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Please critique this design

indevredeindevrede Posts: 33Member
I'm not sure if I've seen this exact design anywhere, but it contains many of the concepts that I learned about on this website and others. The cartoon is below. Here are the system specs:

* Design day heat load 40k BTU/hr (@ 15F outdoor).

* Lochinvar Knight WHN-055

* primary-secondary with closely-spaced tees

* Turbomax 23 reverse-indirect DHW piped as a series buffer in the boiler (secondary) loop, on the boiler supply

* Thermostatic mixing valve for DHW (115F setpoint)

* System (primary) loop has a Grundfos Alpha in delta-P mode, probably running @ 14 gpm when all three TRVs are open

* Each of the three 50-ft baseboard runs will be "zoned" with TRVs, one on each run

* A single indoor-reset thermostat will be set a few degrees above target, used as a high-temp cutoff (i.e. ODR will be the main feedback)

* The boiler pump will be controlled by the boiler (0-10V signal) for constant 35F delta T, probably running between 0.5 and 2.5 gpm

* The boiler will control its supply temperature with outdoor reset (160F on design day, minimum 145F)

* The boiler will sense the Turbomax temperature and treat it as a zone, calling when the tank temperature drops below 145F

* The boiler will disable the system pump whenever the DHW zone calls

* I didn't draw the air separator and Y-strainer, but they will be included

* (side note: there will be a drain water heat recovery system)

I think the advantages of this design are:

* No short cycling, even with 50ft fin tube zones at 110F

* Return temperatures will vary between 110F and 126F (design day), so the boiler will condense most of the season and maybe beat 90% efficiency on design day

* Even heating within a zone: Design day baseboard temperature of 134F has 8F delta T

* Primary/secondary piping satisfies Lochinvar's recommendation

* Only 2 pumps needed

* No expensive controls beyond what's included with the boiler (unless you count the built-in controls on the pumps)


* (un)availability of small, inexpensive, efficient circulators with 0-10V control?

So, what do you all think? Glitch, fix, or something else? Please critique. Note that this is for a retrofit, so the baseboard is a design constant (obviously radiant would be preferable).


  • EastmanEastman Posts: 927Member
    boiler pump

    The boiler pump would be configured to always run with the system pump somehow?
  • heatpro02920heatpro02920 Posts: 991Member
    edited March 2014
    I think I would

    Replace the Lochinvar with a Triangle Tube solo 60

    Replace the T-23 with a smart 40 and no mixer {I don't like putting indirects on a unit this small, but I have done it and it works}

    Install a g-foss alpha on the primary

    Install 3 t-stats and use bumble bees for each 50ft run

    Install a bumble bee on the indirect

    Install a taco zone control for the 3 heating zones and let the tt' control handle ODR, DHW, ect...

    But we all do things differently, your design will work, I don't normally use trv's with baseboard or mixers with indirects, I use trv's for radiant panels and fhw rads, and mixers on DHW with tanklesses and undersized indirects... I am not sure what your DHW needs are but a smart 40 fits most residential properties...

    Looks like a fun project, good luck with it...
  • indevredeindevrede Posts: 33Member

    The boiler should operate the system pump whenever there's a space heat call ( and turn off the system pump if there is a dhw call).
  • indevredeindevrede Posts: 33Member
    Why not trvs?

    Thanks for the feedback. Just curious, Do trvs and baseboard not go well together? I was going to use the remote style.

    When you install a system as you described, is there no issue with short cycling? My design day heat load is 40k btu/hr ( 35k if I do a good job with air sealing and insulation).
  • heatpro02920heatpro02920 Posts: 991Member
    edited March 2014
    Nothing wrong with trv's

    I think designated delta t circs would be a better option... I dont see how you will short cycle... say a single zone calls for heat, it turns on its designated circulator, ramps up for a minute, then gets its reading, the circ slows down to match your preset delta say 15-20 degrees, Now the unit will modulate to match the load {the solo 60 goes down to 16K btu's}. With the odr taking care of the water temps, your burner taking care of the fire rate, and you circ taking care of the flow rate, you will get the most out of your run times...

    The only issue I see {which I have installed a smart 50 with a solo 60 and it works} is that the low amount of btu's these small mod cons put out can be an issue if there is a lot of dhw being used...

    The customer I installed the 60-smart 50 setup with, said when it was very cold out and he had house guests, he had to play with his t-stat set back, because they were set to heat the house up in the am when they woke, and that is when everyone was trying to take a shower, so now the house wouldnt heat up until the tank was satisified, this could take a while when you have 2 showers with 5 people trying to showers back to back...

