RUNNING INFLOOR OFF STORAGE TANKS?

[jimdeq]

Hi all, I want to hook up my infloor heat tubes to the boiler/storage. The infloor is in a shop 150 feet from the house. The boiler and 3 vertical storage tanks are at the house. 1-1/4" Thermopex makes the transit to the shop. Heres the questions. I have a stainless shell tube heat exchanger in place where the thermopex comes into the basement. The storage tanks that I have in place have fittings near the bottom of the tank. The temperature at the very bottom of the tanks rarely get above 140. The tanks are plumbed in reverse return and I get almost perfect flow through the three tanks. The supply from the boiler flows into the very top of the tanks and return is about 2 feet of the floor. Does it make sense to pipe the shop loop off the port that is 8 inches lower than I have my return coming out. The reason I didnt use these ports for return on the original setup is because they were in the back of the tanks and it would have been challenging to obtain equal pipe lengths. Anyway, I will have to have mixing valve to decrease temps before sending it to the infloor. Wouldnt it make sense to draw cooler water from the bottom of the tank and return it to the bottom of the tank instead of mixing down 180 degree water from the top of the tank?

 

[Hunderliggur]

You may want to consider sending less 180F water to the shop (lower flow rate) and mixing at the shop with the floor return. I am sure someone can chime in with the relative benfits of sending hotter watter and underground heat loss but with well insulated PEX it should not be a problem.

 

[jimdeq]

OK, but why would I want to use any 180 degreee water that I could use for my house load, when I could use 120 water from the bottom of the tank and mix it down to 1oo degrees for infloor use. It seems to me that the shop would impact my storage duration and BTU capacity alot less if I did it this way compared to using 180degree water all day and then mixing it down? Just my thoughts need the pros to chime in?

 

[jebatty]

Wouldn't it make sense to draw cooler water from the bottom of the tank and return it to the bottom of the tank instead of mixing down 180 degree water from the top of the tank?

If I understand this correctly, I might agree, subject to some additional comments. 180F from the top of tank mixed down (100-120F ?) and returned to 2' above the bottom of tank will come in at a temperature lower than the 180 but higher than the mix temp, and this warm/hot return water will have the effect of possibly raising the temp at the bottom of the tank and mixing down the stratification column to the top of the tank. The impact of this will relate to how many btu's you are pulling out of the 180F supply water and therefore the actual return water temp.

If you supply the infloor from the ports 16" above the bottom of the tank, where will the return be? ... the ports 2' above the bottom? If so, tank mixing most likely will occur only within the bottom 2' column of water.

Another question. Do you have temp sensors in the tank(s) that shows the water temp below the 2' return ports or at bottom of tank? The reason I ask is that my tank is supplied by a port horizontal into the tank and 6" above the top of tank, with return also horizontal and 6" above the bottom of the tank. I have temp sensors top of tank, 1/3 down, 2/3 down and bottom. When charging the tank it is difficult to raise the temp of that bottom 6" of the tank above the return port. Your return is 2' above the bottom, and if you supply infloor from a port 16" above bottom, you may find that supply water is quite a bit cooler than at the 2' level. Check it out. Otherwise, sounds like a plan.

Regardless, if you can supply infloor with cooler water that meets the infloor demand, that seems to be a good idea. Also, if you don't need 180F water to meet your other needs, the tank mix down may not be important. It's not important at all for me, as I heat only infloor, mixed down to 100F. Tank mixing that occurs during drawdown is irrelevant in my application, while stratification during charging is important to maintain the delta-T between boiler supply/return water.

 

[afblue]

I would say your theory completely supports the fact of pulling heat loads off the tank at temps that support what you are trying to heat. I think it will defintely help with stratification. What I also like about your plan is, sending 180F water out the top of the tank and sending it low velocity out to the garage is going to waste alot of heat. Heat loss is a product tempurature differential, so the higher the line temp the more heat loss. I also support that you would want to have a mixing valve in the garage to regulate your supply temp.

 

[woodsmaster]

I think the water on the bottom of the tank will become cold very fast and you wont have warm enough water to heat the floor. If you could draw from the middle of the tank I think It
would work better.

 

[jimdeq]

Ok boys thanks for the imput, I took some measurements this morning. The tanks are just under 8' tall with suppy out the top. The return enters a port 2' off of the floor. I have another port that is plugged at 31" off of the floor. I think I could use this port which is 7" above my exsisting return for the supply for the low temp infloor and then just return back to the exsisting return. I think I could zone it with a circ and push it to the back wall of my basement where the heat exchanger is located. Water would pass through the exchanger and back to the tanks. I think there would be very little mixing of the three tanks as I want to keep my stratification for the high temp house load. The shop area thatwill be heated is 1200 sq/ft and very well insulated. I dont have a heat calc on it,but i wont have the thermostat ever over 55. I dont know how much of a drain this will put on my system,but I think the impact could be less doing it this way. Looking for more opinions?

 

[benjamin]

You mention a heat exchanger so do you have glycol in the shop system? If you're pumping already cool boiler water through a heat exchanger and then through the shop, you're talking about a lot of pumping for a little bit of heat. If the shop load is as small as you say, then it may stay 55 with water that's only 70 or even less, but that load could easily bring the bottom of your storage down to 75 and there will be very little heat transfer and the pumps will just keep running.

