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How much hydronic baseboard?

I'm renovating two rooms in a 230 yr old house. One is a bedroom in the NW corner (our cold weather direction), with about 25 ft of exterior walls, essentially NO wall insulation, about 35 sq ft of single-glazed windows, with well-fitting exterior storms. The room is about 185 sq ft / 1290 cu ft, with a heavy attic cap. There is currently about 25 linear feet of hydronic baseboard in the room. Is this adequate? I had recently removed about 8 feet and I am reluctant to add any more, as I am already at, if not above the 67 ft guideline for 3/4in baseboard and have lost two boilers prematurely from acid corrosion.
The second room is a newly renovated bathroom, about 70 sq ft / 490 cu ft. One exterior wall of about 7 ft. One new energy star window of about 11.75 sq ft. All walls, including interior walls will be insulated. Heavy attic cap. Heated slate floor and radiant ceiling heater / fan. I am planning 10 ft of hydronic baseboard, and expect this will be more than sufficient.
What are your thoughts?

Re: How much hydronic baseboard?

If you plan to do this as a diy project we would need more info in order to to adequately respond to your question.

One of your previous posts indicates you live in south central NH, which is a cold climate; can you post the total sq.footage of the house, and the rated output of the boiler---this would be on the nameplate at the front of the boiler; a 230 y.o. house harks back to colonial days---is it made of stone, or standard wood frame with wall cavities??? What are the typical low overnite temps there in January/early Feb.??? (design temperature).

If the house is very large or more than one story, how is the boiler piping set up; are the different sections of the house zoned off with zone valves/circulators, and separate wall thermostats for different rooms, or is the boiler piping connected as a single pipeline with the baseboard/radiators as a single pipe circuit using only one wall thermostat for the whole house???

Thinking back to last winter, did the present system heat the house well, with the boiler quickly producing 180 degree hot water that heated the rooms promptly, or was the boiler constantly firing all day & nite burning a lot of fuel to try & keep up with heating the rooms??

Are there also hydronic cast iron radiators connected to the boiler piping?? If you have bleeder valves on the baseboards, bleed out any entrapped air, using a screwdriver or pliers with a small cup to catch any water that comes out.

You are asking if the new baseboard will be sufficient to make the bedroom & bath feel warm; have you had problems heating these rooms in the past with the amount of convectors (baseboard) you had then???

Could you comment as to why you had to replace 2 boilers due to "acid corrosion"; do you have hard well water with a lot of minerals?? What reasons is your boiler service person giving for the boiler failures???

Questions about how many feet of baseboard are needed to heat a room are specifically based on the heat output of standard baseboard, which is approx 500 btu/hr per linear foot when the water temp is 170 degrees & the pumped flow rate of the water going through the baseboard is at least 1 gallon per minute---but then a HEAT LOSS CALCULATION has to be done to calculate how many heat BTUs/hour are exiting the room thru the windows/walls to the exterior of the building, depending on how much insulation is in the wall cavities, & what the walls are made of.

A less scientific method is to assign perhaps 40 btu/sq.ft. heat loss for a building/room such as the one you describe in your post, multiply this by the room's sq.footage (185 sq.ft. x 40 btu/hr/sq.ft heat loss = 7,400 btu/hr room heat loss) & compare that with the calculated btu/hr output of the 25' of baseboard (500 btu/hr/lin.ft @ 170 degrees X 25' of baseboard = 12,500 btu/hr heat output for the bedroom); given this scenario it looks like the amount of baseboard footage will do the job; but this makes a lot of assumptions that may not be true---is the room heat loss really only 40 btu/sq.ft./hr or is it more like 50,60 or 70???---considering the low temps of central NH, the lack of insulation in the walls, the boiler water temp being pumped thru the room, being possibly closer to 120 or 140 degrees, instead of 170, etc.; the heat output of the baseboard would only be approx 290 btu/hr/lin.ft. @ 140 degrees (290 btu/hr X 25' baseboard = 7,250 btu/hr baseboard heat output), which would not be quite enough to keep the room adequately warm.

Given your familiarity with the building, you may have to go thru perhaps at least part of the winter to see if the added baseboard will keep the room warm; you can always tweak the other elements of your system to get the room warmer----1) make sure the boiler water temp on cold days is in the 180-190 degree range when the boiler's circulator is pumping the hot water thru the baseboards---this is a simple adjustment on the front of the boiler; 2) as you noted in your post, definitely add insulation to the exterior wall cavities of the room, or have it blown in by a contractor; and definitely add blown in insulation to ALL PARTS of the house (exterior walls & attic) before the snow flies---an insulation job is low-cost & will save you tons of money in heating costs; a full insulation job will change & improve dramatically the dynamics of the heating system, trapping more heat inside the house on cold days, instead of allowing it to fly thru the uninsulated walls to the great outdoors--it's not how much heat the boiler/heating system produces, it's how much heat is trapped inside the house by the insulation envelope that will make the house feel comfortably warm & save tons of money on reduced heating costs---you'll notice a considerable drop in annual heating fuel usage as well.

In regards to the bathroom, the heating btu/hr requirement is always doubled for bathrooms---that's because people don't feel warm stepping out of a shower on a winter's day unless there is lots of heat in the room; thus a 10' X 10' bathroom that would ordinarily need 35 btu/hr/sq.ft for heat (10' X 10'=100 sq.ft. X 35 btu/hr/sq.ft.= 3500 btu/hr heat needed), would actually need (10' X 10'=100 sq.ft X 70 btu/hr/sq.ft.= 7000 btu/hr heat needed/divided by 450 btu/hr per ft. heat output of baseboard= 15.5 ft. of baseboard needed.

Many of these calculations for the bathroom baseboard are based on the assumption that the supply/return piping for the bathroom baseboard is not located "at the end of the line", just before the main piping reconnects to the boiler--in such a case many bathrooms are cold because the boiler-heated water has given up all its heat to the other rooms' convectors by the time it reaches the bathroom baseboard; it's for these reasons that the bathroom is often put on its own separate thermostat---a zone valve is often installed (easy with hydronic heating) that will heat only the bathroom convectors, controlled by the bathroom's own t-stat.

Going to the boiler room, can you see where the supply/return piping from the bathroom baseboard connect to the main boiler supply piping???? Try turning up the T-stat to get heat & use your hand to feel the supply/return piping of the bathroom baseboard when the heat comes up ---if the circulating water is approx 180 degrees (as it should be) you won't be able to keep your hand on the pipe for more than a second or two; if the piping is only lukewarm, something will have to be done to get more heat to the bathroom; while you're at it, go up to the bedroom in question & also feel the heating pipes with your hand to determine if you're getting 180 degree water in there, or something only lukewarm.

Given the heated floor in the bathroom, you probably could get by with 10' of baseboard in your case, but GIVEN THE RESTRICTED SPACE that is present in nearly all bathrooms, baseboard is rarely used due to lack of space---instead put in a one-square foot KICKSPACE HEATER at the base of the sink vanity, or even into a wall cavity & hook it up to your hot water heating system piping, using the connections of the existing baseboard; another alternative is to install a small cast iron or stainless steel radiator----these take up much less room & put out a lot more heat; another option is to install HIGH CAPACITY baseboard with a heat output of approx 700 btu/hr/lin.ft (7000 btu/hr/divided by 700= 10' baseboard needed--2 separate sections of 5' baseboard, if room allows); if only 5' of floor space is available for baseboard, two 5' sections of high capacity baseboard can be stacked one on top of the other to get good heat output.

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