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2 temp v. 1 temp

ChrisK_2
ChrisK_2 Member Posts: 11
I’m planning on using a TT Excellence 110 with outdoor reset as my boiler. I’m going with radiators in the basement and underfloor for the main floor.

Initially I was planning on a 2 temp system with the boiler’s outdoor reset for the radiator supply temp and a Taco RMB with outdoor reset for an extruded plate underfloor radiant. Since the boiler rest is set for the radiator’s higher supply temp, the underfloor supply will always be mixed down and I won’t see the big efficiency gain from the extruded plate’s lower water temp. I understand the extruded plates are normally the way to go with underfloor, but not sure for my case. It seems like I’m adding unnecessary cost and complexity of the extruded plates and mixing block to get the underfloor “dialed in”.

I evaluated a 1 temp system supplying the radiator at my underfloor supply temp of 110*F, but the radiators become way too large and costly.

So I’m wondering if it makes sense to go with 3/8” suspended tube for the underfloor. This would require a 155*F supply temp. I could do a 1 temp system with reasonably larger radiators. It would be cheaper to buy, install and maintain.

Also, unintuitively, it seems that this one “middle” temperature system may be more efficient than the original 2-temp system since the boiler will be running at a lower reset curve.

Does this make sense? Is all I’m giving up is response time? What do you think?

Thanks

Chris

Comments

  • hot_rod
    hot_rod Member Posts: 16,289
    maybe

    can the load be met with suspended tube installation?

    If the output temperature at design still allows the boiler to run in condensing mode, the efficiencies should still be high.

    I'd consider a product like UltraFin if you are in fact "suspending" the tube. My experience and infared camera views show UF provides a very even floor temperature. Much better than bare tube stapled to the floor which tends to stripe.

    The answers is in the numbers .. if 155F at design can move enough energy to heat the space, consider it.

    hr
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • hot_rod
    hot_rod Member Posts: 16,289
    maybe

    can the load be met with suspended tube installation?

    If the output temperature at design still allows the boiler to run in condensing mode, the efficiencies should still be high.

    I'd consider a product like UltraFin if you are in fact "suspending" the tube. My experience and infared camera views show UF provides a very even floor temperature. Much better than bare tube stapled to the floor which tends to stripe.

    The answers is in the numbers .. if 155F at design can move enough energy to heat the space, consider it.

    hr
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Jamie_5
    Jamie_5 Member Posts: 103
    dumb mixing valve

    Unless you have the ability to tweak more than the slope of the reset "curve," I'd use a manual mixing valve for the second, lower temp and stay with the plates.
  • Mark Custis
    Mark Custis Member Posts: 539
    I am with Jamie

  • Brad White_191
    Brad White_191 Member Posts: 252
    I think that you may find

    that the under-floor system, especially if staple up, may require the higher temperature and that the radiators would be better off on a mixing valve and require a lower temperature.

    It seems counter-intuitive, but then, your radiators are in the space and the staple-up has to drive heat up through some strata first...not an absolute but it has been demonstrated.

    Perhaps you have already done this so forgive any repetition. What you need to know is what your heat loss is and what your radiators are capable of doing for that and at what temperature. Of course, plates, no plates, suspended, insulation, etc. are all variables affecting this.

    I have found often enough that the cast iron needs say, 110F and an under-floor system 125+. Enough to justify mixing valves.

    Both will condense or contribute to such at the boiler of course. But both need to be managed and the highest temperature emitter set actually known.

    Lately, after monitoring some systems, I am beginning to favor mixing valves regardless rather than just running off the combustion side (boiler) directly. The spikes, even with modulation, could be better tempered and constant via the valve. This means that a mixing setup might be a good idea for both floor and radiators... Again, your situation is specific, my comments are general.
  • Mark Custis
    Mark Custis Member Posts: 539
    Thanks Brad

    I had pushed the button before I was finished typing.

    I learned all about pumping away from a mixing valve over the weekend.
  • ChrisK_2
    ChrisK_2 Member Posts: 11


    I'm using Watts RadiantWorks software and it's saying 155F on the design day for a suspended tube (not staple-up).

    Thanks

    Chris
  • scott markle_2
    scott markle_2 Member Posts: 611


    I have implemented simple means of using return from high temperature areas as supply for lower temperature loads. I had great results on an 11 radiator 3/8 home run piped job. I used the individual radiator return lines with extruded plates to give some supplemental floor warmth to the kitchen and bath. The diverter valves on the panel rads provide a nice means of permitting some flow through the radiant regardless of what the TRV is asking of the radiator. I intentional oversized the radiators and in the end I don't think the requirements for the radiators was much different from what the floors need. I think sizing the radiators for a common water temp is a better investment than a motorized mixing valve.