What about running the slab heat as a secondary storage? heating up your tanks and then dumping the remaining heat into the slab? or running the slab heat on a one hour timer when you start the fire?

The problem is that your house and DHW both need hot water, and the slab only needs lukewarm water so it will ruin the effectiveness of your house storage if you let it.

 

[jimdeq]

As Jebatty asked ive been checking the very bottom temp gauge which is 6" lower than the return. Jebatty , it is noticeably cooler during the entire burn cycle and while charging the tanks ,but what I also noticed is that overnight the tanks have a tendency to equalize temperature. After about 8hrs it seems that I lose my stratification. I think most of it is due to the alpha pump slowly pumping 2 gpm to the sidearm on the DHW.
Woodmaster , You thought I should draw more from the middle of the tank. I can do this using the port I described above. You were also concerned about the water getting too cold? I dont know if there is a answer without trying it?

Ben, Yes I was going to add glycol to the underground and shop loop. I also like your timer idea. I guess the whole issue is that Im trying to design so that I can use the already cooler or unheated water to my advantage?

 

[Willman]

Should be easy to heat calc the shop. From there you will have a fairly good idea on btu's required for your target temp. Then you can figure the gallons of water to provide that temp. Prolly a formula to accomplish this task. There might not be enough btu's in the center lower portion of the tank at all times of the heating season.Might only be a few really cold days that it won't work.

Will

 

[benjamin]

I guess the whole issue is that Im trying to design so that I can use the already cooler or unheated water to my advantage?

You may be making things more complicated than necesary when the storage just won't heat everything for long enough to be worthwhile. A btu is a btu, whether it is at 180 or 100, if you remove too many btus from your storage it won't make hot water or keep the house warm.

If you had the ubercontroler that could predict when your next fire was going to be and compare the temps at different levels in your tanks, and decide when it was worthwhile to pump heat to the shop and when it wasn't... that would be a different story.

Using the timer, you decide you want to work in the shop the next day so that night you build your fire, set your timer, heat all your storage and the slab and then you go out to the warm shop in the morning.

 

[jebatty]

Lots of pieces of good advice here. Maybe I missed this, but how much storage do you have? I saw 1200 sq ft shop, well insulated. What height for the sidewalls?

Here is my experience which may help you determine what will be best for you. You are in NE WI which may have somewhat similar weather to me in north central MN. Typical mid-winter temps -10F to +5F; really cold spells might be a couple of days at a time, down to -20 to -40F. My shop is 1500 sq fit and I consider it well insulated: R19 walls, R40+ ceiling, 2" foam under entire slab and perimeter, also 2" foam down 30" around the perimeter. I have 1000 gallons of storage, horizontal tank. My Tarm is rated at 140,000 btuh. I use a 5 x 12 x 30 flat plate hx, as the floor is antifreeze, while the boiler and tank are water.

Taco 009 supplying 6 x 300' loops of pex in the slab. Flow is 2.75 gpm to floor. I set the slab mixing valve at 100F. Floor sensor for heat to the slab is set at 61F, with 1F differential. Interior air temp ranges between 50-58F at outside temps of -20 t0 + 20F. Actual heat load ranged between 11,000 - 18,000 btuh. Delta-T on slab supply/return is 30F +/- (100F supply, 70F return), which at 2.75 gpm = 41,250 btuh. Typical 1000 gallon tank draw-down on a slab heating cycle is about 12-15F, which is 100,000 - 125,000 btu's.

Comments:
1. Valid point that your near bottom of tank temp may be too low to provide the heat you need for the slab. You will need to determine how much low temp water you have, therefor how many low temp btu's you have available to heat the floor.
2. I did not find it practical to play around with the slab temp. It takes a long time and lots of btu's to heat the slab up if I let it drop below the 61F set point. In your case it may be practical to have a lower set point, but I would be cautious in letting the slab drop much below the setpoint.
3. Your low-flow circ on the sidearm is resulting in tank mixing, as return water from the sidearm likely is quite a bit higher than near bottom of tank water. When my slab is being heated, my tank mixes almost completely, as again hot supply water mixed down to 100F results in a little less hot return water to the tank. This is not a problem for slab heating, but it would be a problem if the need is for high temp water for space heating.
4. A btu is a btu, regardless of where you draw it from, and it will need to be replaced from your boiler. This is a guess, but I would expect that unless you have a lot more than 1000 galllons of storage, your stored btu's will not get you very long between boiler burn cycles. For me, tank 185 - 100F is 709,000 stored btu's, and at average 15,000 btuh heat load for my shop, that gets me two days between boiler firings. All of this past winter my firing rate was every other day or longer, depending on outside temp. You need to know your total heat load, determine your stored btu's, and then decide whether trying to use low temp water makes any sense. How low can the hot water supply for the house get before you need to re-fire your boiler? Again, unless you have lots more than 1000 gal of storage, if you can't go below 160F, that's only 167,000 btu's for a 1000 gal tank (adjust for your storage gallons), divide that by your space heating load, and you will have your answer. If you have to re-fire your boiler frequently anyway, you might want to re-think your approach.

