Electric or Hydronic underfloor heating on bathroom renovation.

speedfox_uk

I originally posted this on the a r/HomeImprovement subreddit, and someone told me this would be a good place to re-post it.

Hi, my partner and I are looking to get underfloor heating added to our bathroom as part of a complete bathroom renovation, and are deciding between hydronic and electric ad we are interesting in hearing some opinions on this. Here are the details.

1.) Location: South East England.

2.) the final bathroom will be about 7m^2

3.) There is already a radiator on the wall that is fed off the property's central heating system that uses a normal combination boiler, so I presume no need to run new pipes for heating (?)

4.) The bathroom will have *really* high ceilings.

5.) The bathroom is on the first (top) floor of the building (2 stories total)

Some specific questions:

a.) I presume that if we went hydronic we would not need a radiator on the wall because the floor would be fed off the central heating, but would we still need a radiator if we went electric, or would the hearing from the underfloor heating be enough?

b.) If we did go with electric as the sole heating source in the room, would that be more or less costly then running a radiator from the central heating system in there?

c.) Would the best course of action be to simply put in a new radiator if/when the time comes. My worries stem from the fact that there seems to be a big push coming over her to get everyone to replace gas boilers with heat pumps, and that heat pumps work best with underfloor heating (I don't want to have to get everything ripped out of the bathroom in a couple of years to put that in). I guess if there are appropriate radiators available I don't need to worry about this now(?)

Thanks in advance.

Jamie Hall

On the relative costs of the two ways of heating the space -- you really will have to do your own number crunching! Things are changing so fast, and relative costs on either side of the pond are so different...

Now. That said. I personally would not count on the undefloor heating being able to keep the bath nice and cosy and comfy. Moderately warm, yes. But not much better than that. We normally figure on being able to get about 20 BTUh per square foot out of a floor heat installation, which works out to somewhere around to only about 60 watts per square meter. So I think you would be very wise indeed to keep the radiator as well as the floor (might as well keep the radiator!). Further, I would make the floor hydronic, as you already have the piping there. However, I don't know what temperature your radiator is operating at, and that is really important as you don't want to overheat the floor. You may want to put a thermostat in the floor and a control valve on the floor loops, so that the floor is held at some reasonable temperature (say around 45 C). Down the road, if you do get to (have to!) switch to a heat pump for the whole system, the floor will fit in well with that, although you may need to change to a different radiator, as heat pump hot water systems run at a lower temperature than conventional gas systems do. But if you do it this way, you will have the options.

speedfox_uk

Jamie Hall said:

Down the road, if you do get to (have to!) switch to a heat pump for the whole system, the floor will fit in well with that, although you may need to change to a different radiator.

Ah, so you think that no matter if we go with electric or hydronic we will need to have a radiator as the primary heat source, and underfloor heating will just be for keeping the feet warm?

Jamie Hall

speedfox_uk said:

Jamie Hall said:

Down the road, if you do get to (have to!) switch to a heat pump for the whole system, the floor will fit in well with that, although you may need to change to a different radiator.

Ah, so you think that no matter if we go with electric or hydronic we will need to have a radiator as the primary heat source, and underfloor heating will just be for keeping the feet warm?

I expect so, unless the room is uncommonly well insulated.

STEVEusaPA

Maybe you consider radiant ceiling. Much easier to control. You get to use the entire ceiling to heat the room. Plus if it's a high ceiling, and no fancy moulding, you can insulate right on the face of the ceiling, and build your radiant ceiling below it, and cover with drywall. Radiant will warm the tile, but keep in mind there is a big difference between radiant heat and floor warming. With radiant heat your floor wont be 'warm' all the time. If it were, your room would overheat.
Being in the UK, I'm assuming you are using lower temperature water, so you can ditch the radiator and add a towel warmer or slim profile panel radiator to the wall-everything properly piped & controlled of course.
Well insulated as @Jamie Hall mentioned is always a good idea. I have a full bath, super insulated, and even though I put a toe kick heater under the vanity, I never hooked it up, and never needed it.

speedfox_uk

STEVEusaPA said:

Maybe you consider radiant ceiling.

I hadn't even heard of that before. Thanks.

hot_rod

do a quick load calc on the room. Any outside walls? Windows? It could have a fairly low demand.
Nice about electric it can be fired up anytime of the year for warming or drying the floor.

Jamie Hall

hot_rod said:

do a quick load calc on the room. Any outside walls? Windows? It could have a fairly low demand.
Nice about electric it can be fired up anytime of the year for warming or drying the floor.

