Sizing a radiator

lfoehrenbach

We are renovating a kitchen/dining space and are trying to figure out radiator size/location. Our home is 1930's woodframe. The new space is 28'L x 12'W x 8'H. It has one exterior wall with about 65 sq. feet of new double insulated window/doors. The whole exterior wall has been reinsulated, tyvek wrapped and sealed, and clad with new shingles. Our heating system is single pipe steam run off of an oil fired steam boiler. We live in central Vermont where the average outdoor daytime temp is around 30°F in winter, and rarely falls below 10°F but we can swing down to -5° on occasion. We also have a wood-burning insert that supplements the heating system, but we don't burn wood all day, every day. The room is above an unfinished concrete basement with exposed steam pipes (so, heated?) and below heated bedrooms.

Can anyone advise on how many BTU's I'm looking at for a radiator? Ideally, I would like to use one long radiator on a centrally located interior wall. (I have one that I can repurpose if it meets the heat requirements). Can I get away with one radiator in such a large space? There is a large opening from the dining area to the LR that has another radiator and the wood burning insert.

We are also installing an electric toe-kick heater in one corner of the kitchen to use on chilly mornings, but due to the expense of running it, I'd prefer not to rely on electric heat for the space to be comfortable. I've used several online heat loss calculators, but am getting results that range from 3,800 BTU's to 12,000 BTU's. Any advice appreciated!

EdTheHeaterMan

This is a difficult one. You can select a radiator based on the actual BTU load calculation of that room. If you do that, and the rest of the radiators in the building are 50% oversized, then that room may never get enough heat. That will be because the rest of the home (if evenly heated) will satisfy the thermostat and shut off the burner. That new radiator will then stop heating just as quickly as the other radiators in the home. But that radiator may not have the room up to temperature as fast as the oversized radiators in the rest of the building.

If the radiators are 100% oversized then your problem will be even greater. So to solve that you would want to oversize the new radiator the same percentage.

Say you guess at oversizing the new radiator at 50% to match the home, and the other radiators are actually the correct size for each room. Now your new kitchen radiator will overheat the renovated Kitchen. Now the Kitchen will keep the heat longer since the rest of the house is not up to those more efficient standards.

In order to get a better idea, someone needs to do a load calculation of the entire house and compare it to the heat output of a total of all the radiators . Now you know how oversized the radiators are compared to the needs of the house. Once you know that then you can decide on the percentage you should oversize the new radiator.

OR

I just may be making something out of nothing. And you just pick one that is a little oversized for the room. You can always turn it off if the room gets too hot.

Do you have a similar size room in your home? How big is that radiator? Does that room have 2 radiators? You can ballpark it that way, and maybe just get one a little smaller because of how well the room is insulated.

Do you know a mechanical engineer that does heat loss calculations for a living? Do you know an HVAC contractor that does load calculations for a living? Since you are getting different loads based on the same room using dIfferent methods, you really need to get someone that does this all the time and can make a more accurate calculation. 

dabrakeman

How much radiation (sqft or btu) did you remove from the kitchen/dining area in the renovation? Also, usually radiators are placed next to exterior vs interior walls to improve comfort by better equalizing temperature throughout the room. We don't know how your boiler is currently sized to the total standing radiation in the house. As @EdTheHeaterMan said if you are already oversized and you reduce the standing radiation your balancing and control issues will get worse. I personally would stay on the larger side of your estimates
(closer to the original sqft of radiation you had in that living space) and start off with an adjustable vent on the radiator. If too big it can be slowed down a bit. If it is too small and you are feeling drafts on the far side of the room you are SOL.

lfoehrenbach

Can you recommend a reliable heat loss calculator that would give me a pretty good idea of how to calculate BTU's needed for this room? I could then apply the same calculator to the other rooms to see if, indeed, the radiators are oversized. I do not know of a HVAC specialist that could do such a detailed calculation.

The radiator I wanted to use for this room previously lived in the dining room, on an exterior wall under a drafty set of old windows. The new location is on an interior wall, in a larger but more open plan/better insulated room.

According to my research, the re-sized radiator will output approx 8000 BTU's.

I used a couple of different heat loss calculators to approximate the needs for this room. Not sure which of these is providing the most accurate results.

Castrads says I need 3800 BTU's to heat the room

https://www.castrads.com/us/heat-loss-calculator

Omni says I need 5800 BTU's

https://www.omnicalculator.com/construction/heat-loss

EdTheHeaterMan

Both Omni and Cast rads are limited in the load calculation parameters you can provide to do an accurate calculation. There was a time that many of us here in HeatingHelp.com would recommend a free calculator provided by SlantFin boiler company. Slant fin has since sold the baseboard division and closed the boiler division. there is still an App available for the load calculation but I have not used it. Maybe others herein have some input about it. Here is the place to start

https://slantfin.com/slantfin-heat-loss-calculator/

delcrossv

https://forum.heatinghelp.com/discussion/comment/1813722#Comment_1813722

@EdTheHeaterMan Sorry Sir, the links go nowhere. Slant Fin is gone

EBEBRATT-Ed

Get the square footage of the rest of the house excluding the kitchen/dining area. Then calculate the EDR of the radiators in the rest of the house and total that up.

This will give you and EDR/square foot for the house not including the kitchen/dining area.

Then get the square footage of the kitchen dining area 336 sq feet and install the EDR that your EDR ratio for the rest of the house shows.

There is no way to get this exact but it should be close enough. As others have mentioned you can go a little larger and slow the venting or use a TRV

EdTheHeaterMan

@delcrossv , Hmmm. Then I guess they need to pay for WrightSoft or Elite like all of the professionals did when I was in business. I still have an older version of one of them on an old computer. I can't get it for my MacBook without paying for the license again. So when I need to do one, I crank up that old Dell laptop and hammer one out and email the results to my Mac. I also have some hand written forms that were put out by the Hydronics Institute. (remember when it was called The Institute for Boiler Ratings I=B=R years ago?).

These forms give you more choices to select from, you can do a room with 1/2 of the ceiling exposed to the exterior and the other half under a conditioned space. Each room is more accurate that way. Since there is no air condition load, you didn't need to tell the difference of the North or South or East/West walls. Less work needed for a heating only system size.

bburd

I=B=R was the Institute of Boiler and Radiator manufacturers if I recall correctly.

I'm not that old, but I've read a lot of old books.