Radiant heat design details – Bosch or Navien combi boiler?
I am getting into the details of our radiant and DHW design and was looking for some feedback on the boiler choice. I have been planning on using a combi boiler and so far favored a Bosch model, but have started to wonder whether the Bosch would short cycle in my planned setup.
These are the two Combi Boilers that I am considering:
The property details are:
- ZIP code: 94941 (California)
- Two stories, 2,358SF
- 5 BR (3 actual bedrooms, 1 Office/Guest, 1 Guest), 3.5 BA
- New in-wall insulation (Roxul Comfort)
The radiant heating system was calculated and designed with the following parameters and results:
- Title 24 calcs: total output (Btuh): 75,000 – total system load: 49,738
- Lower level via embedded in slab (729SF)
- Upper level via joist trak plates below sub-floor (1,812SF)
- Total Zones: 6, total RH circuits: 16
- Water temperature: 95.3
- Total flow: 5.37
- Head loss: 7.0 (not yet including the plumbing from boiler to manifolds)
- Radiant design calcs: total heating load: 25,013 Btu/hr
- Single pump for all zones
Aside from the best Combi Boiler for the above, I’m also wondering what recommendation for a single pump there are? I am considering the Taco 008 Cast Iron Circulator, 1/25 HP
I would appreciate any feedback on the above!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Parse the meaning of this line for me:
"Title 24 calcs: total output (Btuh): 75,000 - total system load: 49,738"
A total oad of 49,738 BTU/hr for a 2,358' house is a RIDICULOUSLY high number for a location with a 99% outside design temp of 38F.
(See p.42, PDF pagination for Mill Valley's 99% outside design temp)
Even an uninsulated house with tightened up wood sash single panes would not be that high. With code-minimum double panes and R15 Roxul and at least R19 in the attic you're probably looking at a whole-house load under 20,000 BTU/hr @ +38F, nowhere near the " ...total heating load: 25,013 Btu/hr..." number.
Who designed the radiant?
The fact that you have it cut up into 6 zones is an issue. I suspect you'll be much better off with a tank-type combi heater with it micro-zoned like that.
Calculate the mass of the water in the tubing on each zone and report back the masses by zone.
Using an old-school low efficiency pump like a Taco-008 on a multi-zoned system would be a mistake. An ECM-motor smart-pump such as the Taco VR1816 set up for constant pressure using zone valves would probably pay for itself in reduced power use in short years.
The line "Title 24 calcs: total output (Btuh): 75,000 - total system load: 49,738" is from the original Title 24 calculation, which was based on a smaller remodel plan as well. Basically, the house does not have any insulation in the walls, all windows are still the original windows from 1953, and the attic insulation - fiberglass insulation - has degraded to being useless.
The actual radiant design then took into consideration our revised plan for more efficient windows as well as putting in wall insulation in all walls as well as adding the R30 insulation in the attic.
Radiant design was done by Uponor Engineering as I am planning on using Uponor materials.
I don't have the water mass and wouldn't know how to calculate it. Parameter details I can offer per Zone: Area, Heated Area, Total Tubing, Required Temp, Unit RH Load, RH load, Total Load, Est. Peak Output.
I still don't know what the meaning of that sentence is.
A "total output" of 75,000 BTU/hr means what? Was that the output size of the original heating unit?
A "total system load" of 49,738K means what? Is that somebody's calculated Manual-J on the heat load prior to improvements?
The radiant design says total heating load of 25,013 Btu/hr, which would be a credible number for a house that size with almost no insulation. Those other numbers would require leaving some windows open. For a tightened up 2x4/R15 house with R30 in the attic it's still suspiciously high.
For reference, my house has crummier insulation than that, about the same 2400' of fully conditioned space plus another 1500' of insulated basement (on a not-very-efficiency sprawling footprint mostly 1-story with a lot of corners). My house comes in at about 17,500 BTU/hr @ +38F, maybe 20,000 BTU/hr in a gale-force wind.
