Radiant Heat Runtimes

[Smrtpunk]

I have a smallish garage (16*37) that I’m heating via a radiant heat system using a 6 KW electric boiler.

In my area of the world we’re subject to Time of Use billing, meaning electricity rates almost double during peak times. The cheapest times to use during the week are after 7pm straight until 7am the next day. I usually go into the garage around 7PM until 930-1000PM Sunday to Saturday.

The issue is, it takes too long to warm up the space. So, I end up trying to heat the space during the day leading up to me going into the garage. That said, the peak electricity rate is from 5-7PM, so I don’t want to run it then.

What I end up trying to run the system here and there (via thermostat programming) for small amounts of time to try and maintain my desired temp of 13.5 Celsius.

Question #1:
In your experience, are you setting a temperature and holding it throughout the day? Or are you running the system like myself, here and there to try and obtain a desired temperature for when you are actually in the garage?

Question#2
Do you think that turning on the heat here and there is actually using more electricity as you have to recover the temperature in the slab at a much lower temp than if you are just adding a little heat bit by bit as needed to maintain a constant temp?

I want to buy a Nest Thermo so that I could pull metrics and see how much the system is actually running but they’re not cheap!!

 

[homebuilt burner]

Mine is set and turned on in Oct. and off in April. I let the thermostat run it as a constant temp. We had a programmable thermostat in our house and we saw no significant savings setting the temp up and down, only a cold house half the time.

You are in a different situation as I am on Nat Gas and the price is the same all day.

 

[Randy in Maine]

I would just set the thermostat so that it actually only heats the slab during the "cheap times". See how that works. Good insulation really pays off.

I just turned on my "garage zone" that is set to 50ºF, but I don't think the boiler has actually started sending hot water out there yet.

 

[James-W]

Would it be possible to switch to natural gas or propane? The heating system you have is probably the "Cadillac" of heating systems, but it doesn't work well if you keep turning it down or turning it off all the time.

I would think that putting in as much insulation as possible in the walls a ceiling and then turning the system down to a setting that you can still work reasonably comfortably in and letting it run all the time would be the most economical way of doing it. But I am not a HVAC person and I may be wrong on that so take what I said with a grain of salt.

 

[Smrtpunk]

Interesting read from: http://www.concretenetwork.com/radiant-floor-heating/questions.html

What is Thermal Mass?

"Thermal Mass" refers to the ability of a material to retain heat. For instance, a heated stone will remain warm much longer than a block of wood. This is because the stone is denser, thereby containing more mass. The mass of the earth can be used as a flywheel when it is heated under a radiant concrete slab. This storage of heat can carry a building through a time when energy is not available. Where "off peak" electrical rates are offered, using a radiant floor in conjunction with the thermal storage of the earth beneath the slab can produce some very low electric bills.

 

[theoldwizard1]

In your experience, are you setting a temperature and holding it throughout the day? Or are you running the system like myself, here and there to try and obtain a desired temperature for when you are actually in the garage?

This only works if you have excellent insulation, including under the floor.

Because you use it such a short of a time, you would be better off to leave the temp around 5-10C and then use some other form of heat to help bring it up to temp when you go in, even if that is an electric heater.

 

[Firebrick43]

+1 on the old wizard. Nothing good comes from wild swings in a high mass radiant system.

 

[Smrtpunk]

This only works if you have excellent insulation, including under the floor.

Because you use it such a short of a time, you would be better off to leave the temp around 5-10C and then use some other form of heat to help bring it up to temp when you go in, even if that is an electric heater.

I failed to mention this in my earlier post but I do have the thermo set to not let the building fall below 5 C and as far as I can tell, with my current schedule of heating, it never has (yet... first winter!).

In an attempt to use it only during the less costly periods, my thermo schedule looks like this (1600-1700 = 13.5) (1900-2200 = 13.5) (4 hours of 13.5) the rest of the day, I'm allowing the building to go to a minimum of 5C before engaging the system.

 

[finn]

I use my oil boiler and in floor heat to keep the shop at 42-45 degrees. I turn on the overhead propane heater to bring the building up to working temperature as required when I'm actually in the shop.

My house has a propane boiler and in floor heat also. Response time is too slow to fiddle with the thermostat.

My personal opinion is that in floor heat is over hyped. It is comfortable, but not flexible, and is expensive to run as a result.
I

 

[Shop Specialties]

Floor heat is not expensive to run, in fact you only need a 1/3 of the BTU to heat the same space as compared to forced air.

The problem is people messing with the thermostat and having large temp swings. It is easier to maintain temp with 1/3 less BTU than it is to turn the thermostat up/down constantly.

 

[finn]

Floor heat is not expensive to run, in fact you only need a 1/3 of the BTU to heat the same space as compared to forced air.

The problem is people messing with the thermostat and having large temp swings. It is easier to maintain temp with 1/3 less BTU than it is to turn the thermostat up/down constantly.

I'd like to see the thermodynamics to support that assertion.