    You might find this interesting: it's a specialized mixing valve that is designed for the purpose of using high temperature return water for low temperature loads but can adjust between the two depending on requirements.

    http://www.baunach-usa.com/#


  • not much wiggle room there if anything is wrong with your calcs. how solid are they?
  • ChrisK_2
    ChrisK_2 Member Posts: 11


    The Watts RadiantWorks PRO software seems to provide good heat loss values. The different areas in the radiant floor zone have heat loss values between 12-15 BTU/h*sqft (depending on windows and exterior walls) using 24F for a design day (Seattle). Is the supply temp RadiantWorks provides reliable... no idea.

    The subfloor is 1.25" with 1/2" eng wood floor on top of that. What would a reasonable suspended tube supply temp be for 2 runs in each 16"OC joist bay? It will all be insulated up to snuff and heated space below.

    Thanks


  • I mean how confident are you in your input's accuracy and ability to model the reality of your home and flooring usage, etc.

    the output curve for suspended tube is nearly flat, which is to say, you have to turn up temperatures a lot to get a little more output, so you don't have far to go here. If your, say, infiltration estimate is off, or there is a good area rug you didn't take into account, that could make the difference between a system that works and one that works "most of the time".
  • Andrew Hagen_2
    Andrew Hagen_2 Member Posts: 236
    One Temp

    I prefer to keep things simple, save the extra circulator, and run everything at one temperature. Size your radiators for the lowest temperature practical (typically 140-150°F). Extruded plates work fine this way. I would recommend using pex-aluminum-pex in the plates to minimize the possibility of expansion noises.

    I look at extruded plate radiant floors as simply a large panel radiator.

    edit: Somehow I missed the suspended tube part in the original post. I am not a fan of suspended tube installations. While they can work, they still require high temperatures and have limited output that may not meet the load in colder climates.
  • ChrisK_2
    ChrisK_2 Member Posts: 11


    I've included all vents, fireplaces, recessed lighting, etc into their heat loss calculator. The spaces vary from .5 ACH up to 1 ACH depending upon the penetrations. The house is a full gut remodel so will have all new insulation and windows. 60% the house will have the original leaky shiplap sheathing, but 30% of that received tyvek house wrap. The interior envelope of the house will be air-sealed.

    So confidence that it is modeled correctly? Hmmmm, I *hope* so. I entered all the penetrations, but not sure is the baseline of .5 ACH is appropriate.

    The total home heat loss is very similar to values calculated by forced air subs, for what that's worth. Of course their smallest system at max burn has twice the design load.

    I hear you in that I'm leaving little room for error by using suspended tube. I was hoping to save $$$ and complexity with a one temp radiator/suspended tube system, but maybe it's just better to have a two-temp extruded plate system so I have the additional horse power if I need it.

    Thanks Rob


  • If all the new insulation, 0.5 ACH is my standard 2x4 construction assumption.

    1.0 ACH is a very, very leaky room though. If that's an area with new, well installed insulation and such, then you probably have some wiggle room in these numbers.
  • ChrisK_2
    ChrisK_2 Member Posts: 11
    on the margins

    Thanks Rob,

    The 1ACH is due to a filreplace.

    I feel fairly confident in the 12-15 BTU/sqft. I've read in another one of your posts that per Uponor's charts a 15 BTU/sqft a 3/8 suspended tube system would only require 135-140F supply.
    http://radnet.groupee.net/eve/forums/a/tpc/f/4771065301/m/3411057263?r=4501038263#4501038263

    I haven't been able to find that Unopor charts online. The Watts RadiantWorks software is telling me 150F supply for the same 15 BTU/sqft. My total floor buildup is 1.75" as opposed to the 1.5" that the Uponor chart represents.

    Do you think I'm designing this right on the margins, or do I have enough extra bandwith to satisfy the real world?
  • Bob Bona_4
    Bob Bona_4 Member Posts: 2,083
    me too

    I'm not a fan of suspended. I would say bite the bullet and do two temp. I almost can guarantee you will say "I'm glad I did" rather than "I wish I did".

    Suspended has SO many pitfalls, down to trying to maintain that consistent air gap.


  • I think you're probably comfortable... of course that's a blind WAG, but based on what you're saying I would not raise an alarm.
  • scott markle_2
    scott markle_2 Member Posts: 611
    Cascading Temperature

    Did anybody check out link I posted on the Multi-path Mixing Distributor?

    This idea makes a lot of sense for condensing boiler applications.

    I think marketing for comfort is over. If the "performance" benefits of two temperatures has no effect on energy optimization, then we are talking about complexity and expense with out any eco creds.
This discussion has been closed.