    He called me about the issue and I told him not to use a setback while he had house guests, OR to set the setback to catch up before they tried to shower... He let me know that that remedied the issue perfectly, and he never noticed an issue again...

    I always warn my customers, when using setbacks and priority HW, the unit can not do 2 things at once....

    I am not super familiar with the knight boilers, but I prefer the TT units over them, although the important part is to get a contractor that knows what he is installing... So I would take a knight installed by a knight tech over a tt installed by a knight tech {although anyone familiar with mod cons will get it done, future service is a concern}....

    The trvs will work and they are nice, but I like designated circs rather than a single circ doing all the flow management...

    This is really just my preference, I like to use as few mechanical valves as possible, if I can get around external valves I will... I dont like mixers on DHW {unless it is code and needed}, I dont like mixers on radiant {I would rather use a separate plate exchanger and aquastat}, I just prefer to go with out them...

    Do the math on materials, Im not sure if you are doing the install yourself or have a contractor, but Ill bet the bumble bees vs trv setup and single circ is not a big price difference....

    I hnoestly would look into a rinnai tankless too, if you can fit it, the cost wont be considerably more than the indirect and you will not have any DHW issues... Sounds like a medium/small home so a tankless may be a good idea... I have done a bunch of mod cons and installed tanklesses rite next to them..
  • ZmanZman Posts: 4,811Member

    I am not sure how you will get the boiler to condense.

    With 145 degree setpoint and minimum boiler flow rates I don't think you will get the low return temps.

    I am not a fan of reverse indirect and low temp heat.

    The lochinvar is a good choice for mixing although you have to add a mixing module.

    Never use a smart tank for reverse indirect. They are steel on the outside.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • indevredeindevrede Posts: 33Member

    Re: short cycling: I just considered the smallest zone output (50 ft * 160 BTU/ft/hr = 8k BTU/hr) and the boiler minimum output (11k BTU/hr) and the mass of the water in the loop. Without a buffer, the mass is about 4 gal, so with a boiler setpoint differential of 10F,  10F * 4 gal * 8.3 lb/gal / 3k BTU/hr = 7 minutes. Not horrible, but not great. The reverse indirect buffer adds over 200 lbs of mass, so short cycling goes away.

    Re: night setback, understood. I wasn't planning to use it, thanks for the confirmation.

    Re: running out of hot water, I'll be using a drain water heat recovery unit, so my 55k boiler will be effectively about a 90k boiler, enough for endless hot water for a 2 gpm shower.

    Again, thanks for the advice & insight.
  • Paul48Paul48 Posts: 4,492Member

    What happens when the DHW is at 140*, and there is a call for heat?
  • indevredeindevrede Posts: 33Member
    edited March 2014
    Setpoint = boiler supply

    Reply to Zman's post titled "Condensing?"

    I'll have the boiler control the boiler supply (hot water coming out of the boiler), not the system supply. Because of the higher flow rate in the primary loop, the system supply will be 134F on design day, but the boiler supply will be 160F. Return temp will be 126F. With ODR, the boiler supply will vary down from 160F to 145F minimum.

    Again, the system supply is not controlled directly, but instead will be a function of

    * the boiler supply temperature (controlled directly by the boiler)

    * the boiler loop flow rate (controlled directly by the boiler for 35F delta T)

    * the system loop flow rate (controlled by the delta P pump)

    Example 1: in the shoulder season ODR calls for 145F boiler supply. If 3 zones are calling, that's 150 ft* 160 BTU/ft/hr = 24k BTU/hr. The flow in the boiler loop is 24k / 500 / 35 = 1.4 gpm. The system loop is at 14 gpm (max for alpha in delta P mode), so its delta T is 3.5F. So, the system supply is 113.5F and the return to the boiler is 110F.

    Example 2: again, shoulder season and 145F boiler supply, but this time only one zone --> 8k BTU/hr. Boiler loop is 0.45 gpm. The delta P pump would probably be 7 gpm (50 feet of 3/4 copper has 6 ft head loss at 7 gpm). So the delta T in the system loop would be 35*0.45/7 = 2.3F, and so the system supply would be 112.3F.
  • indevredeindevrede Posts: 33Member
    Good question

    Depending on set differential and target 145F DHW tank temperature, the boiler may get a call for DHW (as a zone) when the tank is at 140F. In that case, the boiler will turn on the boiler circulator and supply whatever temperature is set by ODR (between 160F and 145F) until the DHW is satisfied (modulating according to PID, of course). While the boiler is satisfying The DHW zone, the system circulator will shut off (this is key, and I'm really thankful that Lochinvar allows this configuration).

