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Community and Q&A

Radiators for Condensing Boilers

NearCoastalBC | Posted in Mechanicals on

My current small house (modern, well-insulated) has a Viessmann boiler and standard rads. Unfortunately the installers had little clue about the niceties of this system, so it’s not operating as well as it could.  Little can be practically done at this point but I may be moving in a year or two and although heat pumps are “the way to go”, I may opt for the same type of system as the backup power requirements are lower. In a power outage, my modest whole-house battery backup can run basics + the boiler for 4-6 hours. Not so with a heat pump drawing 1,500+ watts.

I have learned  quite a bit recently about these boilers, including the performance improvement “slope” as the return temp. is reduced below 130ºF. There will be a practical limit to this, but I figure about 145º out and 115ºF return might be realistic.  In looking at baseboard specs, there are various “high output” units such as  Slant Fin Multipak 90 + H1 elements, which put out as much heat @150º in as standard units do at 170º, which is a good start in “losing” more heat.  I notice that the same elements installed in a taller cabinet (Series 350), produces even more heat, presumably because the taller cabinet creates more convective air flow.  I have also been reading about “convective radiators” (like these from Stelrad: )which are tall and thin, with single, dual or even triple elements, but these seem to be only available in the UK (?) It appears these combine both tall vertical fins (good convection) and a flat panel. With the numerous “thermally-challenged” homes in the UK, they need all the BTUs they can get.

There will be other design considerations aside from this, of course, but it seems “losing” as much heat before the water returns to the boiler is an important first step.  Any comments or recommendations on this would be appreciated.

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  1. paul_wiedefeld | | #1

    You've got the gist - lower return temps are best, so lower average water temperatures are better. You can do this two ways: lower the temperature needed and/or reduce flow to widen the difference between supply and return temperatures. Steel panels, cast iron radiators, baseboard, radiant surfaces it's all the same. Outdoor reset will get at least some condensing out of most systems, so you're adjusting that % up or down.

    These journals describe it better than I can:

  2. Expert Member
    Akos | | #2

    The ROI on replacing any existing rads is basically never. I would focus on what you can tweak in the existing system.

    Setting up outdoor rest properly is the first step. I have seen Viessmann units that were installed, cranked up to 180F and left there basically turning it into a non-condensing boiler.

    Next check pump flow rates. It is pretty common to have these system way over-pumped. You want to run as low of a flow rate as low as the unit allows. There is very little efficiency lost when the unit supplies hotter water as long as the return temperature is low, for example I'm running older cast iron rads on a condensing unit with about 40F delta across them.

  3. Expert Member
    DCcontrarian | | #3

    From your description, I'd say there's a good chance your existing radiators are oversized anyway. For the installer, it's a lot easier to install one that's too big and never worry about it.

    The standard assumption is that all radiators behave the same way, there is a linear relationship between temperature and heat production. The zero-intercept of that line is room temperature.

  4. NearCoastalBC | | #4

    Would have replied earlier but had some login problems.

    Tweaking the current system isn't really an option. I was enquiring about what to look out for if I were to move and want to retrofit an existing building or do a new install. If I was a plumber I might be able to modify my current system, but "I'm not going there" and at $150/hr. I'm not getting my plumber in to do it, aside from the regular maintenance he already does.

    AFAIK, the existing rads are about right, given the HLC and the length of elements installed, based on ~170ºF water temp. But it's moot at this point, as is the way the 6 rads on the main floor (the main zone) were plumbed. I did consider having my plumber replace the rads on the main floor, but the cost (Slant Fin Multipak 90, ~$1750.00 + labour) isn't worth it, plus, even at 150º EWT, the RT probably wouldn't be low enough to make a significant difference anyway. The ROI wasn't the main factor; more a matter of principle of using less NG, as we are being encouraged to do.

    Paul- thanks for the link. As to "Steel panels, cast iron radiators, baseboard, radiant surfaces it's all the same", I'd say,"Yes, but the amount of heat each emits differs." My thought, if I were to move and refit or a new install, is to use rads that have a high output per foot in order to "lose" as much heat as possible before returning to the boiler.

    I believe the pumps are set for 1 GPM and I believe my plumber balanced the flow rates. I am suspicous of the low loss header, as I think that can screw things up as well, but there are a few other oddities in the original install that are probabl;y not worth messing with unless I were to replace the whole installation with a newer boiler, which I don't want to do unless it gets beyind its service life and also if I plan to stay here long enough!

