot_rod
    hot_rod Member Posts: 18,157
    High flow rate, like how many GPM? If you have 2.5 gpm, which blends with some cold, how large is the tub? 30 gallons? What is the flow rate of the valve on the tub?

    If you want large flow fast, two choices really either a tank that can store that dump load, or enough BTU to generate it continously.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • doughpat
    doughpat Member Posts: 36
    hot_rod said:

    High flow rate, like how many GPM? If you have 2.5 gpm, which blends with some cold, how large is the tub? 30 gallons? What is the flow rate of the valve on the tub?

    If you want large flow fast, two choices really either a tank that can store that dump load, or enough BTU to generate it continously.

    I'm not sure of the tub size -- I'll go check soon. Its a "small" standard tub.

    The faucet has not been chosen/purchased yet....I'm guessing I could find a relatively low-flow faucet which would limit the BTU's required?
  • hot_rod
    hot_rod Member Posts: 18,157
    How much/ how fast is the bottom line.

    A standard depth 5 foot cast or steel tub, probably around 80 gallons at 105F to get a comfortable bath.

    next determine how quickly you want to fill that 80 gallons.

    At 2 gpm??? probably not reasonable.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • doughpat
    doughpat Member Posts: 36
    Alright...had a well-reputed heating company out to look at the setup and give me some advice.

    They are proposing the Lochinvar Noble 110K BTU, no buffer/indirect tanks -- just hooked up directly.

    They're saying they've used them in lots of houses similar to mine, in my area, and have had smooth sailing. They said that there isn't going to be a problem with short-cycling (outdoor reset), and that a buffer tank isn't necessary.

    They are also saying that a small bathtub should be fine, and to keep in mind that you aren't filling the tub with 120F water -- it'll be mixed with some cold as well. They do say to get a faucet that corresponds to the GPM rate of the combi-boiler.

    I have to admit I like the simplicity and the low-footprint of this setup, as its a small house. Also I can't help but think the indirect tank + boiler is adding quite a bit of complexity and cost, neither of which I really can afford right now.

    Am I going to regret this?
  • hot_rod
    hot_rod Member Posts: 18,157
    I've lived with 3 Combi boilers at my property, one for 15 years. My wife takes a tub often, The 110 Nobel I have keeps her in hot water just fine, 55° well water.
    You could crank the Nobel DHW temperature to 120, run full hot and adjust the flow at the tub filler to keep up with the gpm flow and temperature you want, if the tub flow rate outruns the Nobels output.

    A buffer could be added later, if you feel it cycles to often.
    With modulation and firing limit this is light years ahead of the old "single speed" boiler we applied to radiant for years.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Larry Weingarten
    Larry Weingarten Member Posts: 2,777
    Hello, With venting, there are many ways to pet that cat. For example, you could run it out a sidewall and then run it up over the roof. Then build a chase to enclose it. Maybe even build an exterior shed for a tank?

    About "regretting this", I like to have shower water come from a tank. This way you're not depending on controls keep a very tight temp control. Showers are no fun if the temperature fluctuates much at all. Having adequate storage would also eliminate the problem of slow tub filling.

    Yours, Larry
  • doughpat
    doughpat Member Posts: 36
    hot_rod said:

    I've lived with 3 Combi boilers at my property, one for 15 years. My wife takes a tub often, The 110 Nobel I have keeps her in hot water just fine, 55° well water.
    You could crank the Nobel DHW temperature to 120, run full hot and adjust the flow at the tub filler to keep up with the gpm flow and temperature you want, if the tub flow rate outruns the Nobels output.

    A buffer could be added later, if you feel it cycles to often.
    With modulation and firing limit this is light years ahead of the old "single speed" boiler we applied to radiant for years.

    Thank you for pointing out that a buffer could be added later. I think I will give it a go without a buffer tank, but reserve space for the addition of one later. How large of a buffer tank would be prudent? I know that @hot_rod used a 6 gallon tank. I believe someone else said up to 20 gallons.

    Good to know your Noble 110 keeps up. If one was to "adjust the flow at the tub filler" -- is that done at the faucet level? I've only done a little bit of plumbing repair, but I don't recall seeing that type of adjustment. I haven't bought the bath tub faucet yet (though the rough-in is done), so I'll need to keep an eye out for that ability.
  • doughpat
    doughpat Member Posts: 36


    About "regretting this", I like to have shower water come from a tank. This way you're not depending on controls keep a very tight temp control. Showers are no fun if the temperature fluctuates much at all. Having adequate storage would also eliminate the problem of slow tub filling.

    Yours, Larry

    Hi Larry - thanks for the input. I suppose I'll have to just hope the Lochnivar maintains a decently steady temperature. I have heard that some people will actually put a small electric tank downstream of a tankless water heater....which seems a bit silly, but I suppose would help to smooth out the temp a bit. Just don't like the idea of spending all this money to avoid standby losses only to introduce them with an electric tank!
  • hot_rod
    hot_rod Member Posts: 18,157
    HW temperature will fluctuate a few degrees, I have not found it to be much concern. It is most noticeable on a small flow as the boiler cycles up and down more. With a 1.5bvgpm shower head, the output is very stable.
    Not a concern at all with a tub filler.
    Many shower valves have temperature control built in and some more expensive have temperature /pressure balance control response.

    A small electric 120V point of use could be added like the Eemax. Those are often added to boost low temperature DHW sources.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • doughpat
    doughpat Member Posts: 36
    I am intrigued by the point of use fixture. I'm curious how much of the heating load it ends up providing. It "seems like" it might end up providing quite a bit, considering how often the kitchen faucet gets used for just brief periods. Washing hands, a quick dish-wash, etc.

    I suppose that it doesn't help reduce the number of on/off cycles on the Lochinvar combi. The combi is going to fire up any time hot water is called for. The point of use heater will just reduce the amount of time you have to stand there and wait for the hot water to arrive. In fact, I could see it being even harder on the combi, as the water will be shut off even faster....before the combi really even gets going?

    Do they require a dedicated circuit? I've got the walls open now and running a circuit (even 240V one) would be relatively easy now...