Hydronic baseboard: is longer or shorter better as regards furniture placement?

VAsparky

I'm in a small Cape Cod (34' x 24' with a full basement and finished upstairs), 8 miles west of Washington, DC. Ductwork is inadequate for central A/C, especially upstairs. I'm considering hydronic baseboard heat with a mod/con boiler and mini-splits for cooling.

Because the rooms are small (e.g. largest bedroom is 12' x 12', living room is 17' x 12'), much of their perimeter is occupied by furniture. So, I'm wondering if I'm better off designing the system:

(1) with the intention that the space in front of the convectors should be unobstructed, and therefore I should use the shortest possible baseboards (or maybe go with panels) in order to leave the maximum amount of wall length free for furniture placement,

OR

(2) with the premise that furniture can be placed anywhere, and that I should make the baseboards as long as possible to compensate for the reduced convection in areas behind furniture.

Any thoughts on this?

Ironman

You need to size the emitters based upon the heat loss of the house, not furniture.
If you're going with a mod/con, then you should size them for a maximum SWT of 140 - 150*. That way the boiler will condense under any normal outdoor temps and be at its peak efficiency. The cooler the return water to the boiler, the more efficient it will be.
If you wanna conserve wall space, then look at Smith's Heating Edge low temp baseboard or consider panel rad's.

Brewbeer

I went in the other direction. I ran a lot of baseboard, sized for heat loss on the design day using 130 degree supply water temp. I've got furniture tight in front of baseboard in parts of the house, but there is enough of it that it doesn't matter.

The problem with running a lot of baseboard is it can get expensive, especially if you use the Smiths HE, which was running $50/ft when I was crunching the numbers. There are other baseboards which have high ratings, just a little lower than the Smiths HE, at less than 1/2 the unit cost. Don't forget to factor installation costs in your estimations.

VAsparky

Thanks, Ironman. Yes, I understand that heat loss is paramount in sizing the baseboards. I'm thinking of the tradeoff between longer length with lower output per foot versus shorter length with higher output per foot, considering supply temperature.

For example, if that 12' x 12' corner bedroom needs 5,000 BTU/H at design temp, I could use 10' of an emitter that puts out 500 BTU/ft at 150F SWT, and keep the area in front of it entirely clear of furniture, or I could use, let's say, 24' (the entire exterior wall length) of a 300 BTU/ft emitter at a lower water temperature, with the excess capacity compensating for the fact that some of the baseboard will be blocked by furniture.

Gordy

Panel rads may be a better option if there is an amount of wall space that will allow its existence with out blocking output from furnishings.

Your idea of being able to block the baseboard by using more of it is counterintuitive. Base board works by convection, and needs air flow to accomplish this.

VAsparky

Gordy, my thought was that I would install an excess length of baseboard, knowing that a certain amount of it would be rendered partially or completely ineffective by the furniture blocking its airflow. The alternative would be to prohibit furniture in front of the baseboard, and use the shortest possible baseboard so as to minimize the impact on furniture placement.

I like the panel radiator alternative. In my example bedroom, there are two windows, about 4' wide. The window sills are about 2' above the floor, so I probably wouldn't want to put furniture there anyway. Allowing a little margin on either side of the window, I could dedicate 10'-12' of perimeter wall to the radiators.

VAsparky

Brewbeer - your first paragraph sums up the tradeoff better than I did!

I agree that cost becomes a consideration when oversizing the emitter, but in my case it's a small house, DIY install, and probably using a relatively inexpensive baseboard.

Gordy

I find panel rads a bit more elegant than base board, and can take up less space perimeter wise.

It also depends how tight the furniture will be to the baseboard, and if air can get under the furniture. Example some sofas have skirting that drops to the floor.

Zman

More radiators is better in any case as it allows for lower water temps and higher efficiency.
Sizing all the radiation to the heat lost of the spaces using the same design water temp makes things simpler.

NY_Rob

What are your plans for zones and what is your choice of mod-con?
You have to keep sort cycling considerations in mind.

Brewbeer

what @Zman says !
If this is DIY and the budget permits, consider installing as much radiation as you can afford. You'll be able to run that mod-con way down in the condensing range to maximize efficiency.

VAsparky

NY_Rob said:

What are your plans for zones and what is your choice of mod-con?
You have to keep sort cycling considerations in mind.

Good questions - I haven't really gotten that far in the design.

Probably three zones: basement, first floor and second floor. Short-cycling would definitely be a concern given the small size of the house; I haven't done a thorough Manual J, but I guesstimated around 50,000 BTU/Hr before I add much-needed insulation (essentially none now) and air-sealing.

Gordy

You can size for low water temps until the cows come home, but you can not ignore blocking that output from the baseboard. Essentially it’s like throwing a bear skin rug on a radiant floor.


I don’t disagree with sizing for low water temps to benefit the mod con, but your emitter type, and zoning needs to be addressed. Don’t size zones less than the minimum modulation of the mod/con. Which is 8k

hot_rod

It is tough to completely shut down fin tube output, as long as it gets some air movement it will move some heat.
Closing the damper or covering the fins is about the only way to shut them down.

Or a thick mat of pet hair on the fins.

I'd second panel rads as you get convection and radiant output, nice to stand in front of. Add TRVs for ultimate adjustability.

NY_Rob

VAsparky said:

Probably three zones: basement, first floor and second floor. Short-cycling would definitely be a concern given the small size of the house; I haven't done a thorough Manual J, but I guesstimated around 50,000 BTU/Hr before I add much-needed insulation (essentially none now) and air-sealing.

Do you plan on heating the basement?

What's the total approximate square footage of the heated area of the house?