If you know the total amount and sizing of tubing on a zone, you can calculate the water volume. (We'll keep it in English units, since we're working with BTU)
Half-inch PEX runs about 0.9 gallons per 100' of tubing. That's 8.34lbs/gallon x 0.9= 7.5lbs per 100'.
If it's 3/8" PEX it's about 0.5 gallons per 100', or (x 8.34=) 4.17 lbs per 100'.
But to get back to your initial question, the Greenstar 100's min-fire input is 34,600 BTU/hr (http://s3.supplyhouse.com/product_files/ZWB-28-3-brocuhre.pdf ), and at 95% combustion efficiency it's minimum output would be about 33,000 BTU, which is way over your DESIGN condition load. according to the radiant designer's calculations. That means that even if it were all one zone it would never modulate, only cycle on/off, and given that it's cut up into 6 zones, most of which are low-mass underfloor types, it's going to be a short-cycling disaster. If what I expect is true, that your load is under 20,000 BTU/hr, even at min-fire it's getting onto 2x oversized for the load. Cross this one off your list- it's totally inappropriate on all counts.
With a min-fire input of 12,000 BTU/hr the NCB 150 is better, but not a heluva lot better. At 95% combustion efficiency it's min-fire output is going to be ~11,500 BTU/hr. If the house was operated as a single zone that would be sorta-OK, it would still modulate at least some of the time. But if the calculated 25K load is true, the average zone load is about 4200 BTU/hr, which means the min-fire output is over 2x oversized for any individual zone even at the 99% outside design temp, probably more 5x oversized for any one zone for the average wintertime outdoor temperature. If it's not going to modulate and only cycle on/off 90%+ of the time, there's no point to a modulating boiler.
The only thing that can save the NCB from a short-cycling itself into low efficiency on zone calls is thermal mass, which is why we need to get a handle on the amount of thermal mass in the zone radiation. I'm skeptical that it's sufficient.
One potential solution is to install a thermally massive buffering hydraulic separator such as a Boiler Buddy instead of Navien's ready-made manifold, but a simpler cleaner solution might be to go with a tank type combi such as an HTP Versa.
As a general rule "wall hung combi boiler" and "micro zoned heating system" are almost never compatible due to the limitations of modulation on combi boilers. At your low whole-house load even the smallest mod-con boilers probably won't balance well enough on a single zone call, the only credible solution to which is thermal mass. You can either put the mass in the radiation (the slab zone is probably fine), or in a buffer tank (or buffering hydraulic separator. Combi boilers are rarely the "right" solution to a multi-zoned system unless they're inherently self-buffering, like the Versa.
With your low loads and micro-zoning, in the hands of the right hydronic designer you can probably do just as well or better for less money using a condensing tank hot water heater and an isolating heat exhanger between the potable and heating system sides.
Hmm, that doesn't sound too good. I would really prefer to keep footprint and design as small and simple as possible. Are there alternatives to the Versa products? (In case the Navien does not work out.)
And the Versa come with a massive 100,000 BTU for heating. Would that really end up being more efficient?
Zones 10X are embedded in slab. Zones 20X are joist Trak plates.
The following are my mass calculations: (all 1/2" Pex)
Zone 101: 22.05lbs (Total tubing: 294)
Zone 102: 26.03lbs (Total tubing: 347)
Zone 201: 114.90lbs (Total tubing: 1,532)
Zone 202: 35.85lbs (Total tubing: 478)
Zone 203: 20.70lbs (Total tubing: 276)
Zone 204: 39.53lbs (Total tubing: 527)
Total volume about 259lbs.
Dana, in addition to my previous post with the mass calculations I did, I still owe you an explanation of "total input". But instead of trying to explain what "total output" might mean, which I'd only be doing based on my assumptions, I thought I'd just upload the Title 24 analysis report that I was handed when I bought the home a few months ago. (The previous owners had started the remodel planning, but had never finished it. I bought the house, changed the remodel plans and also added radiant heat to it.) I took it from the last page of the report.
According to the report, the design heat load is 49,738 BTU/h, and the design cooling load is 44,155 BTU/h. (The design cooling load is the sum of the sensible load and the latent load).