    After the DHW is satisfied, the system pump will turn on in order to satisfy the space heating call (if there is one). If no space heating call, then the system pump won't turn on, and the boiler pump (and blower) will turn off.

    I really appreciate these questions, thanks. And, anyone with experience with Lochinvar SMART control configuration please chime in to make suggestions or corrections to my plan.
  • Paul48Paul48 Posts: 4,492Member

    Once the DHW is satisfied, it will then switch to heat if there is a call. But according to your drawing, it will continue sending hot water through the indirect.
  • Paul48Paul48 Posts: 4,492Member
    Please explain

    "Design day heat load 40k BTU/hr"

    You say you have 24k btus of emitters?
  • indevredeindevrede Posts: 33Member
    Always through the indirect

    The Turbomax 23 is a reverse indirect, so it's acting like a 26 gallon series buffer in the boiler loop. Every time the boiler fires, hot water gets pushed through the tank. If no one's using DHW, then the output of the buffer = input temperature. Even if the bottom of the tank is "cold", i.e. 35F below the top, it will only take a few minutes to heat the entire tank to the same input temperature (160F to 145F depending on ODR) . Once the bottom of the tank = the top, then you're looking at a conventional primary/secondary setup.
  • indevredeindevrede Posts: 33Member
    Three 50ft zones of fin tube

    Three 50 ft zones of fin tube. At 110F, they each put out 8k BTU/hr. At 130F they each put out 13.3k BTU/hr.
  • Paul48Paul48 Posts: 4,492Member
    Mixing Valve

    The setting for the mixing valve for domestic is different at different temps. How do you ensure no one gets scalded?
  • indevredeindevrede Posts: 33Member
    Thermostatic mixing valve

    Thermostatic mixing valve
  • Paul48Paul48 Posts: 4,492Member

    Got mixed up with your shoulder season example. It's an age thing, I guess.
  • indevredeindevrede Posts: 33Member
    Almost like a reverse indirect hydraulic separator

    I've convinced myself that the design in the first post is exactly the same as a reverse indirect (like an Ergomax) piped as a hydraulic separator / low loss header / buffer that connects the boiler loop to the system loop.

    So, nothing really new or unique here, and it should work fine. Now I just have to figure how to add 0-10V speed control to a Grundfos Alpha...
  • GordanGordan Posts: 891Member
    How is the DHW zone heat call generated?

    If aquastat, you're going to have a conflict between ODR and the aquastat. Either way, there's a conflict between the fixed temp operation of DHW and ODR operation of the heating system.

    There's a major difference between a dual-tapping buffer tank and what you're considering here; namely, the return water from the system will mix with and dilute the hot water at the top of the buffer tank if you pipe it in the usual supply-at-top fashion. You will be continuously tempering the buffer tank.
  • ZmanZman Posts: 4,811Member


    I am following your logic.

    I have real concerns about the low flows you are running through the boiler.

    For one, the heat exchanger could be damaged. This is why they recommend primary secondary.

    Secondly, If the flow goes too low,the flow will become laminar rather than turbulent.This will trash efficiency.

    I would run this one by Lochinvar.

    I guess a worse case scenario would mean you have to raise the temps and lose some efficiency. (actually the worst case would be a damaged boiler that is not warrantied)

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • indevredeindevrede Posts: 33Member
    With a lochinvar temp sensor

    In the DHW thermowell.

    Re: continuous tempering, the DHW tank setpoint (aquastat) and the DHW boiler supply setpoint will both be 145F, which is the lower limit of ODR. I think that should work perfectly, see this section of the installation manual: (I'll be configuring the DHW as a zone with system pump "forced off")
  • indevredeindevrede Posts: 33Member

    Here's the relevant page from the installation manual. Nothing about laminar flow there, and it appears to show flow all the way down to 0 gpm even for the largest WH boilers. But I think you're right, I should confirm with Lochinvar.
  • GordanGordan Posts: 891Member
    So let me see...

    During the shoulder season you potentially have both DHW hysteresis and ODR hysteresis to worry about. If (DHW setpoint - DHW differential) < (ODR setpoint - ODR differential) this would result in a DHW call during a space heat call, which would force the space heating cycle to be interrupted so the priority DHW call can be satisfied. This could work out pretty well, but you have to sweat the details. If, at some future point, you wanted to bump up the DHW setpoint in order to get more first draw capacity, this would have a bearing on all of your parameters.
  • GordanGordan Posts: 891Member
    Actually, upon further reflection...

