Heat Loss From Storage

[ewdudley]

We all know that storage in the conditioned space is a good thing in winter, and that it's a bad thing in summer. Likewise storage in an outbuilding is a bad thing in winter.

But how good and how bad?

Assuming:

• 1000 gallon LP tank, 41 inches in diameter and 194 inches long, 180 degF
• inside temperature 70 degF
• outside winter temperature 20 degF
• R-50 insulation

(These should be somewhat pessimistic assumptions: outside temperature low, R-value low.)

Surface area of tank:

(2 * (pi 20.5^2) inch^2) + (41 pi 194 inch^2) = 191.87 ft^2​

Heat gain with tank indoors, 110 degf deltaT:

191.87 ft^2 * 110 degF / 50 Rvalue = 123.71 watt = 422.12 btu / hour​

Heat loss with tank outdoors in winter, 160 degF deltaT:

191.87 ft^2 * 160 degF / 50 Rvalue = 179.94 watt = 613.98 btu / hour​

So with tank in outbuilding in winter, total heat loss would be about a 1000 btu per hour, 600 lost to the great outdoors plus 400 not gained indoors. My heat load in winter averages about 25,000 btu per hour, so for me having storage outside the house would be like throwing away 4% of my fuel. Hurts some, but not the end of the world.

In summer the only disadvantage to having storage inside is heat gain, on the order of 200 watts, or about 0.06 ton of refrigeration.

So my conclusion is that these are pretty small numbers. If you need to put storage in an outbuilding, insulate well and don't look back. If you can put storage inside, so much the better.

And if you're using indoor storage for DHW in summer you won't be sweating because of it.

 

[huffdawg]

I have 2- 500 gal tanks in my shop ,they have close to 4" of closed cell foam sprayed on them and then they're inside a framed box with close to a foot of fiberglass insulation wrapped around them. Believe it or not it is still the coolest spot to hang in, in the summer time. The ceiling is close to 11 feet high though.

 

[PassionForFire&Water]

Great topic!

This is the on-going discussion in Europe with modulating pellet boilers.
For instance, the Windhager BioWIN that I represent, does not require thermal storage.
Also the manufacturer discourage storage for residential applications 10kW to 26 kW (34,000 BTU/hr - 89,000 BTU/hr)
Their research showed that the heat loss from a thermal storage tank is to significant.

Windhager believes in "make the heat when you need and bring it where you need it"
Again this is for their residential pellet boilers, not their gasification boilers.

Wondering what these numbers look like for an outdoor wood boiler with 4" fiberglass insulation, with 165F boiler water and -10F or -20F outside

 

[MarkW]

It's good to see the math laid out. I knew the losses would be minimal if properly insulated though I'd not run the numbers.
Some additional losses would be incurred through piping for installs at a distance for out buildings. Still, minimal if insulated properly.
So, the most significant loss would seem to be money in insulating the run and separate building. Primarily why I want mine indoors, when I can.
Plus, radiant losses off the boiler are intended to warm the proposed garage install.

And finally, I don't have to go outside when it's cold.

Wondering what these numbers look like for an outdoor wood boiler with 4" fiberglass insulation, with 165F boiler water and -10F or -20F outside

I don't want to know. Right now, my install is running insulated not much better than that. I'd probably cry if I saw the numbers. I know it's significant just from the wood usage.

 

[PassionForFire&Water]

And if you're using indoor storage for DHW in summer you won't be sweating because of it.

Way to often when I'm on the road to meet with potential customers I see oil boilers with an integrated DHW coil in a small boiler room or in a basement. These oil boilers typically are very poorly insulated.
Almost every time I come across a setup like this say June to Sept it's close to 100F in that boiler room or basement and for sure there will be an air conditioning unit running full blast.
Speaking of heat loss and inefficiency.

 

[MarkW]

Almost every time I come across a setup like this say June to Sept it's close to 100F in that boiler room or basement and for sure there will be an air conditioning unit running full blast.

Exactly why I want to run DHW off solar in the summer so the boiler's not fired.

 

[huffdawg]

I think I might have more heat loss out of my flue than I lose from my storage. Spartan mentioned he had been monitoring my system temps through my public display link and he noticed that when my boiler was at idle my flue temps mirrored my storage top temps. Has anyone else had this problem. I'm guessing its quite a significant amount.

Huff

 

[maple1]

Exactly why I want to run DHW off solar in the summer so the boiler's not fired.

I think I would go HPWH before solar.

