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Old 08-14-12, 07:59 PM   #11
Vlad
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opiesche,

Be careful when you select your HWT. All HWT have their minimum temperature unless it is a condensing one. If you just get regular DHWT and try to run it @ 80F you will end up with condensation problem.

For this reason if you use regular DHWT (non condensing type) you have to keep it @ minimum allowed temperature an install mixing valve.

I used special mixing valve I050C2R-2 - Taco I050C2R-2 - 1/2", 2 Way Outdoor Reset I-Series Mixing Valve w/ Sensor which works as thermostat as well. It is pricey but worth every penny. Check eBay for better deal but make sure you get exactly this model.

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Old 08-31-12, 02:58 AM   #12
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Just a quick update on the temperatures. We're experiencing an unusually cool late summer - it's not even quite September yet, and outside temps are already at 55F around 10:00pm. Interior is 73F at that time, and between then and about 1am go down to roughly 72.5F, with the exterior at about 53.


So, in 3 hours with a dT of -2F exterior, I'm seeing an interior dT -0.5F with a total dT of 20F between exterior and interior.
So, that tells me that, roughly for every degree difference between interior and exterior, I'm seeing 0.015F of decrease of interior temperature per hour.

In the winter, if we're assuming 35F at night, the dT exterior-interior would be about 35F, and assuming a linear relationship (not sure if it is, more data will tell), I should expect a decrease of 0.53F per hour in the house, or the same as raising the temperature by 0.53F per hour to keep it at 70.

Now, that's only if the relationship between the exterior-interior dT and the heat loss is really a linear equation. Anyone know if that's actually the case?

Crawlspace insulation will of course change that equation again, and it'll also be interesting to see how effective it is once installed. I've got a few months of data logging ahead of me (there's something I could put my Raspberry Pi to good use for!), but 0.53F per hour sounds like a piece of cake for a 980sqft radiator
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Old 08-31-12, 11:48 AM   #13
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Quote:
Originally Posted by opiesche View Post
J...So, in 3 hours with a dT of -2F exterior, I'm seeing an interior dT -0.5F with a total dT of 20F between exterior and interior.
So, that tells me that, roughly for every degree difference between interior and exterior, I'm seeing 0.015F of decrease of interior temperature per hour...
opiesche,

I really like your approach here. I have wanted to do something like this in my house.

Keep up the good work and keep us posted.

Best,

-AC
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Old 09-21-12, 08:55 PM   #14
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Another quick update: After building a temperature logger with a Raspberry Pi, a couple of digital temperature sensors, a bit of wire and a few dozen lines of Python code, I now have proper temperature data to share!

This is currently all without heater, and it shows a few interesting things. First of all, down to about 50F exterior temperature at night, we can comfortably do without heating at all. We've got about a 4-5F interior drop over eight to nine hours at night, which I'd consider pretty decent.
It also shows that there's about a two hour lag between exterior temperature drop and the interior following suit. That means, that when I measure an exterior drop, I've got plenty of time to turn on the pump keep interior temperature at target. Of course, that time will shorten as exterior temperatures get lower, but it gives me a good indication that it'll be fairly easy to keep the interior comfortable.
Here's some graphs from my custom built logger:





Exterior in green, interior in red. All temperatures are in degrees Celsius. Please ignore the downward spikes to 0, those are just failed readings from the sensor. In the current iteration, I'm sampling temperature every 5 minutes.

(I'm not sure how this forum handles the image links, so it's entirely possible that these immediately hit the webserver on my little Raspberry Pi. Please be gentle!)

Last edited by opiesche; 09-21-12 at 09:25 PM..
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Old 09-21-12, 09:06 PM   #15
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Forgot to mention, the exterior sensor wire is going out a window, which is then shut as far as possible and the gap is padded with rolled up bubble wrap to create a reasonably airtight seal. The interior sensor is close to an outside wall, so the interior temps are probably half a degree or so lower than in the center of the room, but I'd say the difference is probably negligible.
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Old 09-21-12, 09:48 PM   #16
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Quote:
Originally Posted by Vlad View Post
opiesche,

Be careful when you select your HWT. All HWT have their minimum temperature unless it is a condensing one. If you just get regular DHWT and try to run it @ 80F you will end up with condensation problem.

For this reason if you use regular DHWT (non condensing type) you have to keep it @ minimum allowed temperature an install mixing valve.

I used special mixing valve I050C2R-2 - Taco I050C2R-2 - 1/2", 2 Way Outdoor Reset I-Series Mixing Valve w/ Sensor which works as thermostat as well. It is pricey but worth every penny. Check eBay for better deal but make sure you get exactly this model.
Thanks for the info, Vlad! I've been thinking about a mixing valve, but then I'd have to worry about what to do with the additional water I'd introduce into the system. In the end, I think I might go with a tankless natural gas water heater. They usually have energy factors of around 0.8, so are pretty efficient - and since I'm only going to need <30F rise of water temperature, a small model should do the trick. I'm looking at this one:

Eccotemp-FVI12-NG Indoor Tankless Water Heater

Which allows for individual control of gas and water flow, so I should be able to adjust this very well for my target temperature. This heater turns on when the water flows, so my pump circuit would automatically also turn on the heater if the water temp is below target.

Has anyone here had experience with tankless heaters for hydronic floors? I'd be interested in hearing some experiences!
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Old 09-23-12, 03:42 AM   #17
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Quote:
Originally Posted by opiesche View Post
Thanks for the info, Vlad! I've been thinking about a mixing valve, but then I'd have to worry about what to do with the additional water I'd introduce into the system. In the end, I think I might go with a tankless natural gas water heater. They usually have energy factors of around 0.8, so are pretty efficient - and since I'm only going to need <30F rise of water temperature, a small model should do the trick. I'm looking at this one:

Eccotemp-FVI12-NG Indoor Tankless Water Heater

Which allows for individual control of gas and water flow, so I should be able to adjust this very well for my target temperature. This heater turns on when the water flows, so my pump circuit would automatically also turn on the heater if the water temp is below target.