Quite true. It also may be, given your current gas and electric prices, much the least expensive option!

Dave Carpentier

Our main floor bath is about 15 sqm and just has radiant in the slab. Its on it's own zone with a thermostat in the room. When I kept the t-stat at "room" temperature (~19c), the tile floor would be.. acceptable, but not "warm" and cozy.
Last winter we bumped the t-stat up to 24c and wow.. talk about happy feet when stepping out of the shower. Plus the tiles dry up quickly at those temps. I should note that we keep the door closed, otherwise that higher-than-normal temperature would just sneak out of the room when I wasnt looking, Im sure.

I wonder about your higher than normal ceiling.. warm air tends to rise. Wet warm air tends to stick to things.. watch for mold taking hold up where you cant reach it. Proper ventilation would take care of the issue though.

HVACNUT

When you say "there's already a radiator on the wall that is fed off the property's central heating system that uses a normal combination boiler", is this a separate private dwelling or a multi dwelling building?
Either way I think you'd need to know how the system was piped to see if you could just tap off the pipes from the existing radiator. Unless it currently uses constant circulation and a separate thermostat or TRV for the bathroom, you're looking at piping a new zone and controls at the boiler. 
Even if it is constant circulation, you'll still need a thermostatic mixing valve and pipe in an injection loop. 

PC7060

Another very popular option is Schulter Ditra Heat which is installed under the tile. Much less expensive than in floor radiant and works very well paired with the panel radiator. 

Floor temperature is controlled with a thermostat mounted to the wall and can be programmed with heating profile based on time of day. 

https://www.schluter.co.uk/schluter-ditra-heat_1.aspx

speedfox_uk

HVACNUT said:

is this a separate private dwelling or a multi dwelling building;

Multi dwelling building, but each dwelling has its own boiler (it's a large old house that's been converted into 4 flats). There is no communal heating system like in a large block of flats.

ariccio

There is an angle here that has not been discussed:

Some people (e.g. me) would want bathroom floor heating not for a primary space heating purpose, but for a combination of drying and luxury... Improved drying means less of a slip hazard for aging family members and also less growth of mold and bacteria that necessitates cleaning (also by aging family members, one of whom has arthritis). That's not a fancy luxury like a Ferrari, but it's a luxury to have less work to do! For this, I wouldn't need enough heat to keep the room warm - this is done fine with the supply register in the bathroom - just enough to drive off moisture. Heck, no reason it couldn't be in the walls or the tub too! Sure, this would also feel nice and cozy.

Electric resistance heating is... Less than a good choice when trying to turn an old 80s home into an efficient modern home. I'm not exactly expecting to reach passivhaus levels here, just to not waste thousands of watts.

Hydronics of some kind seem like an obvious engineering choice, because in theory they can be connected to a heat pump with a custom heat exchanger. The dream is actually to have minimal additional energy expended to heat the floor during the cooling season. A heat pump water heater dedicated to the bathroom radiant system would definitely function, but it's a huge challenge to find space for a dedicated HPWH.


The obvious solution to me would be to integrate it into the space cooling/heating system. In the summer, all the heat being rejected from the rest of the space is more than enough to heat the bathroom surfaces for "free".

The question becomes then: does anybody manufacture systems that integrate nicely with the fancy newer multi split systems?

I know Mitsubishi has some amazing manifolds for controlling the flow of refrigerant in many different ways throughout a home. I also know that Tesla pioneered a similar concept with their octovalve, to provide arbitrary 8 way control of heat flow in the onboard refrigeration system. It's been done in mass production in other contexts.

It seems to me like the industry is in the right place for the first time ever for plug-and-play systems like this to start showing up on the market.

Indeed, if I can find a non-custom solution here, I suspect it will also lead to other non-custom solutions to places where we'd be forced to use electric resistance heating, like resistance heaters in ERVs used to prevent core freezing.

hot_rod

If you are just trying to warm or dry the bathroom floor, 75 sq ft, a small electric mat would be my choice.

On a cold design day 75 sq. ft. X 10 btu/ sq. ft = 750 btu/hr. A tiny load to run through a boiler. Any day less that design that load is smaller.

Plus you have a radiator in there adding heat?

Your bigger issue in warming that floor will be over-heating the space.

Anytime the floor surface temperature exceeds the room temperature you are adding heat.
Keeping the floor at 80°F, room temperature of 70F will add 10 btu/ sq ft.

You get about 2 btu/ sq ft per degree difference between floor surface and ambient air temperature.

To "feel" a warm floor, it needs a surface temperature above your skin temperature.