Replacing cast iron radiator with smaller one

A smaller radiator will offer you less heat. Is the kitchen too hot? …then reduce the size of the radiator.

If your kitchen is just the right temperature when the heating system is operating, then installing a smaller radiator with less heating output, will result in your kitchen will be colder.

Doesn't look as though you have much room to move, but yes — since this is hot water it's not all that hard to move…unless that drain pipe isn't as much in the way as it looks.

That’s a tough situation. Obviously you need the kitchen space. I wouldn’t sacrifice comfort or simplicity though. What if you had a shorter, deeper radiator the same width and radiation capacity? That may allow you to continue the counter in front of the window and maybe have some storage underneath without inhibiting the radiator’s function.

What about putting a cart in front of the radiator? You could build a cabinet around the radiator if you leave access to service the radiator and give it some place to pull air in at the bottom and out at the top. there are all sorts of wall hung cast iron radiators too. you want to match the edr of that radiator to the new one.

I understand we can't replace it with a compact aluminum radiator because that would require a lower temperature. Any advice?

who told you that? Also you could use a steel one (also lightweight).

steel radiators or other low mass radiators will heat differently and will be very difficult to impossible to balance with the rest of the system unless they are on a separate zone which would present other problems because of the small size of the zone.

I see a complete kitchen reno with staple up radiant.

That's an H.B. Smith Imperial 3-column rad (see page 76 of @DanHolohan 's book "E.D.R."). Whereas the usual 3-column unit is rated 5 square feet EDR per section, these are 6-1/2. So this rad is rated 65 square feet, or 9,750 BTU per hour when the boiler is at 180 degrees F.

Since all the rads in the house were sized using the same method, if you install a rad in the kitchen with less capacity, the room will be cold. You have to work within the parameters the original installers used if you want it to work well.

If you can find a used four-column, 8-section rad that is 38" tall, or a 6-large tube, 11-section rad of the same height, it will carry the load while taking up a bit less floor space, though it will probably stick out from the wall a bit more.

Also, do not reduce the pipe size to the radiator. or the water will not circulate well. Again, all the piping was designed the same way, and if you add resistance by using smaller pipe, the water will go elsewhere.

The easiest way out of this would be to leave the rad as it is, and rework the cabinet design.

Can you send pictures of the boiler and associated piping? Totally separate zone will give you significantly more options

I've replaced an rad like that directly with floor heat. No extra zones, just plumb to the new floor heat to the existing rad.

This was with UltraFin install from bellow. These can be sized to run with the same temperature water as your exisiting rad then you match the sqft of heated floor to be the same output as the existing rad. Depending on how restrictive the existing rad piping is, you might need multiple short loops in parallel, if doubt go with a small local manifold with multiple short loops.

If you want to get fancy, you can even add a TRV or zone valve on this for local temperature control but simple adjustment on the manifold is good enough.

@EdTheHeaterMan

You have to do a bit of design but it works.

Say that rad is about 6000BTU output. If you don't want the floor too hot, you are looking at 20btu/sqft of floor, so to replace it you need 300sqft of heated floor.

If you go to the ultrafin install manual:

https://www.ultra-fin.com/wp-content/uploads/2022/03/Ultra-Fin2013manual-WebHi.pdf

There is the table for high temp water. From there you need 24" OC of fins to get reasonable floor temps with 180F water.

The issue I have seen is most rads are oversized for the space and they never run that hot, so a direct sized floor heat will be not get toasty.

That may work when you design the entire system to use the same tubing and ultra fin and have each loop properly sized to match the rest of the pressure drops of the other loops. Your statement about just using the 3/4" or 1" or 1-1/4" supply and return from the removed CI radiator, and the same circulator, and the same thermostat is what I have a problem with. As I said previously, "It may even work in some rare cases."But in this case I believe this is bad advise. How do you make sure that the 1/2" PEX tubing gets the proper amount of flow?

Assuming the system on the Left is operating properly and is relatively balanced. When you remove a gravity designed cast iron radiator that has very little resistance to flow, and replace it with 100 to 200 feet of 1/2" PEX tubing, there is not going to be enough pressure differential to successfully move the GPM necessary to heat the floor. I guarantee it!

This is why I have a problem with your advise to @amaiale. Unless you are going to actually do the job and guarantee the result, then this advice is not in the best interest of our fellow Wallie. Just my educated and experienced opinion.

No offense intended @Kaos. I just want the facts out there and the best advise for the members.

I understand how to make an insulated plenum out of each joist space by insulating the rim joist to form a cavity without any possibility of air movement in and out of the heated plenum of the joist space. This is important with most designs of staple up and Ultra Fin.

When you said "I've replaced an rad like that directly with floor heat. No extra zones, just plumb to the new floor heat to the existing rad." It brought back memories of a finished ceiling that needed to be torn up in order to make a failed "Just connect it to the existing pipes" floor heat system in order to redesign the piping with its own pump and zone circulator to get it to work. So, as I said before, unless YOU are doing the project personally and guaranteeing the result, it is bad advice, because I would not assume that the person doing the job would have the understanding that you and I have.  In fact, I would guess that the average plumber could really screw this up using your advice. IMHO.

Know your audience!

and the room probably needs less!

does that new cabinet design allow access when needed to that radiator.

@amaiale, Can you post a floor plan for your new kitchen cabinets in including the radiator location. In an earlier live I worked for a lumber company that sold kitchen cabinets and was involved in designing kitchen cabinet layouts. With this experience I was able to save one of my heating system customers the cost of replacing a washer and dryer set with a stackable appliance or apartment combo unit in the utility room. The existing boiler was in that room and when they wanted to replace the Oil Heat with Gas Heat, I was able to redesign the small room by moving the heater location from one side of the room to the other side. This left room on the opposite wall for a washer, a dryer, and narrow laundry tub. Something the wife always wanted was a laundry tub, but it would not fit with the oil burner on that wall.

If I could look at the floor plan of the new Kitchen cabinets, I might see something that the cabinet design folks do not see, regarding how the radiator can be incorporated into the design.

???

My guess is the original poster considered this sites advice (while good and well-intended) and plans to proceed with the recommendations of her on-site cabinet designer per her last post

Regards,

RTW