Has anyone here had experience with tankless heaters for hydronic floors? I'd be interested in hearing some experiences!
Using tankless HWT for low temperature application is going to be a real pain. It will short cycle on and off. With tank you just take as much water as you need and return water goes back to tank. Water mass in the tank will be like a buffer it will prevent short cycling. Short cycling is always bad because usually equipment efficiency is reached after some time not right away after starting.
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Old 09-23-12, 02:43 PM   #18
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Quote:
Originally Posted by Vlad View Post
Using tankless HWT for low temperature application is going to be a real pain. It will short cycle on and off. With tank you just take as much water as you need and return water goes back to tank. Water mass in the tank will be like a buffer it will prevent short cycling. Short cycling is always bad because usually equipment efficiency is reached after some time not right away after starting.
Thanks, Vlad! A tank heater it is. I've found a few that list minimum temps of 80F, it's just a matter of picking the right one.

With the recent temperatue graphs, by the way, I can also calculate my total heat loss:

The temperatue graphs back up my previous estimate of 0.15 degrees of temperatur decrease interior for every degree difference between interior and exterior (in the graphs I'm seeing about 1C per hour for 10 degrees C difference).
So, let's be generous and assuming 0.2F per hour per degree F, or 2 degrees per 10 degree difference ext-int.
The total air in my house is roughly 2200 (sqft) * 9 (average ceiling height) = 19800 cubic feet.

The density of air at sea level is about 1.2 kg per cubic meter = 2.64lb / m^3. 1 m^3 = 35.31 cubic feet, so 0.07lbs per cubic foot, which means the weight of the total air in my house is 0.07*19800 = 1386 lb.

Specific heat of air is 0.24 BTU/lb/degree F. I'm losing 2 degrees F per hour at 10 degrees difference between exterior and interior, so

0.24 * 1386 * 2 = 664 BTUs per hour of heat loss, if the interior is at 68F and the exterior at 58F. Our coldest nights are about 30F and the interior should stay at 68. Assuming a linear relationship between heat loss and temperature difference, that would make the heat loss on our coldest nights about 0.24*1386*(0.2*(68-30)) = 2528 BTU/h.

Someone double check my math, please? I'm new to all this

Last edited by opiesche; 09-23-12 at 02:51 PM.. Reason: Corrected a calculation
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Old 09-23-12, 04:32 PM   #19
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Default Heat loss graphs

So, I couldn't help but tinker a little more. I've added another bit of code to my temperature logger based on the above calculations, so now I'm plotting heat loss against the exterior-interior temperature differential:



The blue curve is the temperature difference between exterior and interior (scaled by a factor of 100). Positive values mean it's warmer inside than outside. The red graph is the calculated heat loss in BTU/h. Negative values mean heat loss, positive are heat gain. I'm taking temperature differences within 5 minutes, calculate the BTU heat loss from them, and multiplying the result by 12 to get BTU/h.

This is today's data, so unless the forum caches the images, this plot should update live as long as my Raspberry Pi is online. The data is a little noisy despite smoothing, because sometimes between two 5 minute samples, the interior temperature doesn't change at all, which makes the heat loss graph spike towards 0 for that interval.

It shows one very interesting thing: The house gains heat much more easily than it loses it. Observe the temperature differential in the left half (about 10F) with a heat loss of about 1800 BTU/h on average. On the right, we're looking at a temperature differential of about -5F, but a also heat gain of roughly 1800BTU/h on average.
That tells me that I should probably invest in some attic insulation and radiant barrier foil to keep summer temperatures more comfortable

Last edited by opiesche; 09-23-12 at 04:58 PM.. Reason: Fixed calculation, updated post to reflect
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Old 09-23-12, 05:34 PM   #20
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Going a little further with this: say we more than double the heat loss for our coldest nights, to 4000BTU/h.
I've got 980 ft^2 of heated floor surface, that means I'll have to put at the most 4 BTU/h/ft^2 into the floor.

Each square foot of flooring contains on average 13 inches or so of tubing, at 1/2". The volume of a cylinder is pi*r^2*h, so 3.14*0.25*13 = 10 cubic inches or 0.163l of water.
At 1kg/l, that's 0.163kg or 0.35lb of water per square foot.

Water has a specific heat of roughly 1 BTU/ lbdeg (1 BTU for each pound of water that is 1 degree warmer than ambient). So, to get 4BTU out of 0.35lb of water, it'll have to be 4/0.35 = 11 degrees warmer than the ambient temperature. If I'm considering 70 degree ambient temp, I'll need the water to be at least 81 degrees to satisfy the highest heating demand - take losses into account, and the 85 degrees I was originally considering don't sound too far off

Again, I'd appreciate if someone could double check my math here.

Also, what does that mean for my water heater? I'm seeing that most of them rated somewhere between 30,000 and 60,000 BTU/h - seeing how my need should be around 4000-5000BTU/h, and taking the energy factor of 0.6 of most gas water heaters into account, I shouldn't need more than 7000BTU/h in gas input during the coldest outside temperatures.
My guess is, to prevent short cycling as Vlad and AC_Hacker mentioned, that I should go with the lowest rated water heater in terms of energy input, that I can find. The 75,000 BTU tankless I mentioned would definitely not be a good choice - a 35k BTU tank water heater would probably do the trick nicely. Any thoughts?

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