The 75,000 BTU/h number is simply the output of the existing heating equipment.
The usual problems are evident: the heat load calculation seems to be high (due to errors made by the person who entered the data used to make the calculation), and the HVAC contractor who selected the heating equipment grossly oversized the equipment. Happens all the time.
One think to add to my post with the mass calculations is, Zone 202 and 203 are Kids bedrooms. I could combine them into a single Zone, increasing the mass, if that would make a difference and allow us to utilize just a min. 12,000 BTU boiler, such as the Navien, and allow us to avoid a buffer tank. By doing that, Zone 204 would then be the smallest Zone in terms of mass I guess.
BTW: I'm assuming that Zone 101 and 102 aren't an issue as they are sitting in the slab and hence would not cause short cycling?
The Versa works at higher efficirncy because has a ~450ls lbs of thermal mass to work with, and a 10:1 turn down ratio on the 100KBTU/hr heating module- it can dial back to about 9500 BTU/hr -out.
First let's do some napkin math on your radiation & loads and the NCB-50. We'll make the simplifying assumption sthat if you design-condition load is 25,000 BTU/hr, your average load will be half that, or 12,500 BTU/hr, and that the load per zone is proportional to the tubing or water mass in that zone. Without looking it up, lets assume the NCB will swing around it's setpoint within about a 5F range. (It may be programmable for a higher differential than that, but you'd have to dig that out of the documentation.) Without running the numbers separately we'll assume that if properly pumped the slab zone have sufficient thermal mass to keep the short cycling bounded.
The largest zone # 201 is about 115lbs out of 260lbs total, so the average load is about 12.5K x 115/260= 5529 BTU/hr
The minimum output of the NCB is about 11,500 BTU/hr so the zone has about 6000 BTU/hr, or 100 BTU/minute of "extra" heat dumping into it with the boiler at minimum fire. It takes 1 BTU per lb to raise the water temp 1F, so to swing the full 5F differential between the high & low on the boiler with 115lbs of mass takes 115 x 5= 575 BTU. At 100 BTU per minute that takes 5 minutes 45 seconds. It's roughly a 50% duty cycle, and you'll get about 5 burns an hour serving just that zone, which is OK, both on burn duration, and total burns per hour.
But the smallest zone #203 has an average load of about 12.5K x 20/260= ~960 BTU/hr. With a minimum output of 11.5K and radiation that's emitting only ~1K there is about 10,500 BTU/hr, or (/60=) 175 BTU/min of extra heat being being dumped into 20lbs of zone mass. To swing 20 lbs tthrough a 5F differential only takes 20lb x 5F= 100 BTU. So the burn times will be about 100/175=0.57 minutes, or 34 seconds long. That's an extreme short cycle. It's only about a 10% duty cycle, which means it would overlap the cycle the bigger zone's cycling only about half the time.
The other zones aren't much better. Ideally you'd want all burns to be longer than 3 minutes, (5 minutes would be better) and at most 5 burns per hour. The solution is to add mass, or use a boiler with a lower minimum modulation (like a Navien NHB-080, or an HTP UFT-080W, either of which modulates down to about 7500 BTU/hr out, which would pretty much guarantee that burn cycles for the zones would overlap, most of the time.)
If you installed a 30 gallon Boiler Buddy with the NCB in lieu of Navien's pre-fabricated hydraulic separator you'd be adding 250lbs of thermal mass to the system, and it would be involved with every burn, extending the minimum burn times, and even the 18 gallon/150lb water mass version would probably do it. It takes 150lbs x 5F = 750 BTU to swing the full 150lbs through 5F, so at 175 BTU/minute of excess heat it would deliver burns approaching five minutes.
Reality isn't quite as simple as the crayon-on napkin analysis- it's better to do the real math in a more refined way before going forward, but if your goal is to use the NCB-150 you can probably get there with a Boiler Buddy BB-18 as the hydraulic separator, and definitely get there with the BB-30. The BB-18 is about 4' tall, and narrower than the NCB-150. It may fit underneath the boiler if you have 8' or taller ceilings where it's being installed. The NCB needs only 9" of clearance from the ceiling at the top, and it's only 24" tall. The BB-30 is about 6' tall and would probably have to be off to the side.
