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newbie radiant heat system questions

I've learned a LOT in the past week but apparently not enough. I am designing a system for what will be new construction. I will need to do the work myself -- I can solder and I am an engineer, other than that this should be a tragic comedy.

I am building in central Maine and I am assuming a 0 degree mean low. Various calculations of heat loss for the house range from 44000 to 50000.

I want four zones and have managed to come up with a sub-circuit layout that will give the requisite heat to each room with a uniform 130F water source and a max head through the 1.2" PEX of 2.2 feet. The min head is 1' in the basement zone (0.5 gpm @ 1.3', 0.45 gpm @ 1' and 0.53 gpm @ 1.42' circuits in parallel.)

What do I need to do to balance these heads? I calculated that if the basement ciruits were just connected the net flow and head would acceptably balance heat and the hotter rooms would in fact be the ones where more heat was preferred... The real question is the range from this effective 1.3' head to the 2.2' head.

If I balance the heads of the zones using a restrictor of some sort, i assume this will naturally balance the flows. I further assume that if the flow rates are uniformly too high that the only effect will be less heat dissipation into the rooms because of the lower residence time in the pipe and a higher return temperature. Lower flow rates would mean more heat transfer due to slower flow and a lower return temp. Right? Design is for a 20 degree loss in the loops and a 110 F return temperature.

Am I right in assuming that at a source temperature of 150F to the mixer valve that I would be using a 50/50 mix of return water and source water for an net hot water use of about half my design flow rate?

My heat source will be a direct-vent gas system and I am debating between a 95% efficiency boiler with tank or an 85% efficiency Takagi TKD-20. What are the pros and cons of boilers vs. DWH systems?

One big pro of DWH is I feel like I understand it but a big con is that at 5 gpm it has a 52' (18 psi) head in the heater itself. Apparently I need to learn primary-secondary systems if I use a DHW heater?

Finally there is the open loop vs. closed loop question. What little I understand about boilers is that they are inverently closed loop systems (indirectly firing the DHW through a heat exchanger) and while the boiler is more expensive, it is well integrated...

I liked the open look system off the DHW heater but was told that the cold make-up water would take down my heat unacceptably. I intend to use grey water recovery on the showers and dishwasher but not the laundry.

Any specific LP gas unit references are also welcome. I expect I will be able to find someone to professionally install the GAS and tweak the system without having to contract out the entire job.


  • David Van Wickler
    David Van Wickler Member Posts: 35
    balance through

    manifolds with individual flow meters. If you know the heat loss you know the flow rate. flow meters will dial in the required flow rate. Balancing through pressure drop is achieved simultinacially (sp?).

    Other than that pick a pump for the total flow rate + 15% and and the highest pressure drop circuit +15% and let 'er rip.
  • Steve Ebels
    Steve Ebels Member Posts: 904
    A better way

    Typical engineer. Getting all in a froth over the wrong stuff. (Grin)

    From the looks of what you are talking about, you are designing the system for one constant temp. (130*) day and night, fall, winter and spring. Therefore you are likely going to use room thermostats on your zones to call on the boiler and pumps or ZV's. Correct??

    Here's a different method that I have used with much success. It starts with an intelligent boiler and control set up. This is of course, a Viessmann product. (No, I'm not employed by Viessmann)

    No mixing valves or injection stuff. Just a boiler and control that are able to modulate temperature to match the weather. Shoulder seasons will probably need 100* water or less from what I have seen. This of course is assuming you're utilizing a poured floor. (gyp-crete or concrete over your tube.) Staple up will run a little higher but the modulation does the same thing. The net result of constant circulation and modulating the water temp is that you have "cruise control" on your system. It's not firing at full output on any call for heat. Your system is "heating" all the time, it's just matching the output to the load. The modulating boiler will turn the burner down to match what's going on and the water in the floor still flows but changes temp to match demand. This eliminates the problems that come with an on/off type system. Such as flywheel (mass) caused overshoot and lag. Better comfort, better efficiency, less wear and tear on equipment.