 

[peakbagger]

Exactly why I want to run DHW off solar in the summer

That used to be the way to go and the way I get hot water in the non heating season. Unfortunately the front end cost of a SNW system makes it far more attractive to go with heat pump hot water heater.

 

[BoilerMan]

Heat pump water heater is THE WAY TO GO! The best thing I've done since burning wood!

I can now pull heat (and humidity) out of the house and concentrate it in the DHW tank. On those days when I tank two showers a day because it's so hot, I air condition the house that much more!

Sorry EWD for the derailment. As you can see insulation is the best investment on storage, in the house, well pretty much everywhere!

TS

 

[BoilerMan]

Way to often when I'm on the road to meet with potential customers I see oil boilers with an integrated DHW coil in a small boiler room or in a basement. These oil boilers typically are very poorly insulated.
Almost every time I come across a setup like this say June to Sept it's close to 100F in that boiler room or basement and for sure there will be an air conditioning unit running full blast.
Speaking of heat loss and inefficiency.

Marc, don't you just love America and the days of cheap oil! No one ever really thinks beyond installation.......

TS

 

[stee6043]

Heat gain with tank indoors, 110 degf deltaT:

191.87 ft^2 * 110 degF / 50 Rvalue = 123.71 watt = 422.12 btu / hour​

Heat loss with tank outdoors in winter, 160 degF deltaT:

191.87 ft^2 * 160 degF / 50 Rvalue = 179.94 watt = 613.98 btu / hour​

This equation should be described as a ballpark estimation of heatloss. And it's a rather large ballpark.

Wrapped at R60, on days I'm not drawing heat and not running my boiler, I will get roughly 10 degrees of temperature drop per day. 10*1000*8.6/24 = 3,583 btu/hr. This is with tanks in the conditioned space (my basement). I'd describe my insulation as "about as good as you can do" without spray foam. I'm actually closer the R90 above my tanks, R60 everywhere else.

I won't suggest that having your tanks outside your house is bad. In my case I do not use my boiler in the summer so it's a no brainer to have them inside. Either way, that calculation above should be considered "directionally correct" at best.

 

[ewdudley]

Wrapped at R60, on days I'm not drawing heat and not running my boiler, I will get roughly 10 degrees of temperature drop per day. 10*1000*8.6/24 = 3,583 btu/hr.

That your storage system performs poorly doesn't somehow change how heat loss is calculated, nor does it undermine the methodology. There must be some reason that your experience does not jibe with the formulas.

 

[huffdawg]

This equation should be described as a ballpark estimation of heatloss. And it's a rather large ballpark.

Wrapped at R60, on days I'm not drawing heat and not running my boiler, I will get roughly 10 degrees of temperature drop per day. 10*1000*8.6/24 = 3,583 btu/hr. This is with tanks in the conditioned space (my basement). I'd describe my insulation as "about as good as you can do" without spray foam. I'm actually closer the R90 above my tanks, R60 everywhere else.

I won't suggest that having your tanks outside your house is bad. In my case I do not use my boiler in the summer so it's a no brainer to have them inside. Either way, that calculation above should be considered "directionally correct" at best.

That seems like a lot of heat loss ,I think mine is half that maybe more . I will check again in the summer season.. the closed cell foam makes a big difference I think.

 

[heaterman]

Great topic!

This is the on-going discussion in Europe with modulating pellet boilers.
For instance, the Windhager BioWIN that I represent, does not require thermal storage.
Also the manufacturer discourage storage for residential applications 10kW to 26 kW (34,000 BTU/hr - 89,000 BTU/hr)
Their research showed that the heat loss from a thermal storage tank is to significant.

Windhager believes in "make the heat when you need and bring it where you need it"
Again this is for their residential pellet boilers, not their gasification boilers.

Wondering what these numbers look like for an outdoor wood boiler with 4" fiberglass insulation, with 165F boiler water and -10F or -20F outside

Especially after 2-3 years worth of mouse activity chewing through the insulation..

 

[Spartan]

I think I might have more heat loss out of my flue than I lose from my storage. Spartan mentioned he had been monitoring my system temps through my public display link and he noticed that when my boiler was at idle my flue temps mirrored my storage top temps. Has anyone else had this problem. I'm guessing its quite a significant amount.

Huff

Part of my job here at the NSA is to spy on boilers.....*laughs nervously*

 

[Spartan]

Great topic!

This is the on-going discussion in Europe with modulating pellet boilers.
For instance, the Windhager BioWIN that I represent, does not require thermal storage.
Also the manufacturer discourage storage for residential applications 10kW to 26 kW (34,000 BTU/hr - 89,000 BTU/hr)
Their research showed that the heat loss from a thermal storage tank is to significant.