Slantfin has an app that can help you calculate your heatloss without too much effort. It's better to have a handle on your heatloss before designing the radiator layout.

VAsparky

NY_Rob, I apologize for the slow response.

Thanks for the tip on the SlantFin app! Looks very nice and easy to use. I've just started working with it, but so far, the heat losses are coming in pretty close to my rough Manual J numbers, and the resulting suggested convector quantities are very workable (I'll probably be using more, in fact).

The basement is about 740 SF with 5 ft. below grade and 2 ft. above, 8" block walls, unfinished (no insulation). First floor also 740 SF, 4" brick, gypsum sheathing, 2x4 studs, plaster interior, no insulation. Second floor about 612 SF under gabled roof, will have (probably) R24 insulation.

pecmsg

Designing for where the furniture goes is fine but that changes (at least here yearly)

Cape Cod Style leaves room in the attic for central air / Heat Pump with a H W Coil on the 2nd floor. With a heat pump lower energy costs in the Fall, good part of the winter and spring. No worries where the furniture goes.

VAsparky

Good points about the heat pump. I've been considering mini-split ceiling cassette units for the upstairs, since the rooms need A/C anyway (just using window units now) and I want to maximize storage space in the kneewall area.

pecmsg

Heat Pumps or Minis a Back up source of heat is recommended.

That's why i like the HW coil in the duct system. A few precautions need to be taken due to the cold environment but nothing that cant be done!

mikeg2015

A coil in the ductwork piped in series can give you more output with less baseboard footage and still hitting the efficiency you are looking for.

Consider using baseboard under windows but use higher water temps. In bedrooms, radiant wall panels may work better and feel better.

Radiant ceiling panels can work well in family rooms if you have spaces upstairs.

Baseboard is cheapest but occupies more space. Some of it is low profile so you can still put furniture in front of it.

Lastly, don’t feel locked in to a particular water temp for all zones. If you use return water temp control, you can use a variable flow primary set on delta T control. Then use circulator for each zone and pipe the highest temp zones first on the primary then each lower temp in order. DHW would be first and automatically get priority.

Gordy

If the ductwork is inadequate for AC, how will it be for heating?

VAsparky

Just wanted to thank everyone for your very helpful comments and suggestions. I'll keep you posted as the design process (slowly) progresses.

Intplm.

When you are designing the baseboard lay out, You might want to look into different types of high capacity or high intensity baseboard elements. They can come in shorter lengths then common baseboard, giving you more wall space while satisfying heat loss calculations.
The Slantfin company , as earlier mentioned, as well as other manufacturers have those recommendations. This might be helpful toward some of your furniture concerns.

Jean-David Beyer

When I replaced my old hot water heating system with a new one, I wanted to use a mod-con with outdoor reset. Downstairs is radiant slab at grade, and upstairs is baseboard.

I got a new mod-con boiler, split the baseboard and radiant slab areas into two zones (they needed considerably different water temperatures), got natural gas into the house, and so on.

The baseboard zone had 3 feet of baseboard in each room, and was always too cold. I had the contractor (I am not a heating professional) put 14 feet of baseboard in each of the two rooms. This was the width of the rooms on the wall where the windows were. I no longer have the itemized bill, but as I recall, the cost of materials was less than the labor, overhead, and profit for the job. In retrospect, I wish I had put ten more feet of baseboard in those two rooms. I could use even lower supply temperatures then.

Keith M

I am the commercial product manager for Slant/Fin. We rate our baseboard and finned tube heat outputs using the Hydronics Institute test method. There is a product for sale in this country that used a European test agency to rate their "low temperature" product and it IS NOT the same test method as we use. For example, the have a "mixing fan" blowing air at their product and this turns it into a type of fan coil/baseboard hybrid. Using the IBR test method the baseboard must develop it's own convective air movement, it does not use a fan at all. I encourage you to get a copy of their test report and review it yourself. It is free. We have tested some of these products in our test room and found the actual output is slightly higher that 50% of what they publish. This is using the Hydronics Institute testing procedure. (now part of AHRI). If any heating professional wants a tour of our main office and production facility, just call me at Slant/Fin and we can set a date. Please note, this invitation does not extend to my competitors.
That being said. I recommend designing baseboard and finned tube systems using a 140F water temperature if you plan on using a mod con boiler. With improvements in insulation, window and doors, sometimes homes that had baseboard installed 30 years ago, homes with upgrades in insulation, windows and doors now have ample baseboard to properly apply a mod con boiler.
However, when replacing a cast iron boiler with a mod con you should always do a heat loss. Not only for proper boiler sizing but look at the amount of active fin in the baseboard cover to ensure there is enough active fin to heat, under design conditions, within the temperature limits of the mod con boiler. generally a cast iron boiler can heat up to or higher than 220F, while mod cons may have a limit of 200F, 190F or perhaps lower.
Baseboard and finned tube is "perimeter" heat and should be installed on exterior walls, under windows or close to doors (without interfering with movement of the door) and covering as must of the exterior wall as possible. This is to maximize comfort.
Review the "low temperature" baseboard test the other company used to justify their ratings. Research yourself, and do not just rely on what I state. I am plagiarizing another company in stating "an educated consumer is our best customer"...this is an accurate statement.
We publish outputs for our baseboard and finned tube down to 110F water temperature.
Keith Muhlmeister
Slant/Fin Corporation
516 484 2600

VAsparky

Intplm,

Thanks for pointing out the availability of different baseboard ratings. There is indeed quite a range; for example, Slant/Fin's Fine/Line 30 puts out 320 BTU/H/LF with 140F water at 1 GPM, while their Multi/Pak 81-A delivers 440 BTU/H/LF under the same conditions.

Intplm.

Good to hear. Thanks.