Replacing Kickspace Heater with Radiant

[Gary in NJ]

I have a 10 x 13 bathroom (with a 5 x 13 adjacent closet) that is currently heated by a Embassy Industries HAV-48 Kickspace Heater (4800 BTU). We typically keep this unit on the low setting (2400 BTU) and it adequately heats the space (there is one outside wall that is 10'). I'm planning to update this 20 year old bathroom and I would like to put in some in-floor heating. What we have works (it even heats the tile in front of the vanity), but radiant heat would be quieter and better distribute the heat.

If I kept the radiant heating on the existing zone, would I simply need to add a 4-way mixing valve where the copper goes from 3/4 to 1/2 to feed the HAV-48? Obviously with the HAV-48 removed I would create loops in between the floor joists with 1/2 PEX.

Am I under-thinking this? I know these types of questions come up all the time. I did a search and didn't find what I was looking for. If however you've answered this question before, simply provide me a link and I'll read up from there.

 

[Fitter30]

Good manual https://www.google.com/url?q=https:...FjAAegQIBhAB&usg=AOvVaw01hjLEsXmZqTRsawpRsKsC
Need 70' of tubing for 2400 btu's

 

[Sponsor]

 

[Gary in NJ]

Thanks for that document fitter. I should have no problem running 70' of PEX in the space I have. I will probably keep with the 1/2 size that the current HAV-48 is piped with, so I will certainly install double the suggested 70'.

 

[Fitter30]

Might want use a thermostat that incorporates a floor sensor.

 

[Gary in NJ]

The plan is to have the bathroom radiant heat remain part of the current zone.

 

[Dana]

The plan is to have the bathroom radiant heat remain part of the current zone.

To get it right when including it all as one zone you'll need to do a room-by-room load calculation for the bathroom and other rooms, and compare the ratio of load to radiation, and make the output of the bathroom's (now floor) radiation comparable to that of other rooms. There is no way to know if the kickspace heater's output was/is actually proportional to the rest of the zone, but it's more important to get that right when using the floor as a radiator. There is almost NO way the actual heat load of a 10 x 13 bathroom is anywhere near 4800 BTU/hr (unless the window is left open). Use a Manual-J-ish type load calculator such as LoadCalc or CoolCalc. The absolute accuracy isn't super high on these freebie online tools, but the loads will still be close to proportional, at least sufficient for specifying the radiation.

What boiler is driving this system, and what type of radiation (& how much) is there in the rest of the house?

 

[Gary in NJ]

Thanks for the reply Dana.

I keep the kickspace heater fan on low, so it only delivers about 2400 BTU. Also, the HAV-48 heats both the 10x13 bathroom as well as the adjacent 5x13 closet, so it's really heating a space that is 15x13 with 2400 BTU.

The boiler is a WM Gold 115k (maybe 120) BTU unit that heats 4 zones (including DHW) in a house that is 4,200 sq ft.

The bedroom loop (the one that this bathroom is on) is approximately 144 lf and heats about 1,150 sq ft of the home.

Unfortunately, the bathroom is the first stop in the loop, so I don't think I can simply add a 4-way valve and be done. At the very least I will need to return the radiated loop directly back to the boiler so I don't drop the temperature of the entire zone. I'm also wondering if this should also have it's own pump. I really don't want to create another zone just for this small space.

 

[Dana]

Thanks for the reply Dana.

I keep the kickspace heater fan on low, so it only delivers about 2400 BTU. Also, the HAV-48 heats both the 10x13 bathroom as well as the adjacent 5x13 closet, so it's really heating a space that is 15x13 with 2400 BTU.

The boiler is a WM Gold 115k (maybe 120) BTU unit that heats 4 zones (including DHW) in a house that is 4,200 sq ft.

The bedroom loop (the one that this bathroom is on) is approximately 144 lf and heats about 1,150 sq ft of the home.

Unfortunately, the bathroom is the first stop in the loop, so I don't think I can simply add a 4-way valve and be done. At the very least I will need to return the radiated loop directly back to the boiler so I don't drop the temperature of the entire zone. I'm also wondering if this should also have it's own pump. I really don't want to create another zone just for this small space.

A 15 x 13 space still doesn't need 4500 BTU/hr at NJ type design temps, and most won't even need 2400 BTU/hr unless there is a lot of window area. Most bathrooms have minimal window area, and most closets have zero. It still pays to run the load calculations for all rooms to get this right. You probably only really care about heating the bathroom, the closet not so much (?), which would be fine running 3-5F cooler than the bath(?).

Is the linear feet the length of the plumbing + radiation, or just the radiation? Is it ~8" tall ~2x2" fin tube or something else?