Windhager believes in "make the heat when you need and bring it where you need it"
Again this is for their residential pellet boilers, not their gasification boilers.

Wondering what these numbers look like for an outdoor wood boiler with 4" fiberglass insulation, with 165F boiler water and -10F or -20F outside

A bit redundant to run storage when you already have controlled output. Although one has to get the sizing right and expect a little too much and a little bit less a few days a year.

In my house, I usually need 25,000 btu average but a week ago it was minus 25'C and howling. First time ever my high efficiency 80,000 btu furnace went for hour long runs. So I lowered the thermostat by 3 degrees and we became friends again. If I was running a pellet boiler, I would size it 20 to 60k btu and live with the few extreme days of cold once every ten years.

 

[jebatty]

... when my boiler was at idle my flue temps mirrored my storage top temps.

I see something like this also when my boiler is off between firings. My tank is horizontal right next to the boiler, thermo-siphoning takes place between the tank and the boiler, and there is some heat loss through the boiler to the flue. I haven't minded this too much because this also adds some heat to my shop and my flue runs up about 11' to the ceiling, so much of the heat radiates into the shop anyway. I only use the boiler during the heating season.

My 19' x 3' horizontal tank in the heated shop space is about R-40 on one sidewall and endwall against outside walls, about R-30 on the other two inside walls, the top facing into the shop and the bottom. The bottom is on the radiant slab with a set point normally about 62F. Except for loss through the outside walls, any heat loss is into the heated space and is of no concern.

The heat loss equation is very interesting and informative.

 

[heaterman]

Regarding heat loss and....... btu output vs design conditions..........

First, the heat loss from indoor storage. The elephant in the room there is nothing more or less than how well the storage is insulated. It should be a minimum of R-19 and more importantly, air tight and sealed up. Loosely wrapped tanks of any kind or even a "box" around the storage that's filled with batt insulation is less than desirable. There is not much more to it than that.

We had the opportunity to "play" with a big CB (7260?) and do some rough calculations on heat loss while no load was present. It was just sitting there with no circulation to the buildings while other work was being done inside for a couple days. Long story short, the heat lost from the boiler just sitting there was in the ball park of 5,000/hour. That was at an outdoor temp of between 10-15*. I think that model is second to largest in the CB line so a typical 5036 or 60 series would be a bit less but still just plain awful. This is pretty typical of all outdoor units.

BTU vs actual heat loss.......
Just cleaned the Windhager again (4th time since new) and did some calculations on pellet use and btu per hour of run time.
The clock showed 2739 hours of burn time and pellet use was 6.84 Tons.
So we have a total fuel use of 112,860,000 btu given the 16,500,000/ton figure from the DOE website
112,680,000 divided by 2739 hours = 41,204 btu/hour average
41,204 X 85% efficiency = actual heat delivered of 35,024 per hour.
The design condition heat loss for my house is a little over 90,000btu/hour so you can see the average heat loss is about 1/3 of that.
This matches up with what I'm seeing for firing rate on the Windhager display. It will typically sit around 40-60% "throttle" and I have it dialed in to work at the 70,000 btu maximum rate.

Design loads are much different than what is needed for normal winter operation and that should be kept in mind when selecting what size equipment you are going to install. There are significant $$ to be saved by sizing for 90% conditions rather than installing something that will meet 100% of the load 100% of the time.

 

[jebatty]

Design loads are much different than what is needed for normal winter operation and that should be kept in mind when selecting what size equipment you are going to install. There are significant $$ to be saved by sizing for 90% conditions rather than installing something that will meet 100% of the load 100% of the time.

I would agree, but with some exceptions. The ability to modulate to a considerable extent, like a pellet boiler or a boiler like a Froling, allows a range of flexibility. Also, as was true for me, the choice was to oversize (140,000 btuh rating; 35,000 btuh design heat loss at -30F) because I have 1000 gallons of storage and I wanted to be able to fully meet demand, regardless of conditions, but more importantly charge storage quite quickly and then draw from storage. Normal operation therefore is one firing of about 4-6 hours every other day. That makes life pretty easy with minimal boiler tending for normal operation, and the ability to actually be absent for about 10 days during the winter without a need for backup.