But using the same analysis on the 10:1 turn down ratio & thermal mass of a 55 gallon Versa Flame you can see how that becomes a lot simpler- it's effectively short-cycle proof.
Looking at the Energy Anlayisis Reporta, a heat load of 49,738 BTU/hr @ +38F for a 2407' house is simply not a credible number despite the ~11-12K of low-performance window load. A house that size & condition that came in at 50KBTU/hr @ -5F, but not at above-freezing design temps, at least not without some windows open.
I don't have time to sift through the whole thing, but I'd believe the 25K number the radiant designers came up with, using high air leakage assumptions. The 3.5 tons of cooling load is also suspiciously high, but not impossible.
If you have a heating history on the place it's where-is-as-is condition you can use the existing heating equipment to get a realistic load number based on fuel use.
Could you post the Uponor reports you'd have received from the fine gentlemen in Minnesota . Or was it performed by a local rep ? Of particular interest would be the Radiant Panel Report and Detailed Heat Loss Summary .
Dana is 100% correct about your need for mass . I would not suggest anything in the low mass HX category for your project and would point out that although your slab does have plenty of mass that can only really be counted on to buffer the system a couple times a year coming out of non heating events .
If you can post those reports myself and others will be able to offer better help . I am not just opinionated on radiant but I am a Uponor Certified Designer / Installer ( check zip 08755) besides being on the educational , codes and standards , certification and Technical committees of Radiant Professionals Alliance . I would also suggest that you contact RPA to see if there is a member in your area to perform your work .
With the NCB 150 I really think you'd do OK with the BB-18 once you dialed in the system, but let a competent pro run the numbers using hydronic design software tools just to be sure. (Richard would be able to make that call with higher certainty than me.)
The difference in price of Navien's hydaulic separator manifold kit and a BB-18 is about $500, and you're really not out of the woods on short cycling without it until there's at least 100lbs of water per zone. The BB-18 would allow you to run different rooms at different temperatures- it's really not a bad solution, but then again neither is setting it up to run off an HTP Phoenix Light Duty condensing tank with an external heat exchanger for the heating system. There are several ways to skin the cat, but it's easier and more reliable if you have more thermal mass to work with.
I'm not sure if Navien ever specifies just how far of a differential swing around the outdoor reset setpoint the NCB controls will deliver. Some boiler manufacturers allow that differential to be programmed, others don't. Those that don't typically run between 3-7F of swing.
The UFT-080W is not a combi boiler though it could arguably be used that way (not legally in CA though), since it has a separate port pre-plumbed to support an indirect tank. This unit is built in Korea by Kiturami, where the water volumes in apartment heating systems are small, and they seem to get away with using this series as non-isolated combi boilers there. With the volume of water in YOUR system, and the amount of water stagnation in the radiation you'd have for half the year it's probably not a great idea to use it that way. In MA it would be legal, but only if it met minimum specified circulation per day parameters whether there was a heat load or not. The max fire output of the UFT-080W might be a bit small for serving up hot water in your application too. That would support a single 24/365 shower, but it's quite a bit less than the NCB-150's hot water delivery. In Korea units as small as the -080W would normally serve a 1 or 2 bedroom apartment. If you needed to fill a bathtub at a reasonable rate you'd still want a tank. With an indirect tank it's enough boiler to cover the loads of something like 19 out of 20 houses in the US.
A couple questions if I may ?
Did this design in fact come from Uponor or was it performed by a local rep or wholesaler ?
My experience would suggest that a local rep or wholesaler , as a Uponor engineering Design performed in Apple Valley would include a project name or number at a minimum.
Why were you entertaining using a 008 Circ ?