    At times, during low heat demand periods, your boiler's burner will cycle off while the call for heat is still on. This would mean that the return water that would be tempering the tank would be (considerably) cooler than the 145 F setpoint, potentially causing the DHW call to be initiated even though there's no DHW demand. So here's a circumstance where space heating, absent DHW demand, generates a DHW call and interrupts space heating.
  • remodelremodel Posts: 69Member
    boiler flow

    If you get a response from loch let us know, I doubt they will commit to saying the boiler can go below a certain flow rate even though there has to be a bottom end. I know p. 36 shows the fixed speed pumps at full fire.  It is relevant that the majority of the time you are at low fire and the pumps are oversized.  I have looked throughout the wall and didn't find much on low flow through boiler just tons of discussions on oversized boiler pumps.    
  • indevredeindevrede Posts: 33Member
    Both excellent points

    Gordan, thanks for posting. You're identifying issues that I hadn't fully thought through.

    Following this train of thought (self-interrupted space heat), is this a bad thing? The boiler's not short-cycling, and cold water isn't being pumped through the space heating zones, so what's the problem? The boiler is filling the buffer with heat, then sending some of that heat to the space heating zones, then pausing to fill the buffer with more heat, and so on.

    Or am I missing something? (I'm also wondering if this is why typically people put buffers on the return, not the supply)
  • indevredeindevrede Posts: 33Member
    Maybe it *will* short cycle?

    Still trying to think this through. Maybe the boiler will short-cycle because I've effectively cut out the buffer from the control loop.

    Just a guess, but could moving the control sensor from immediately upstream of the tank to immediately downstream help? One problem I see is that there would be a long lag in the response so I'd guess the boiler would fire at 100% a lot of the time, possibly every time...
  • heatpro02920heatpro02920 Posts: 991Member
    I am curious

    What benefits you are getting over something more conventional, don't get me wrong, I love discussing different alternatives to the "norm" but to compare it to the system I laid out in my earlier post, what benefits are you trying to exploit?

    By all means I like seeing guys trying something more than the normal blah blah blah heating system, but I like to see something get more simple vs adding complexion...

    Just curious as to where you are going?
  • indevredeindevrede Posts: 33Member
    I should have been more clear

    I'm pretty much stuck with the Knight boiler and Turbomax 23 reverse indirect and 150 ft of baseboard. I have 2 pumps (alphas) and I'd rather not buy more or buy other zone/mixing controls unless absolutely necessary. I want a system that lets the boiler turn down and condense as much as possible, but also not short cycle in the shoulder seasons with 50 foot baseboard zones. In my original post I proposed an arrangement that I thought satisfied all my requirements. After reading the responses in this thread I now believe the design is flawed, but I still hold out hope that if I just pipe it properly I can reasonably achieve my goals.

    The fallback plan is to let go of sub-zoning (ditch the TRVs) and to keep the baseboard as one big zone. Then the risk of short-cycling goes away so I can discard the buffering idea and use the reverse indirect like a regular indirect.

    Again, I'm still hoping that there's a simple solution. Maybe use the reverse indirect like a true hydraulic separator? (with tees on the inlet and outlet) In that case I'm not sure what the flows would look like, or how ODR would work, or where I would put the sensors, or if I would get much condensing or turndown, or if delta P or delta T control would be beneficial, etc. Maybe someone else has an idea?
  • Paul48Paul48 Posts: 4,492Member

    well do the emitters match the heat loss, room by room?
  • EastmanEastman Posts: 927Member
    edited March 2014
    delta control apparently only goes to 25

    A forum member named smihaila asked Lochinvar detailed questions about some of the more advanced pump control features on the boiler you are considering.

    From a post titled: Some answers from my Canadian Lochinvar rep

    Q3: What fine-tune parameters are available in the "Smart Control" logic

    for the customization of the "Const Delta-T" operation which seems to

    be supported via "0-10V BLR PMP OUT"?

    A3: The Knight Smart System controller will always try to maintain a 20F Delta T (Default) but has a range from 15 – 25 using the PC Software.
  • indevredeindevrede Posts: 33Member

    Thank you, this detail is really important.
  • indevredeindevrede Posts: 33Member
    Not sure...