Once a decision is made to have substantial storage and bear that cost, there is more cost to upsize but the bulk of the cost for plumbing and storage does not change. Pricing I have seen for a 170,000 btuh rated boiler was about 20% more than for a 100,000 btuh boiler of the same brand. While that is not small change, it may not be a deal breaker when the cost of the entire system is considered, and when rated capacity increased by 70%.

 

[phantomblack]

I have a CB 5036 with 100 feet of run between my house and the boiler. When the lines enter the house they are not insulated to warm the basement a bit. I have standby loss of about 2300 btu's per hour.

 

[Spartan]

BTU vs actual heat loss.......
Just cleaned the Windhager again (4th time since new) and did some calculations on pellet use and btu per hour of run time.
The clock showed 2739 hours of burn time and pellet use was 6.84 Tons.
So we have a total fuel use of 112,860,000 btu given the 16,500,000/ton figure from the DOE website
112,680,000 divided by 2739 hours = 41,204 btu/hour average
41,204 X 85% efficiency = actual heat delivered of 35,024 per hour.
The design condition heat loss for my house is a little over 90,000btu/hour so you can see the average heat loss is about 1/3 of that.
This matches up with what I'm seeing for firing rate on the Windhager display. It will typically sit around 40-60% "throttle" and I have it dialed in to work at the 70,000 btu maximum rate.

I'm a little confused. Do you have heat storage?

If NOT, then what it means you are running it constantly at various rates. What do you do in fall and spring? If it's off, how long does it take to come up to temperature?

I have too much time on my hands, a complete machine shop and infinite amount of free wood (a bit of the wrong type though) thus debating if I want to do something in a land of cheap natural gas. Then there is the question if it's better to chip and burn or mass fire and storage. The slightly higher time it takes to chip is more then offset by the saving in space and cost of a massive water storage system.

 

[heaterman]

I would agree, but with some exceptions. The ability to modulate to a considerable extent, like a pellet boiler or a boiler like a Froling, allows a range of flexibility. Also, as was true for me, the choice was to oversize (140,000 btuh rating; 35,000 btuh design heat loss at -30F) because I have 1000 gallons of storage and I wanted to be able to fully meet demand, regardless of conditions, but more importantly charge storage quite quickly and then draw from storage. Normal operation therefore is one firing of about 4-6 hours every other day. That makes life pretty easy with minimal boiler tending for normal operation, and the ability to actually be absent for about 10 days during the winter without a need for backup.

Once a decision is made to have substantial storage and bear that cost, there is more cost to upsize but the bulk of the cost for plumbing and storage does not change. Pricing I have seen for a 170,000 btuh rated boiler was about 20% more than for a 100,000 btuh boiler of the same brand. While that is not small change, it may not be a deal breaker when the cost of the entire system is considered, and when rated capacity increased by 70%.

Very true Jim. Storage is the great equalizer and usually vastly under estimated in terms of benefits to the overall system operation and performance. Provided it is done right of course.

 

[heaterman]

I'm a little confused. Do you have heat storage?

If NOT, then what it means you are running it constantly at various rates. What do you do in fall and spring? If it's off, how long does it take to come up to temperature?

I have too much time on my hands, a complete machine shop and infinite amount of free wood (a bit of the wrong type though) thus debating if I want to do something in a land of cheap natural gas. Then there is the question if it's better to chip and burn or mass fire and storage. The slightly higher time it takes to chip is more then offset by the saving in space and cost of a massive water storage system.

No storage other than the 80-100 gallons in the system. You are correct in the assumption that my boiler is constantly "on". It shuts off only when the water temperature reaches setpoint in the boiler itself. This happens very seldom as I think the display showed only 600+ starts in the 2700+ hours. It turns the firing rate up and down to match the load presented by my constant circulation heating system. (Panel radiators with thermostatic radiator valves and a variable water temperature radiant floor) The pumps for both of those circuits run as long as outside temperature is below 62*. So my "system" runs from September through May without shutting off. The pellet boiler supplies heat until it approaches the set point and then throttles back to maintain it regardless of how much the emitters are calling for.

The question you pose can only be answered by analyzing your particular set of circumstances. No standard answer for every situation.

 

[stee6043]

That your storage system performs poorly doesn't somehow change how heat loss is calculated, nor does it undermine the methodology. There must be some reason that your experience does not jibe with the formulas.

I suppose. Google heat transfer and see if you can find this equation anywhere other than "rough guess" home builder threads. This is not a formula engineers would use to estimate or calculate heat loss.

I'd also value some real world feedback from folks that have actually observed their tanks in a static condition. My numbers may seem appalling but I haven't seen any other "real measurements" here so far, spray foam or otherwise.