If making it all one zone combined with higher temp hydronics it's pretty easy to go with a mid to high-temp radiant floor approach using finned clamp-on convectors PEX in the joist bays, no down mixing to low-temp required. This has the advantage of being tweakable post-installation by adding or subtracting convectors to get the room to room temperature balance a bit closer. But it still has to be designed (starting with real load calculations) to work well, not merely hacked.

 

[Gary in NJ]

The 144 lf is just the plumbing. On that loop there are 5 2x2 fin-tube convection radiators with a total length of 45' plus the aforementioned kick space heater.

There is just one 46x36 window in the bathroom and one exposed (northern exposure) wall that is 10' in length. The closet space is currently carpeted, so if it is cooler it goes largely unnoticed. However part of my renovation will include new tile flooring in the closet, so the warmth of the floor will become more important.

 

[Dana]

The prescription is still the same: Without comparing the ratio of the room radiation to the calculated room loads there is no way to figure out what type or how much radiant floor would be necessary to heat the room to about the same temp as the rest of the zone.

Even a crude simple I=B=R method load calculation done on a computer spreadsheet (ignoring the loads from air infiltration) would be good enough. A typical 99% outside design temp for NJ is about +15F- the actual 99% design temp in your exact location doesn't really matter for this exercise. Use either the code-min 68F or 70F for the indoor design temp. That makes for a temperature difference of 70F -15F = 55F. Then figure out the U-factors of your construction type, including roofs if the bathroom is under an insulated attic rather than under a second floor room. The basic formula for heat loss is:

U-factor x temperature difference x square feet= BTU/hr

The walls have a different U-factor than the windows & doors, but for argument's sake let's say the exterior walls are 2x6/R19 16" on center studwall type construction, 9' ceilings, the window is a U-0.32 code-min replacement window and the closet + bathroom are on a corner, all above a basement, and below heated rooms above. The losses then break down as:

Windows: (46" x 36")/144" per square foot = 11.5 square feet

U0.32 x 55F x 11.5' = 202 BTU/hr

Being on an exterior corner the exterior wall length is 13' + 15'= 28', so the gross wall area is 9' x 28' = 252 square feet. Subtracting out window area leaves 241 square feet. Looking up the U-factor that type of wall on the table in the links above shows U0.074 BTU/hr per square foot per degree-F difference, yielding wall losses of:

U0.074 x 55F x 241 square feet = 981 BTU/hr.

The total design load is then 202 BTU/hr + 981 BTU/hr = 1183 BTU/hr.

Run that type of calculation (using the U-factors of your actual construction & window types) on every other room on the zone, and calculate the ratio of load/baseboard length in each zone. Exterior doors can be estimated at U0.50, as can clear-glass double pane windows, or clear glass storm windows over single pane windows.

 

[Dana]

(continuing on...)

There will be some variation in the load-BTU/baseboard-feet room to room, but it should fall within a range. Then when it comes time to design the radiant floor you can design it so that it's BTU output yields the same ratio. Since it's on the beginning of the loop it's OK to design it somewhat to the lower end of the ratio-range of the other rooms, eg:

Say the range of the other rooms is 350 BTU of load/foot of baseboard to 400 BTU of load/ foot of baseboard. Typical baseboard puts out about 500 BTU/hr per foot at an average water temp (AWT) of 170F (180F out of the boiler, 160F return). That's between 350/500 = 0.7 load-BTU per BTU of radiation to 400/500= 0.8 load-BTU/ BTU of radiation. So if the design load of the bath + closet is the 1183 BTU/hr, you need to design the floor to emit something between 1183/0.7= 1690 BTU/hr and 1183/0.8 = 1479 BTU/hr. So shooting for anything between 1500-1600 BTU/hr would be reasonable (and tweakable if using finned convectors under the floor rather than plated staple-up radiant at low temp.)

 

[Dana]

There is more, but something on the forum software security is preventing me from posting it all as one response (?).

 

[Dana]

You don't want to heat up the floor directly under the toilet (it can melt the wax seal), or directly under any cabinets (where it'll cook the contents, and is less effective heating the room). Under bathtubs and showers is OK if it turns out you don't have sufficient floor area to actually hit your targeted range using just the fully exposed floor area. Using tighter spacing between convectors than specified in the manual can increase the output per square foot, wider spacing reduces output, so setting up the tubing like the "Modulating, 100-140F" configuration in the instructions gives you a bit more tweaking room, and potentially higher output per square foot. Removing one set of convectors out of the "High Temp 140-180F" configuration to reduce output will leave that part of the floor cooler than the rest, which may or may not be important to you. Think about it, run the numbers, repeat before figuring out which way you space the tubing or the convectors.

 

[Gary in NJ]

Great info Dana. I think I will use those convectors because; it's easier and I can make adjustments by adding/subtracting/moving plates.