    My boiler (Viessmann Vitodens 100, installed in 2009) doesn't have OTC circuitry built in, but I did add a KWE unit designed for it and an outside sensor. I found it somewhat annoying to futz with and disconnected it. As I'm running at ~170º anyway, I doubt it would help

    If I were to move to/build a new place with a basement, the latter would be used only for mechanical and storage, so one way to help "lose" more heat might be to install some baseboard just before the return pipe to the boiler. The basement temp. wouldn't be important as long as it was conditioned and dry. When that day comes, maybe something to discuss with the installers, but I'd do that well before anyone goes in to do an install. "Once bitten, twicw shy" as they say :)

    1. paul_wiedefeld | | #5

      You’re slightly misunderstanding things here. The goal is low return temps. That doesn’t mean losing a lot of heat necessarily, it just means low return temps. You could run 90 degree water out, 88 back and be extremely efficient. In fact, during mild weather when you need the lowest temps, the delta T will close for the same flow rate. Regardless of emitter type, the more you have the lower temp you’ll need. With outdoor reset, even if you need 180 on the coldest day, you’ll be condensing much of the time.

  5. NearCoastalBC | | #6

    Thanks, Paul. I need to do more reading, esp. as to the finer details of OTC.

    In my current system, with 170º out, feeding about 35' of "standard" baseboard - plumbed in series (!) - the return temp. before it goes into the zone header for return water (and then into the rest of the "plumbing" before the boiler) is about 128º, yet the temperature measured at the inlet pipe of the boiler (ie "post-plumbing", low-loss header etc) is about 152º, which doesn't make sense. If I had to replace the boiler system, that could all be done right at the start, which I suspect it wasn't. Not cost-effective to correct that now, though. I'm just gathering info for a new install or retrofit so I can talk intelligently to the installers. I "know just enoughto be dangerous" but I'm trying to correct that :) Certainly the lower the output temp that will work with whatever emitters, the better.

    BTW, I had a good read of the info in that link you semt me. Excellent! Wouldn't have found it on my own.

    1. paul_wiedefeld | | #7

      Yup those are great resources. Sounds like you have primary/secondary piping - the boiler loop has a higher flow rate than the baseboard loop, so the delta T of the boiler loop is smaller.

  6. Expert Member
    Akos | | #8

    The best way to save operating cost a new build is by not going with hydronic.

    The only time you'll have any savings with hydronics is if you are heating with something like a pellet stove or outdoor wood fired boiler.

    For anything connected to the grid, heat pump is the way to go. The heating load per sqft on anything even built to code min is so low nowadays that there is no way to justify the complexity and cost of hydronic. Throw in there the need for cooling for most parts of North America and you are adding ducting anyways so it makes no sense to have a redundant heating system where you can use those same ducts for heat.

    If you think you are in an area that doesn't need cooling, I would look 10 to 20 years out. Lot of folks on west coast or most of Europe didn't think cooling was needed until it was last year.

    P.S. This is coming from someone that built their home with hydronic heat and wall mount mini splits for cooling. If I could go back in time, I would save all the cost, complexity and labor and just install a ducted heat pump.

    1. NearCoastalBC | | #9

      I’ve been doing some research on heat pumps and they are seemingly the way to go, esp. as they provide cooling as well. Even out here, it hit 100°F the summer before last, due to a heat dome , and this last summer, 96°. (The prev. winter it also dipped to 0°F, 14-16 deg. below normal for winter out here!) This last spring I trenched in 120’ of 6” drain pipe, 5-6’ down as an earth tube cooling system, with a fan, and it helped. I’m getting too old for projects like this now, though.

      My one concern about heat pumps is powering them in an outage. I have a whole- house battery backup, good for about 4-6 hrs. at present, and an old 6kw Onan diesel genny. This saved my bacon when an ice storm knocked out power for 84 hrs. a few years ago! A 2 ton heat pump draws about 1600w, so I’ d need a bigger battery system to run the house on the same schedule, without running the genny a lot. But I’m looking into that as well, so it’s certainly not off the table. Heat pumps are improving continually, too.

      And I do have a pellet stove as well. Kept the house warm for two days last week when the boiler developed a leak and it dropped to 13°. I def. believe in having emerg. backup.

  7. JaxFlores | | #10

    Wow, it sounds like you've got quite the setup for all seasons and situations! Your research on heat pumps and the earth tube cooling system is impressive. It's great to see you're prepared with a whole-house battery backup and a trusty generator, especially for those power outage moments.And having a pellet stove for emergencies is a smart move. You've covered all your bases! If you ever find yourself in need of any specific radiator solutions, don't forget to check out Radiator Village ( ). They're experts in all types of radiators and might have some valuable insights to share.

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