    I'm not really sure. It's the typical installation where baseboard is usually along almost the entire length of every exterior wall.

    The main issue is that the finished basement is cold and the top floor is too warm, which is why I thought zoning would help balance. I guess I could put some effort into "permanent balancing", like adjusting flow rates. Also I plan to increase attic & band joist air sealing, so that should help with evening out temperatures.
  • indevredeindevrede Posts: 33Member
    New approach

    OK, forget the reverse-indirect-as-buffer idea.

    The minimum output of the boiler is 10k BTU/hr. I want to avoid short cycling in the shoulder seasons when one 8k BTU/hr zone is calling. The usual strategy is to increase the system mass (maybe with a buffer). But another possibility is to somehow make a single-zone emit a minimum of 10k BTU/hr.

    Is there a cheap & easy way to do this? It would be like temporarily boosting the ODR setpoint when only one zone calls, but returning to the regular ODR setpoint when two or three zones call.

    Another idea is to just manually disable the TRVs in the shoulder seasons and run as one large zone.
  • GordanGordan Posts: 891Member
    Stuck may be a bit harsh

    These are very fine components and will work marvelously. Considering the low, low price of an electric water heater which could be pressed into service as a buffer, it seems like insisting on using the Turbomax for that purpose creates more issues than it solves. Break out the DHW function from the heating system and you're free to optimize both for their usage patterns.
  • indevredeindevrede Posts: 33Member
    OK, I'm listening

    You're right, a 30 gal electric wh is really cheap. Where would I connect it  & what temperature would the boiler control?
  • GordanGordan Posts: 891Member
    Several options

    1) You could get rid of primary-secondary, pump through the boiler, and pipe the buffer in series with the boiler. Supply might be better than return from the point of view of acting like a capacitor and "evening out" the supply temperatures coming from the boiler, so no expansion/contraction at the emitter. Advantage: simplicity, direct return to the boiler minimizing return temperatures. Disadvantage: you have to worry about boiler flow, but with the decoupling of DHW and space heating you now have the ability to tinker with the ODR curve making sure that those TRVs remain open or mostly open. Another disadvantage: during a DHW call, you can't continue to satisfy the space heating from the buffer. Boiler control strategy is ODR with constant circulation.

    2) You could keep primary-secondary and pipe the buffer downstream of the boiler tees. Alternately, you could use the heating element tappings as a second set of tappings to configure the buffer as a hydronic separator. Advantage: the buffer stays online during a DHW call, and you can decouple system flow from boiler flow. Disadvantage: some return mixing may/will result, you have more components, and control strategy is more complicated (though I think that the Knight's control handles this with a system supply sensor, and the boiler pump can be configured to only come on when the burner is ready to fire.)
  • remodelremodel Posts: 69Member
    heat loss room by room

    indevrede...I was in your situation post fact (I am just a homeowner so take it for what its worth) after finding out my boiler was oversized.  Do as Paul48 says and run your heat loss room by room using slant fins program for ipad or their HE2 program (do a google search, I just downloaded it a month or two ago).  Also maybe do some spreadsheet work to determine heat loss per element (flow rate, btu/ft etc to determine your output at your last fin tube) see where that puts you and you might find out you need more element.  Might be able to ad some high output at the end of the loop which might help with delta tee (something I plan to do next season).  Also the loch has some controls to help with short cycling, i.e. ramp delay, large differential offsets maxing out the boiler below 100% of fire.  You can't get away from some short cycling or fussing over condensing all the time, something that took me a little while to realize.       
  • SWEISWEI Posts: 7,356Member
    Lochinvar boiler pump control

    I'd double-check that one if I were you.  I'm pretty sure someone at Lochinvar gave me different info about a year ago, but I don't have notes on the conversation.
  • GordyGordy Posts: 9,264Member
    edited March 2014
    Starting at the finish line

    I will reiterate what others have said.

    Do a room by room heat loss.

    Determine existing base board out put at what water temps you wish to use to be condensing.

    It is possible you may have base board enough to lower the supply temps if envelope upgrades have been made to the structure after the original heating design was installed.

    Or you may fall short with what is existing, and have to add more, or sacrifice a higher supply temp if that is not possible.

    Remember also with ODR your not going to need high water temps most of the heating season even if your base board is just enough at design conditions. So at design you might sacrifice efficiency with out condensing, but be condensing at warmer temps.

    But you don't know any of this with out the heat loss, and emitter outputs start with that.

    I would not ditch the TRVs either.
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