 

[DaveBP]

Isn't this just the sort of situation that primary/secondary piping is designed to address?

Use hottest water for the house 1st, then use that water returning from the house that has cooled down to heat the shell-and-tube system to the shop.

Trying to find the right temp water in your tank will be finding a moving target. There will be times when any given point in the tank will be hotter and colder than what you want.

Or is it obvious I worked 'til 2 this morning and haven't finished brewing coffed yet?

 

[jimdeq]

Jebatty, we have very similiar buildings. The heated space in my shop is 1200 sq/ft with 11 foot sidewalls. Walls have 2 inches of spray foam and then a 3.5" R13 batt. The concrete floor has 1.5 " high density foam along with perimeter. There is 6 270' pex loops. Blown in attic insulation is yet to come. I have 3 windows that are 3x4 and 2 36" service doors. OH door is 12x10 commercial insulated. As I stated before the shop is 150' from the house and 1-1/4" thermopex makes the transit. My tanks are 370 gallons each for a total of 1140 gallons. Could a heat lose be figured off of that info?

Dave, what if the shop was tied into the sidearm? The sidearm seems to almost always be running and I have it set so the Alpha is pumping about 2 GPM through it. What if I was to bump it up to about 5 GPM and then to the shop heat exchanger before returning to the bottom of the tanks?

 

[jebatty]

We are close. I have 2 - 3x4 windows, 1 36" service door, 1 48" service door, and a 12x12 insulated OH. The heat loss calc that I did was 40,000 BTUH at -30F outdoor, 60F indoor. My actual heat loss based on weighed wood burns this winter now ending was 11,000 - 18,000 btuh. See Performance. This actual might be quite close for you.

 

[heaterman]

I see what you're driving at and it will work unless you reach a point where the required temp at the supply point becomes lower than needed to heat the shop. We do something like that with a lot of multiple temperature systems with a primary secondary loop by taking the high temp loads off first and successively lower temp loads tee'd in after that. It gives me maximum temp drop for condensing boilers which is a good thing. The issue in your case is that you do not have constant input temp to work with.

That being said, you can heat a well insulated building with astoundingly low water temps when using a radiant slab. We did a 2,000 sq ft building with 16' ceilings this winter using a Viessmann gas boiler and tube in the slab. At outdoor temp of 0-10* the building heated nicely to 62* with boiler temp running in the 86-90* range. Floor surface temps were 70-73*.

 

[woodsmaster]

I see what you're driving at and it will work unless you reach a point where the required temp at the supply point becomes lower than needed to heat the shop. We do something like that with a lot of multiple temperature systems with a primary secondary loop by taking the high temp loads off first and successively lower temp loads tee'd in after that. It gives me maximum temp drop for condensing boilers which is a good thing. The issue in your case is that you do not have constant input temp to work with.

That being said, you can heat a well insulated building with astoundingly low water temps when using a radiant slab. We did a 2,000 sq ft building with 16' ceilings this winter using a Viessmann gas boiler and tube in the slab. At outdoor temp of 0-10* the building heated nicely to 62* with boiler temp running in the 86-90* range. Floor surface temps were 70-73*.

How long did the circulator have to run a day to heat the building at that temp water ?

 

[jebatty]

You can get close to that answer by taking the difference (delta-T) between the radiant supply temperature and the radiant return temperature, multiply by the gpm flow rate, and then multiply that by 500. That will be the btuh being delivered to the floor. Theoretically if that equals your heat loss at a specified outside temperature, the circ will have to run continuously. If it is greater than your heat loss, then the pump will run until your floor temperature has risen to the in-floor setpoint, when the circ will turn off, and the heated floor will heat the shop until the in-floor temperature drops and again calls for heat, circ is "on," repeat.

In my case, and all of this is approximate from data I have collected, outside temp 0-10F, delta-T = 30F, flow = 2.75 gpm, in-floor setpoint 61F, typical circ run time = about 3 hours, which is 124,000 btu's, but this also resulted in about a 20F drop in tank temperature, which means about 166,000 btu's drawn from the tank. Those additional btu's were heating the shop and "lost" from plumbing, through tank insulation, etc. All of this in turn supplied heat to the shop during the circ run time + about an additional 6 hours. So 9 hours of heat from 166,000 btu's, which is 18,500 btuh heat loss, approximate. I say approximate in all of this because I'm reading this from a low resolution chart I put together.

But keep in mind, if you are "bottom" feeding the floor, your supply temp may be dropping, and delta-T will be changing. If you can maintain a constant supply temp via a mixing valve, then the calculation becomes easier.

 

[Willman]

The heat loss calc that I did was 40,000 BTUH at -30F outdoor, 60F indoor. My actual heat loss based on weighed wood burns this winter now ending was 11,000 - 18,000 btuh.

Jim

Was wondering why the projected loss was more than your actual? What program did you use for calc?

Will

 

[jebatty]

I used Home Heat Loss Calculator. I calculated actual heat loss based on weighed wood burns from November through March.