I understand the air infiltration argument out there, which is promoted by individuals who make their living promoting radiant heat systems, but have yet to see actual thermodynamic and heat balance calculations supporting the theory.

 

[Shop Specialties]

Forced air and radiant in floor are 2 totally different heating systems.

Forced air needs more BTU to heat the air so the wind does not feel cold.
Radiant in floor heat needs less BTU because it is heating you directly.

 

[finn]

A BTU is a BTU. The "wind" you feel in a proper system will be warm air, not cold.

The one advantage of radiant is that your feet stay warm, although the mechanics at one local truck repair shop complain that their feet sweat during heating season, so it's not a panacea.

I wouldn't install radiant again, based on my own experience and talking to several other people with radiant systems.

It's hard to fight internet hype, though.

One advantage is that radiant requires underslab insulation, which helps. I think local codes require the slab insulation for basements now, but not garages, unless they have radiant heat.

 

[kabinenroller]

Electric is not as efficient as gas (or propane) In my first system I tried an electric water heater, it would not recover and made the meter spin. I converted to a propane heater and the system worked much better. That building was existing and I converted it to in floor heat. My new shop was designed with the heating system as the main focus, I use a propane on demand heater and it works excellent. I set the thermostat once and never touch it, the system is never turned off even in the summer. It just does not run because the thermostat is satisfied. I keep the building at 60 degrees, all of the equipment becomes a thermo mass. I have a laser thermometer that I use to check the temperature in different parts of the building, everything including my machines are within two degrees of each other. There is no better way to heat a space.

 

[bfarroo]

Would there be some way to heat some sort of thermal tank during the off peak time and then use that thermal energy during the peak time to keep the space warm?

 

[Shop Specialties]

Yes, a BTU is a BTU but how that is made to make you comfortable are 2 totally different things. You are missing that point. The forced air wind requires more BTU to feel warm than radiant heat radiating to your body. I have been doing nothing but heating for 25 years and every customer that went in floor required less BTU and is happy.

Radiating heat directly to you will require less BTU than heating up air and blowing it on you.

 

[Shop Specialties]

Electric is not as efficient as gas (or propane) In my first system I tried an electric water heater, it would not recover and made the meter spin. I converted to a propane heater and the system worked much better. That building was existing and I converted it to in floor heat. My new shop was designed with the heating system as the main focus, I use a propane on demand heater and it works excellent. I set the thermostat once and never touch it, the system is never turned off even in the summer. It just does not run because the thermostat is satisfied. I keep the building at 60 degrees, all of the equipment becomes a thermo mass. I have a laser thermometer that I use to check the temperature in different parts of the building, everything including my machines are within two degrees of each other. There is no better way to heat a space.

Yes !

 

[finn]

Yes, a BTU is a BTU but how that is made to make you comfortable are 2 totally different things. You are missing that point. The forced air wind requires more BTU to feel warm than radiant heat radiating to your body. I have been doing nothing but heating for 25 years and every customer that went in floor required less BTU and is happy.

Radiating heat directly to you will require less BTU than heating up air and blowing it on you.

Only true if you heat to a fixed temperature 24/7. Heat loss to the atmosphere is strictly a function of how warm you keep the inside temperature and for how long.

Radiant isn't well suited to temperature setback (to reduce delta T with ambient temperature) , so thermodynamics says there will be more heat loss with a fixed level of insulation, and, thus higher energy consumption.

Yes, your feet stay warm at lower air temperature with radiant.

In my own experience, keeping the radiant thermostat set to the low 40s, and installing a hanging propane heater reduced my oil consumption by about 300 gallons over the winter at the expense of less than 100 gallons of propane.

The propane supplemental heater paid for itself in one year.

Radiant is cleaner, quieter, and more even , but it's expensive to install and operate.

 

[Smrtpunk]

So I think the takeaway from all of this discussion is the lot of people with radiant run it at a constant temperature and supplement with another heater if they keep their system at a lower than desired temperature.

No one has been able to answer my second question on their thoughts of what uses more energy to run, a run at periodic times setup (playing catch up on the slab cooling). Or a run at a constant temp with the system running periodically as needed to maintain temp.

I wish I had a way to test but my thermo does not have any data logging option. I suppose I could do small sample times (4 hours) and average it out, other than that, I don't know how else I could track it.

 

[Shop Specialties]

So I think the takeaway from all of this discussion is the lot of people with radiant run it at a constant temperature and supplement with another heater if they keep their system at a lower than desired temperature.

No one has been able to answer my second question on their thoughts of what uses more energy to run, a run at periodic times setup (playing catch up on the slab cooling). Or a run at a constant temp with the system running periodically as needed to maintain temp.

I wish I had a way to test but my thermo does not have any data logging option. I suppose I could do small sample times (4 hours) and average it out, other than that, I don't know how else I could track it.

You will use more fuel having large temp swings than leaving it at a constant temp. Once the slab is up to temp it takes little heat to maintain it.

