When to fire up radiant floor?

[coldh2o]

As it begins to cool off here (central Ontario) I'm wondering when to fire up my hydronic radiant floor in the shop. I have a one year old propane boiler and it was quite efficient last year keeping the shop at 12C (54F). I run the thermostat off an in floor sensor so opening the doors doesn't bump the boiler on.

I could do a heat loss calc and look at degree days, blah blah but the general question is - fire it up as soon as outdoor temps are around 12C and let it chug along, or wait until it gets colder and blast it? Last year I fired it up in late November and it took quite a while (hours) to get up to temp. I wonder if that hurts the life expectancy of the boiler?

 

[mygarageone]

That's a judgement call on your part

 

[coldh2o]

Sage advice, gracias.

 

[pseudorealityx]

You turn on the heat when you get cold. That's about the extent of it.

 

[koditten]

I don't give it a thought. Mine's on a thermostat. When the floor calls for heat, the boiler will turn on.

 

[Rookie2]

hours ? mine takes days (2500sgft)

 

[sprntpshr]

Check the slab temperature and if it drops below 54F fire off the boiler. If you wait until cold weather hits at least the boiler will be closer to its designed heating load and less likely to short cycle. "Assuming" it was sized correctly to begin with. Not saying it wasn't....

If its any help, my in-floor system will be off until mid-late November at least. Last year I started it up Dec 3 IIRC. You look to be a bit further north than I.

A short cycling boiler is unlikely to make it to its full lifespan. Mine is cast iron boiler so return temps gets monitored closely and feeds a storage tank.

 

[roscoe2000]

It would be best to fire up your radiant heating system, once your slab temp is just above or at it's set point. The theory behind heat and cooling is that if you allow the temperature to drop to low (for heating), you will use up more energy to bring your environment temp up to it set point. Think of it like stopping of a hill then needing to merging into traffic going 60 mph, vs doing the same thing on flat ground before having to going up hill. One you will be flat till you can crest the hill, the other you can throttle back...there by using less gas.

Remember you are not just heating the slab, but everything with in those walls....

 

[Thumper68]

I have been playing with mine for 9 years now and have found that it saves me $$$ by turning on the system before it gets too cold. I now fire it up when inside temps are in the mid 50's, I heat the shop to 65F on the weekends and 50 during the week.

The first year it was already bin the high 20's to mid 30's when I got it fired up and it ran off and on for 2 days to get the shop heated up.

The next few years I just left the system on and while I didn't have that big start up cost I did notice that at times in the spring that the boiler would run when I really didn't need it to.

So now once the daytime temps are in the high 50's I shut it down.

 

[anthony666]

dont jinx us mr coldwater .. its still nice out !!

 

[UpstateNY]

It would be best to fire up your radiant heating system, once your slab temp is just above or at it's set point. The theory behind heat and cooling is that if you allow the temperature to drop to low (for heating), you will use up more energy to bring your environment temp up to it set point. Think of it like stopping of a hill then needing to merging into traffic going 60 mph, vs doing the same thing on flat ground before having to going up hill. One you will be flat till you can crest the hill, the other you can throttle back...there by using less gas.

Um, no, it's not that simple. Assume you only use a shop on the weekend. Your thought is to heat the shop all week at or near the set point, then bring environment up the desired temp just prior to use. And while u r doing so, heat is escaping from the bldg throughout the week.

Versus, for e.g., not heating the bldg during the week, then heating it up just before weekend use. This equals no heat loss throughout the week.

It's about heat loss, period. It's not about "you will use up more energy to bring your environment temp up to it set point". That is not true, a btu is a btu, energy is energy. The issue is the total amount of heat loss over a given period of time. More heat loss = more $$$

 

[coldh2o]

dont jinx us mr coldwater .. its still nice out !!

My bad. And good point, I still need to get my driveway paved. Bring on the lovely fall days!

 

[coldh2o]

Um, no, it's not that simple. Assume you only use a shop on the weekend. Your thought is to heat the shop all week at or near the set point, then bring environment up the desired temp just prior to use. And while u r doing so, heat is escaping from the bldg throughout the week.

