Replacing radiator with radiant heat in one room

VilleGuy

Our single-family two-story house has a gas boiler with cast iron radiators throughout the house.

In one room on our second floor (a 5’ x 10’ bathroom) we would like to remove the cast iron radiator and replace it with a radiant heat system to be placed under the vinyl floor planking that we plan to install. (We will remove the current ceramic floor tiles and the subflooring so we can have access to the joist area.)

Our heating system has three zones, and this bathroom is on the zone that includes most of our house.

We're still at the planning stage, and when I have the work done I'll use a plumbing professional, but I first wanted to get an idea of what options and considerations we'll be faced with. Should we use oxygen barrier PEX? Is a joist track plate with a flange the best mounting option? What equipment will we need besides the tubing and mounting hardware: valves, sensors, etc.? Will this room require its own thermostat?

I'm sure I'll have other questions, but these are for starters. Thanks in advance for any helpful advice.

GroundUp

Absolutely need O2 barrier tubing. Plates will be subject to what you're doing, the pro you hire should have a recommendation. You don't need an extra thermostat if you don't want one, but you'll need to match the output of the radiant to the heat load of the room (same as the radiator output, if it was correct) which can typically be accomplished by varying water temperature as long as you have enough tubing installed. With that said, you'll need a lower water temp for the radiant than was fed to the radiator which will require some sort of mixing valve or injection to accomplish. Personally I prefer a 3 way thermostatic mixing valve and an additional circulator pump to accomplish this task

hot_rod

Could be a good application for Ultra Fin let it run at boiler temperature

It would be good to define the load of that room. All the rooms for that matter.

Exposed floor space could give you 20 or so Btu/ sq ft

Use the load valve at SlantFin to determine the load see if the floor can cover that number

Small square footage rooms can be a challenge for radiant floors only

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

VilleGuy

Thanks for your input, guys. If it turns out that I'm inviting trouble by trying to add radiant in just this one small room, maybe it would make more sense to install a baseboard hot water radiator in place of our current cast-iron standup radiator?

I should have noted that two of my initial goals were to remove the radiator and its supply pipes (which current rise from the floor about two feet away from the radiator).

Getting rid of the radiator and its pipes would give us some floor space in this little 5 x 10 room and make it look better.

Q: If we did go with basement radiator, what would the dimensions of that be? I'd like it to be as small as possible while still providing the necessary heat.

hot_rod

Does it have a cabinet for the lav sink? If so a kickspace heater is another option. Out of site, plenty of output, could be controlled separately if piped as a zone. Almost instant heat from them, multi or variable speed blowers.

They can also be used as a recessed wall heater between two studs.

A large heated towel bar could give some good output a bit cleaner look compared to cast iron.

Floor heat is always my first choice for bathrooms if it can cover the load.

mattmia2

Something like a runtal tube radiator might be a better replacement than regular baseboard, it would behave more like the cast iron radiator than fin tube baseboard would. Cast iron base board would be an even better option.

Since it is zoned I assume the system is hot water?

Ironman

There's more to this than most folks realize. The biggest problem in this scenario is getting enough heat out of the radiant floor to properly heat the room.

If you can get 20 btus per sq. ft. as hotrod mentioned, that's only 1k btus. Any where the floor is covered by a toilet, tub, cabinet, etc, is deducted from that. So, realistically, you may only get 700 btus or less. To give a comparison, a 1200 watt hair dryer produces 4K btus.

Baseboard rad's require a higher water temp than cast iron rad's and would require much more lineal footage, so that's usually not a good option either.

I personally think that you'll regret it if you remove the cast iron rad's as we've seen this done by others several times and the customer is always unhappy with their cold bathroom.

Electric radiant is an option that works well, but again, it probably won't have enough floor surface area to heat the room by itself on really cold mornings.

VilleGuy

hot_rod: Ultra-Fin is a very interesting suggestion--thanks.

SuperTech

I agree completely with what @Ironman said about getting rid of the cast iron radiator. Unless you are prepared to do what it takes to make the bathroom heated by radiant flooring I'm pretty sure you will end up with an uncomfortable bathroom.
Cast iron radiators have several advantages over fin tube baseboards and convectors. The only advantage that fin tube baseboards have is the lower price of materials and ease of installation.
Cast iron baseboards are an option, or maybe recessed cast iron convectors would be another option I would consider.

VilleGuy

SuperTech, what are your thoughts on Runtal's UF Baseboard or Wall panel?

https://www.youtube.com/watch?v=SP5VBrBICy8

mattmia2

hot_rod said:

Could be a good application for Ultra Fin let it run at boiler temperature



It would be good to define the load of that room. All the rooms for that matter.



Exposed floor space could give you 20 or so Btu/ sq ft



Use the load valve at SlantFin to determine the load see if the floor can cover that number



Small square footage rooms can be a challenge for radiant floors only



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

You have to figure out the heat loss of that room and the supply water temp (and ideally if the other radiators match their calculated loss so we can try to balance things) before we can help you pick a specific heat emitter.

SuperTech

I really like them both! Comparable to cast iron baseboards, but more modern looking. If you have enough of either for the right output I don't think you will sacrifice much in the way of comfort compared to the cast iron radiator.
I like the cast iron radiators the most because I like keeping the old buildings original, and I love how long the cast iron radiators stay warm.

Steamhead

Those Runtal units are steel, so they'll behave more like fin-tube baseboard than a CI radiator. Not good for comfort.

Also, what size are the pipes feeding the existing radiator? If they're larger than 1/2-inch, it's probably an old gravity system, and the connections on those Runtals won't be big enough to allow proper circulation.

You can still get brand-new cast-iron rads that will give off more heat but take up less space.