Would this be a viable solution to lower the return temp?

pdanaher8 · November 20

Hi all. I've been commissioned by my wife to install a combination boiler in a modular house that will be delivered soon. The house comes with hydronic baseboard heat throughout. I have to design this system under the assumption that these are the type of baseboard heaters that operate with a 180 degree supply temp because I can't get specs on these. My question is, if the desired return temp of less than 130 degrees is not satisfied, could plumb in addition slant/fin heating elements ( as many as it takes ) to achieve an adequate return temp?

mattmia2 · November 20

You could but it would take a lot of it and you'd need to do it in such a way that you keep the output in each room balanced. For example if it is a single zone series loop and you triple the emitters in each room such that it goes back and forth in the first room before flowing to the second room, that first room will get much more heat than the other rooms. It would be a lot better to replace the baseboard with panel radiators sized to the desired water temp.

HVACNUT · November 20

What combi boiler are you using? With ODR and/or system and boiler supply and return sensors, the boiler should MODulate and fire based on recognized temperatures.

If there's enough linear feet of baseboard to heat the space on a design day at 180° SWT, then you're good. The boiler will handle the rest.

You're doing the actual installation and commissioning of the boiler? If it's a DIY job, follow the manual STRICTLY. And find a qualified tech to "commission" the boiler, because the manual also has a page or two dedicated to combustion parameters. Stay safe.

Jamie Hall · November 20

The low return temperatures for a mod/con boiler aren't required. They are permissible, since the boiler is designed and built to handle the highly corrosive condensate, but they aren't required.

Now having said that, a condensing boiler (not necessarily modulating) is going to be more expensive to build and finicky to operate because it can tolerate that condensate. The tradeoff is that, if you can get the low temperatures, you get slightly higher efficiency and thus use less fuel. Unless the whole system is designed from the ground up to use the modulating and condensing features, the extra efficiency is not going to pay for the extra cost.

Now I note that @pdanaher8 actually used the term "combination" boiler — and I wonder if that is what was meant? A combination boiler is used for both hot water heat and domestic hot water, and while they can be modulating ad condensing, they don't have to be.

What they do have to be is carefully sized for the application — and they are almost always badly oversized for the heating side due to having to provide ample hot water, or seriously undersized for the hot water side to try and match the heating load. So the question is what are the two loads in this application, and how was the boiler sized.

Last, as has been mentioned, "commissioning" a boiler is not a do it yourself proposition, unless yourself has the necessary testing equipment to properly set up the combustion.

hot_rod · November 20

I don’t know of any combi unit that can’t be derated on the heating side , so no they are not grossly oversized. The small units 100-120 derate down to 12,000 btu/hr output. Same with the non combi mod cons.

Even if your system needs 180 at design 80% of the season it will run lower swt. So use the ODR function to maximize efficiency

pdanaher8 · November 20

So, it's a Navien NCB-H. The math, on paper at least, seems to fit my needs. The TDR's are impressive if they are being honest. This unit will be used for DHW and hydronic heat. I did a room by room manual j off of the prints and arrived at a +/- 75,000 BTU/H heast loss with my design temp. That is well within the modulation range of this boiler. I think it's sized correctly. The house is two floor, so two zones. Being a high efficiency unit, I am looking to maximize that. The zones aren't terribly large, so if the supply needs to be 180 degrees to achieve the stated BTU/H output given for the baseboard heat, the return temp will probably be well above the 130 degrees needed for condensation. My idea was to plumb in a " cascade " of maybe 4 slant fin elements ( mounted to the wall ) per zone return before that water hits the primary loop. As far as commissioning the boiler goes…. I'm not well versed with it. My limited understanding of these combi units leads me to believe that they are "plug and play" as long as the install, sizing etc.. are done correctly.

DCContrarian · November 20

Do you know how many feet of baseboard it comes with? I would think a modular house would have it all sized out and the baseboard would be sized to the heat load. Figure 500 BTU/hr per foot of baseboard.

Baseboard is usually sized for a drop of 20F. Sizing is usually for 180F water temperature. But that's at your design temperature, which is the coldest 1% of the year. The other 99% of the time you can have cooler water and a cooler return temperature.

Jamie Hall · November 20

They are not plug and play. Every single installation is slightly different — enough so that the combustion needs to be tested and adjusted to obtain good results, never mind best results.

Kaos · 12:34AM

https://forum.heatinghelp.com/discussion/comment/1821373#Comment_1821373

75k btu heat loss of a new build even modular seems on the high side, what is your outdoor design temp and what is the rough construction? Around me (north edge of zone 5) that would the heat loss for about a 5000sqft somewhat air tight code min house with reasonable glazing.

The good thing is if the emitters are sized for 75k, you should be able to run them at much lower temp with a more realistic load. Combine this with a good outdoor reset curve, you should get good efficiency out of the unit.

If possible, one thing that will help is to get a two pipe or home run setup, this would get you more control over individual rooms. It would also let you run at lower temps, thus better efficiency.

pdanaher8 · 1:27AM

Zone 2 ( upstairs ) is 30' of baseboard, zone 1 is 40'. When I ran the manual j calculation, I had to guess on the window heat loss factor as the online specs don't jive with the options on the manual j sheet, so I went with the worst case scenario of single pane. The modular home needs to be built to code and VT just upgraded their's earlier this year, so it's well insulated. The high heat loss number is worst case at the design temp of -11 degrees and the turn down ratio of the Navien should be enough to counter any over zealous math. I would hope that the sizing from the design group is appropriate for my location, but I have trust issues. Plus, I enjoy learning about the technical side of things. I may have to look t getting someone in after the install to optimize everything, but I'm confident in my ability to at least perform the install correctly.

pdanaher8 · 1:41AM

Can you explain a homerun set up Kaos?

HVACNUT · 1:48AM

"Plug and play"? You want to play boiler installer, that's fine if it's legal where you are. But we shouldn't play when it comes to the combustion part. A real Commissioner might have an issue with that. Most people who come here, come after the damage was done, so again, strictly follow the Installation manual, and get a pro to commission and test it. Then it should get maintained and tested annually. I think I read a thread here somewhere where someone rented a combustion analyzer. So if you're feeling froggy, jump.

You're going to stack BB in the living space, or in the equipment room to give off extra BTU's so the flue gasses condense?

Kaos · 1:51AM

You only have 35k BTU of baseboards, so 75k would be an issue. I don't think you can even buy single pane windows, most code min nowadays is double pane low-e.

Min modulation of most boilers doesn't change all that much with sizing and since most will already be oversized for the load, doubt you'll have issues. There is not much point in going above the 55k unit in this case.

Getting a handle on your actual heat loss would be a good idea as it will at least tell you what your expected heating costs would be and if it is worth to pay extra attention to build details and even do some upgrades to reduce usage. -11F is pretty cold.

Would this be a viable solution to lower the return temp?

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