 

[terabitdan]

You could use a water storage tank and heat it up to 180 degrees during the lower cost time. A 60 gal tank could store roughly 60000 BTU. It might not last the full 12 hours but it would lower your cost. Any loss from the tank goes to Heating the air, so that's not relevant. A larger storage tank would store more heat.


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[finn]

You will use more fuel having large temp swings than leaving it at a constant temp. Once the slab is up to temp it takes little heat to maintain it.

Not true.

Do you understand thermodynamic laws? Why do you think setback thermostats were developed? That's like saying letting your car idle all nite saves fuel.

You're describing a perpetual motion machine. Keeping an unoccupied room heated with a continuously heated slab will increase heat loss to the ground and atmosphere. You loose less heat if the slab is allowed to cool when not in use because the delta T is lower. Basic, simplified thermodynamics at work.

It may be uncomfortable because it takes time to reheat the slab, but total boiler run time and energy used will be reduced if the slab is allowed to cool.

 

[bushmechanic]

I want to buy a Nest Thermo so that I could pull metrics and see how much the system is actually running but they’re not cheap!!

They're worth every penny; and they pay for themselves very, very quickly.

If you're balancing costs and attempting to determine when the best times are to run your system, I can't think of a better addition. It does a lot more than provide data every month, and any idiot can use one.

If it's compatible with your system, go buy one. Tomorrow.

 

[roscoe2000]

You will use more fuel having large temp swings than leaving it at a constant temp. Once the slab is up to temp it takes little heat to maintain it.

Since the slab acts as a heat sink radiant heat will have an even longer response time once the slab temperature has been allowed to drop. In your case letting it run over night would provide no cost benefit since now you would be trying to heat the slab when the outdoor temp is at it's coldest. Your best option would be to convert from electricity.

Since radiant heat flooring is the most efficient of heating an enclosed space, you are halfway there. Even with force air, you would not be happy with response time once the cold has been allow to soak in to the walls and the building's contents.

 

[Randy in Maine]

I don't agree that radiant floor heat is expensive to install nor to operate.

I have 1000 feet of 1/2 pex in my insulated 6" slab with 2 4 way manifolds (hot and cold) and a thermostat. I probably have less than $1K in that. I already had the boiler (propane fired Baxi) in the house and it was just another zone that was already planned for. Mine runs very little but I have really good insulation in my detached 28x40 garage and very low infiltration rates. Very comfortable to work in. No fire danger.

The OP is getting really cheap electricity during low usage periods at night. (I pay 15 cents a kilowatt all the time here) I would just raise the slab temp to about 13.5º C (~55º F) or a little more and have it only heat when the electricity is cheap. Everything in there will retain a lot of heat all day. The slab will warm back up to the design temp at night when it is cheap. No need to overthink this one.

 

[LS6 Tommy]

This only works if you have excellent insulation, including under the floor.

Because you use it such a short of a time, you would be better off to leave the temp around 5-10C and then use some other form of heat to help bring it up to temp when you go in, even if that is an electric heater.

+1 on the old wizard. Nothing good comes from wild swings in a high mass radiant system.

X3...

Tommy

 

[LS6 Tommy]

I want to buy a Nest Thermo so that I could pull metrics and see how much the system is actually running but they’re not cheap!!

They're worth every penny; and they pay for themselves very, very quickly.

If you're balancing costs and attempting to determine when the best times are to run your system, I can't think of a better addition.

I can, Ecobee. More functions, features, better reports, easier to hook up and use. The Nest is already almost passe' in the smart stat market...

Tommy

 

[Smrtpunk]

Moving forward, I've made the decision to keep the thermo at 10C all the time minus the peak electricity times 0700-1100 and 1700-1900; in which I've set the temp to 5C.

I feel as though this type of setup will use less electricity since I won't be bringing the slab back from ridiculously low temps; as in my experience that can take hours, maybe using more electricity then maintaining the heat in the slab.

I have a crappy Quartz Heat Lamp heater over top of my workbench where I spend the most amount of time and I'll only use it when I'm out there.

Maybe down the road I'll grab the nest thermo and be able to provide some hard data on runtimes, but my gut instinct from using it this little time that I have, tells me that this is probably going to be the cheaper option.

 

[Shop Specialties]

Moving forward, I've made the decision to keep the thermo at 10C all the time minus the peak electricity times 0700-1100 and 1700-1900; in which I've set the temp to 5C.

I feel as though this type of setup will use less electricity since I won't be bringing the slab back from ridiculously low temps; as in my experience that can take hours, maybe using more electricity then maintaining the heat in the slab.

I have a crappy Quartz Heat Lamp heater over top of my workbench where I spend the most amount of time and I'll only use it when I'm out there.

Maybe down the road I'll grab the nest thermo and be able to provide some hard data on runtimes, but my gut instinct from using it this little time that I have, tells me that this is probably going to be the cheaper option.

I think you are making an excellent choice by starting with a 5* temp difference during your different rate times. This way you can evaluate if the 5* was to much or maybe you can go lower and still be comfortable. You can achieve the same results of a Nest thermostat just by simply paying attention to your bill, thermostat settings and most importantly your comfort levels.