Versus, for e.g., not heating the bldg during the week, then heating it up just before weekend use. This equals no heat loss throughout the week.

It's about heat loss, period. It's not about "you will use up more energy to bring your environment temp up to it set point". That is not true, a btu is a btu, energy is energy. The issue is the total amount of heat loss over a given period of time. More heat loss = more $$$

I think you're simplifying too much in the other direction. Heat lost has to be replaced by energy in. Bringing the environment from a non-heated state to the set point once a week takes huge btu input, versus small inputs to keep it at a constant setpoint. It's analogous to the common opinion of not doing nightly setbacks on a slab because it takes too long to recover, only on a longer time step.

I expect the answer falls somewhere in between, and can only be accurately predicted with a lot more specific input on the system parameters, shop size and usage patterns, etc. I do intend on heating to a constant temp throughout, not just on weekends, and the responses so far (thanks guys!) point to better experiences with an early startup. I know, anecdotes do not equal data, but I'm going to try it this year - I'll track my fuel usage and see how it goes.

 

[yeldogt]

Most radiant systems are not designed correctly ... If it takes days to heat a slab then something is not working correctly. Sounds like bad slab insulation or too small a boiler (unlikely) ... Lost of people don't use enough tubing.

Controlling a high mass system efficiently requires more than a wall thermostat -- but that is what a lot of people use.

The key is constant low temp water. The lower the floor temp can be to maintain the ideal space temp the more comfortable the space.. the more efficient the system.

A high mass system is problematic in the spring and fall ...

 

[EOC_Jason]

Lucky you, we are still sweating balls down here in Houston, TX....

 

[jvitez]

Hmmm, interesting question. Here are two more points to consider.

The RATE of heat loss is proportional to the temperature difference between two surfaces. So, heat loss when outside air is 10C° and your slab set point is 12C° is negligible. Your walls will have solar gain during the day and lose heat at night. Why keep your boiler water warm if the inside shop temp is close to the 10C° you want anyway?

Now a fuel burning appliance is at maximum efficiency when it reaches steady state, so blasting it to bring up the slab temp is actually the most efficient combustion condition for the boiler.

Do you store anything that will freeze in your shop (chemicals, paint, spray cans, etc)? Then turn it on earlier. If not, I'd wait until the nightime lows are steadily below freezing and save some expensive propane.

 

[coldh2o]

Hmmm, interesting question. Here are two more points to consider.

The RATE of heat loss is proportional to the temperature difference between two surfaces. So, heat loss when outside air is 10C° and your slab set point is 12C° is negligible. Your walls will have solar gain during the day and lose heat at night. Why keep your boiler water warm if the inside shop temp is close to the 10C° you want anyway?

It's an on-demand boiler, so no stored water to keep at temperature.

Do you store anything that will freeze in your shop (chemicals, paint, spray cans, etc)?

Uhh, beer!

 

[buddyboy]

you'll need a crystal ball to figure it out.

it will depend on the temp each day from now till heating season.

if you know what the temps will be each day you could calculate expected heat loss and BTU's used if you turned your system on now or waited till heating season.

example: if you knew it would be in the 60's F this week, 50's F next week then 20's F 3 weeks from now till spring, you should wait till the day before it drops to the 20's...LOL and I didn't even use a calculator

i say if it feels cold in there, turn it on. then set it to a degree or two above what you think the outside temp will be the next few days, that way you make the least amount of btu's and have the least amount of potential heat loss. only downfall with that stratagy is if the outside temps incure a massive heat wave 3 weeks from now

 

[jvitez]

Uhh, beer!

Then you MUST be careful not to let it freeze. Which Canuck hasn't at one point in their life gone back out to their vehicle the next morning to find broken glass beer bottles frozen in the middle of a beer slurpee stuck to the floor/trunk/seat? (real Canadian's drink beer from glass bottles not cans.......)

Now since you have an on demand boiler than no issues there. It would be a mess of calculations and speculations to try and determine when you'd be most cost effective in turning your system on, therefore turn it on when the daily highs are around 12C and dropping.