Dana was absolutely correct about this circ in this application . Here is the problem , the 008 is a high head circ and your system , even at design has nothing near what one would consider high head and as the ODT gets higher the problem becomes worse . Circulators operate where head and flows intersect , this will be below the curve in your application even at design . Example : at design your highest head loop is 7 feet and the total system flow is 5.7 gpm . Plotting the system curve against the pump curve we find that the 008 will move approximately 11 gpm . That is if all zones are calling , I promise they will not be and if your 7 foot zone shuts down you're gonna be in much worse shape along the lines of moving 12.7 gpm while requiring only 3.7 feet . There goes your efficient system all the way throughout , boiler , emitters , wire to water , the whole works .
A better circ selection would be as Dana stated ( http://www.taco-hvac.com/uploads/FileLibrary/102-499.pdf )or even better a Taco VT2218 with globe valves on S & Rs to your manifolds to guarantee a minimum head so you do not become a slave to the pump affinity laws . ( http://www.taco-hvac.com/vr2218/index.html )
This is a problem in a system which operates BEST when a designed for Delta T of the fluids is maintained as close to design for the largest portion of the heating season .
Did you request this be zoned this way or was this a recommendation by someone else ? For rooms to be on a common zone they really should meet the following criteria for comfort and efficiency's sake . The rooms should have similar BTU/sf requirements , similar finish floor R values , similar use patterns , similar heat gain characteristics
( rooms with South and rooms with North exposures should probably not be on the same zone) . Most often when these are ignored you end up with what I refer to as The Goldilocks House , one which Martin is certainly acquainted with .
Could you also post a floor plan and show direction of the 4 walls ?
Rules of thumb also are a big hindrance these days , especially since the technology we have at our disposal has not been taught to or understood by 98% of the hydronic industry . You do not NEED a boiler at all , a water heater such as the one Dana mentioned or it's big brother Phoenix with a flat plate heat exchanger and 2 ECM circs would be better and would offer you the mass you need in one piece of equipment . As far as buffer tanks go and going in the crawl space if you must , look at the HTP SSU20-B ( 27" tall) . If you must have a boiler , you could use a commercial SSU-45C and load the tank with one coil wjilst removing space heating fluid with the other , very nice alternative frankly .
Thanks Dana, this all is extremely helpful and I really appreciate the fact that I am starting to much better understand what is about to go into my house!
It looks like for now the backup plan is the Navien + a 30 gallong boiler buddy. Although I'd prefer a horizontal buffer tank in that case in order to be able to move it to the crawl space rather. But I could not find a horizontal buffer tank, only a low height 22 gallon Heat-flo.
Going back to your calculation, however, I am tempted to reconsider the zoning upstairs, with basically two alternative considerations for such a revision:
Combine the two kids bedrooms with our master suite (master bedroom, master bath, master closet), combining Zone 202, 203, and 204 to a total mass of about 96.08lbs. This also leaves then only two zones, which should have a much greater chance of overlapping I'd assume?
Combining the entire upstairs to one big zone, for a total of about 211lbs.
Would the above be more efficient than having to maintain a buffer tank?
I would obviously not have to spend $$$ on a buffer tank and I'd also need only one (or two) Ecobee3 smart thermostat upstairs and simply add sensors in all the relevant areas.
In regards to the Navien's set-point, what I found in the Navien Installation and Operating Manual is that, assuming I am looking at the right thing, the Navien has a Supply Set-point Range and a Return Set-point Range, with pre-defined ranges as well as an option for a custom one. E.g., the Low Mass Radiant preset has a Supply Set-point Range of 80-140°F (26.5-60°C) and a Return Set-point Range of 70-116°F (21-46.5°C).
I am, however, not clear how that translates into a swing value and alos how a swing of >5°F would impact the comfort-level in the house? Would people in the house feel greater temperature swings then?
As for the HTP UFT-080W, is that a combi boiler that can operate without adding an indirect DHW tank? Otherwise this would obviously be an entirely different design route, which I could consider if I can use a horizontal DHW tank, such as this Viessmann Vitocell 300-H. A horizontal storage tank would have the benefit that I could move it to the crawlspace and not have it use up conditioned space. The horizontal DHW tank, however, then would be located about 12' to 15' away from the boiler.
