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Community and Q&A

Radiant bathroom floor and auxiliary heat in the master of our PGH

Matt Mesa | Posted in Mechanicals on

Hi all–I’ve got a couple of questions regarding HVAC in my Pretty Good House (PGH). As you may recall, I built a PGH a little over a year ago and many of you contributed wonderful advice to my queries. We had a nice discussion on how best to heat my house and I decided to go with a single minisplit and a natural gas, heater rated fireplace. This system has worked well for most of the house, save the master bedroom. As you can see from the schematic below, we just don’t get a lot of heat into the master bedroom and, when it gets real cold outside (say below freezing), the bedroom is quite cold. Also, we installed electric radiant heat in our master bath, but we rarely use it. I could use your advice regarding the following:

1. I prewired the master for some kind of heater, and now it’s time to decide. It was actually set up for another small minisplit, but I think it would be too loud for a bedroom. So, for this 14’x15′ room with concrete floors, what type of heater would you recommend that would balance comfort, energy efficiency, and cost? Natural gas heater? Cove heater? Wall heater?

2. As I mentioned, we rarely use our master bath heated floor for two reasons: we’re not in the room very often and the cost of running this system is unknown–that’s where my question comes in. So, is it more efficient to turn the heat on, to say 70, and leave it on? Or, is it better to turn it on and off–using the timer thermostat that we have set up. We’ve found that 70 degrees is quite comfy–you can certainly tell the difference. Is there any way to calculate what it costs to run this system? I’m reluctant to add to our heating costs if this system is expensive relative to how much we use it.

Sure would appreciate any insight and advice you all have on these issues. Looking forward to an informative discussion.

Cheers

Matt

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Replies

  1. Andrew Bater | | #1

    Matt, I see no one has chimed in yet so a comment to bump up your thread.

    My house is probably also what would be considered a Pretty Good House (ICF/SIP/PassiveSolar). A geothermal system provides warm water to the first floor radiant slab and when that floor's thermostat calls those areas are heated evenly. However I also have a masonry heater that I fire twice a day during the winter. That keeps most of the first floor consistently warm and as such the thermostat only toggles occasionally, so the pump for those zones rarely runs. However the master bath is the farthest from that masonry heater and that room often ends up perceptibly (and measurably) cooler.

    Now the somewhat cooler bath is not a huge issue, I don't spend that much time there. However stepping out of a hot shower into a cool room isn't the most wonderful thing, so I bought a 150 watt heated towel rack. That little amount of heat in the room, and in particular putting a warm towel around your shoulders, seems to fix things just fine, at least from a psychological perspective.

    I do have the heated towel rack on a control such that the home automation/security (BAS) system turns if off when not needed; I currently use a temp sensor in an adjacent area that gets comparable solar gain. I am thinking about using a motion sensor too, but haven't figured out how I would do the "anticipator" setting. It also seems a bit weird that the BAS system would have a record of every time I use the bathroom!

    Andy

  2. Charlie Sullivan | | #2

    Your second question is the easier one: the timer will definitely save electricity. The less time it's on the better. If you want to know the actual consumption, you would need some kind of data logger. One option you might consider for that is "the energy detective" (TED) whole house electricity monitor with the option to monitor individual circuits. But unlike the minisplit, there's no disadvantage to setting the electric heat way back when it is not needed.

    The other question: if you want silent, that is either electric or hydronic. Hydronic is a complex and expensive system for just one room, so you are likely to want electric. A cove heater seems like a good choice for a bedroom because the radiant heat from it can reach the surface of the bed.

  3. Matt Mesa | | #3

    Thanks guys for the responses! Seems like the forum is quite busy--not many questions getting much attention. I've been looking into cove heaters but don't know much about them. More research is necessary. Charlie I don't understand your comment regarding no disadvantage to setting electric heat way back. I would think that would be a disadvantage because it would probably take more energy to go from, say 60 degrees to 70 compared to 68 to 70. In other words--should the radiant floor be operated like a minisplit--just set it and leave it?

  4. Expert Member
    Dana Dorsett | | #4

    On the bathroom floor, if it's a low-mass floor that heats up quickly, rather than a dumb timer, an occupancy/vacancy sensor set for a 10 minute time-out can work pretty well. Even if the floor is cool when you'r stepping into the shower it'll be warm when you step out.

    How many watts can the system deliver into the floor, and what is the design heat load of that room?

  5. Andrew C | | #5

    What about an electric toe-kick heater?
    I don't know about high peak current draw, but they work more or less instantly, so you only have to turn them on for the ten minutes that you're in the bathroom in the morning. And I think they're cheap, like $150. I've thought about this before in comparison to an electric radiant floor in the bathroom, but I don't know what the expert opinion is; high current draw is probably an non-starter for off-grid. What about for Net Zero or PGH? I'd be interested to hear expert opinion here.

  6. Matt Mesa | | #6

    Hi all, perhaps I need to clarify a little. And ask for clarification. Dana--it's a slab on grade floor, I think about 4" thick. I'd say lots of mass? I don't understand the dumb timer comment, but I do know that the floor is usually around 57-60 degrees if left alone--and it takes 2 hours plus to get it to around 70. So it's not fast responding. Regarding your other questions, I don't have answers but I think I can find them.

    Andrew--for clarification, I already have electric radiant heat installed in the bath. I don't need another heater there, just trying to figure out the most efficient way of operating it and whether it's a significant addition to my electric bill.

  7. Charlie Sullivan | | #7

    Matt, you say "I would think that would be a disadvantage because it would probably take more energy to go from, say 60 degrees to 70 compared to 68 to 70. In other words--should the radiant floor be operated like a minisplit--just set it and leave it?"

    The answer to that is no. The energy that it takes to go from 60 to 70 is less than the energy needed to keep it at 70 or even 68 for a longer time. That's always true with a simple heat source like electric. It's not true of mini-splits. That's because mini-splits' efficiency varies in a complicated way vs. the operating point. But if it's electric heat, you can only save energy by setting it back.

    The amount of energy you save varies, depending on how much thermal mass there is. If you have a lot of thermal mass, it will never get all the way down to 60, and you won't save as much as you would with a lower mass heater. So if you want to know how much you save, you might need instrumentation. But you don't need that to find out what is the best option--with electric heat, the best option for saving energy is always to leave it off as much as you can tolerate.

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