    I have a few of these in operation and it works wonderfully. Very simple to install. In fact, I had a customer, (a GM engineer) who had gotten in touch with a company on the internet. They had produced a drawing for him and were telling him he needed all kinds of mixing valves, outdoor reset controls, zone valves, pumps and other balderdash. He and I decided it would be best for him(he was going to do as much as possible himself) if we kept it as simple as we could. We installed the Vitodens boiler and he ran all the pex, hung the remote manifolds and ran piping back to the boiler room. Long story short, it provide great comfort, maximum efficiency, minimum stuff to break and he has only one manufacturer to deal with in the highly unlikely event of a failure.

    You can still use zone valves to act as a high limit in selected areas if you wish and/or to compensate for internal heat gains. (cooking up a storm, 20 people over for dinner, another heat source like a fireplace).
    This boiler and control will also manage your domestic hot water production utilizing an indirect tank. You can also program the control for whatever setback temp and times you wish.

    I don't know if you're following what I'm saying here. I'm not real good at explaining myself on paper. Fire back if something doesn't make sense to you. Bottom line is...... Keep it simple.

    You'll be living with this system a long time and you don't want constant headaches with it.
  • Jeff Andle
    Jeff Andle Member Posts: 8
    oil, oil, oil

    I kept figuring my heat needs would be low enough that LPG vs. oil wouldn't matter. We had convinced ourselves that since we insisted on am LP gas stove and an LP gas fireplace that the fuel heat value/price difference wouldn't justify also bringing in oil...

    But on further digging it looks like a difference between oil heat and gas heat will be between $600 and $1200 a year...

    Are there modulating oil boilers? The other way to do this is to get a controllable mixing valve ($700) to lower the heating water temperature as the outdoor temperature increases. In either case, is it necessary?

    Right now it seems that a closed system off the boiler using its circulator and a mixer (Taco?) with either four zone valves or four extremely small (1-2 GPM, 3' head) circulators and the manifold screw valves will be a complete system.

    130 water temp was the max I should need under estimated heat losses and requires insulated slab in the basement on 12" centers and pex in radiant-trak aluminum heat transfer plates under thin laminate "hardwood" over OSB (R<1).

    Is an oil boiler going to stay above 80% (85%+?) efficient at a reduced output temperature or are they optimized around 180?

    (Yes, I get what you're saying -- I slept through thermodynamics but I'm an analog and microwaves guy, not an on/off digital whiz -- I love the old servo loops!)
  • Jeff Andle
    Jeff Andle Member Posts: 8

    if the boiler pump (typicaly a grundfos that is overkill to my application -- 1452?) has all the needed kick, do i just use zone valves and a four zone relay controller with priority control to squash the heat when the indirect tank signals a need?

    Heck, with a 50K upper estimate on heat, do I even need to squash the heat when oil boilers are typically showing in 85-150K unmodulated? Big efficient tank, small efficient boiler and let the tank recovery time be what it is?
  • Terry_4
    Terry_4 Member Posts: 42

    fairly simple option is a Grundfos Mix-i-Mizer variable speed injection pump w/ primary - secondary loops. I have only installed a few of these, but so far they seem to be doing very well. And if the piping arrangement is worked out properly, the radiant system is self bleeding.
    I agree with SE somewhat: keep it simple, but if you are going to do domestic with the boiler and maybe even snow-melt you're going to need higher boiler temperatures, therefore you're going to need some type of low temp device.
  • LP modulating combo boiler

    Unlike SE, not working for Viessmann but loves the Vitodens; I work for BAXI and we have the LUNA boiler. The LUNA is very similar to the Vitodens but with a much higher ouput (approx 25%) avillable to your heating system.
    The boiler has a burner modulating from 105000 BTU's to 35000 BTU's with an outdoor reset capability built into the boiler control.
    The BAXI LUNA has been availlable in North America for around 4 years and has so far seen over 4000 installations.
    I am not trying to say that our product is better than the Vitodens because it is a very well designed and constructed boiler, but we have also a good reliable boiler with a proven track record at a very competitive price.
    Check us out at www.wallhungboilers.com
  • Jeff Andle
    Jeff Andle Member Posts: 8
    so many good gas boilers, but

    oil and LPG are the same price up here so going with gas (NG is not available) is a doubling in heating bills...

    However, if all the direct vent model pricing keeps favoring gas boilers, I may just stick to gas at twice the fuel bill. Even overestimating heat and hot water it's $600 a year difference...
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