Of the above, what would be the recommended solution?
I've also linked the Heating System Detail and Radiant Panel Schedule that I received from Uponor. (Per Richard's request.)
Richard, please see my comments in line below:
I had it done through their $500 flat fee design program. I cleaned up the PDF before I posted it. (Removed project #, name, and location)
Inexperience and only basic understanding of the various interdependencies. When I looked at variable speed pumps, it seems that the flow and headloss curve for my system appeared at or below the lower line of the variable speed range curve of the pump.
I am perfectly happy with your recommendation for an ECM.
I did in fact suggest the rooms to be zoned that way, as it made sense to me from a usage perspective and how we typically roam around in our current house.
The house has a very low roof line (?), meaning a large overhang, which I'd assume mitigates the South exposure issue somewhat. As you will see from the floor plan, there aren't really any south-facing and north-facing rooms combined, except for Living room, dining, room and kitchen, which are all connected without actual doors. And we added the 2nd bathroom to that zone, because we wouldn't any of the kids control that bathroom from their room.
Link to floor plan. FYI: the lower floor south wall is below ground. The lot slopes towards the north.
Which is why I really appreciate your and Dana's responses and insight!
Just to provide some more context, I started out with the idea of having one piece of applicane - a combi boiler - because I liked the concept and the simplicity as well as the low profile / low footprint of such a solution. As I now know, I'd at least need an 18+ gallon buffer tank...
One concern I have is about HTP. I browsed the internet and saw a lot of not so positive comments about reliability of their devices and how it was handled by service reps.
Anyways, I'd be open to a water heater + flat plate heat exchanger based solution. Do you have any design drawings for such a solution? List of components?
I'll keep the Navien + buffer tank as one viable option for now.
I realize 'most efficient' and 'simple, long-lasting, and easy to maintain,' may be somewhat conflicting priorities, but as a homeowner who is trying to understand, install, and maintain the system as much as possible myself, the 'simple and easy' is the higher priority for me.
One thing about the internet is that anyone can say anything . You can find something bad about any manufcaturers stuff on ther internet . I actually identified such a rep agency that did not like to service what they sell against HTPs agreement with them . HTP did not like that too awfully much and addressed it , this rep agency was from the MidWest .
The one problem item I can think of was the Munchkin . That has been discontinued for years , that being said , most issues with anything is installer error . Contractor education sucks to be frank and most of the guys who do the work are lowest bidders . HTP is owned by a sole propreitor whom cares about his product , his reputation and the industry .
I will look at the floor plan and if I can help I certainly will .
+1 on the "Don't believe bad reviews on the internet." bit.
Very few boilers out there right now are junk, and it's all about local support and training. The number of "professionals" that oversize the boiler to the radiation & load and mis-design the near boiler plumbing creating low efficiency and high maintenance problems are many. The kinds of common errors that didn't make a huge difference with old school cast iron get magnified when addressing low mass modulating and condensing equipment, and some mod-cons are more sensitive than others. If I had to guess MOST retrofit mod-cons out there are oversized for the loads, and plumbed/pumped sub-optimally. Installer error is a rampant problem in the industry.
A few years ago Navien had a bad reputation too, again primarily due to lack of installer training and poor support, but over the past 3-5 years they have stepped up their game considerably in the US. (Kyung Dong Boiler/Navien is one of the biggest boiler & water heater manufacturers in Korea, and has always enjoyed GREAT local support in Seoul! :-) ) I'm not sure how good the support for HTP is relative to Navien is in the Bay area.
HTP has some boiler products available through Home Depot, including carry some of the Westinghouse labeled versions of HTP's EFT series (which are UFT boilers set up as combi boilers with internal isolation for the potable side). The LAARS Mascot FT wall hung boilers & combis are identical to the UFT & EFT series under the paint, all manufactured by Kiturami. Unfortunately the smallest of the combi series sold in the US with isolated potable only modulate down to 28,000 BTU/hr in, which is more than your heat load.
I appreciate your post details and comparison of Bosch or Navien combi boilers but I prefer Navien. This is my first choice.