Time to heat storage tank

[woodmaster]

Im wondering how long it takes folks to heat the water in their storage tanks.
Also do you find that your boiler shuts off before the tank is totally up to temp. My boiler cycles on and off when the tank temp reaches 145 or so. This is a 738 tank with (2) 120' coils .

 

[Nofossil]

If you heat with coils (as I do) it's really hard to get the tank within ten degrees of the boiler. I suspect that pressurized tanks might work better in that respect.

Until it gets near max temp, my tank heats at close to ten degrees per hour, a bit less if there are other loads drawing heat. My boiler puts out 60,000 BTU/hr average over the course of a burn, and my tank takes 73,000 BTU to gain 10 degrees.

 

[jebatty]

If you heat with coils (as I do) it's really hard to get the tank within ten degrees of the boiler. I suspect that pressurized tanks might work better in that respect.

Until it gets near max temp, my tank heats at close to ten degrees per hour, a bit less if there are other loads drawing heat. My boiler puts out 60,000 BTU/hr average over the course of a burn, and my tank takes 73,000 BTU to gain 10 degrees.

nofo - checked your numbers and I'm wondering if you actually can achieve this performance. 73,000 btu's will raise 880 gal of water 10 degrees assuming no losses.

If you heat with coils (as I do) it’s really hard to get the tank within ten degrees of the boiler. I suspect that pressurized tanks might work better in that respect.

I haven't yet pushed my pressurized tank to the limit of boiler capacity. I know that the plate hx will approach 3* differential at the high end, so theoretically I should be able to bring the tank to within 3* of boiler output, but boiler output ranges between about 170 with 190 idle between cycles on the high end. I have brought the tank to top/bottom 175/160, and as these temps are approached boiler idling cycles occur regularly. As a consequence, I don't usually push even to this level just to maintain higher efficiency on my burns.

 

[sparke]

Jebatty, The Delta T nees to be bigger. Can you increase the Boiler temp without hurting it? I run mine at 200*+ when the tank is approaching 180*

 

[Bob Rohr]

Even with a pressurized tank connected to my EKO, I never get the tank to match the boiler temperature.

With all radiant loads on my system I can pull my 500 gallon tank down to about 90F. It will run up to 150- 160F just fine. I've never seen the tank equal the boiler temperature, however.

Heat travels to cold and the rate of the exchange is driven by the delta T. I suspect as the tank temperature gets close to the boiler temperature the rate of exchanges slows and it's hard to shove that last 10 degrees into the tank. I know the density of water decreases at higher temperatures and this has something to do with it's ability to transport heat.

hr

 

[woodmaster]

I suppose I could increase the high limit if I felt like babysitting the operation, it seems from other posts that it is difficult to get the tank to equal boiler temps though. My only concern is overheating the tank as it can only get to 180*

 

[jebatty]

I haven't checked to see if the Tarm hi-limit can be set higher than 190. It is adjustable downward. The Tarm also has a 210 overheat/shutdown limit, and I would be concerned about reaching that if the hi-limit is set higher than 190. At 190 the draft fan shuts down, but boiler temp continues to rise, depending on heat demand, to about 195 before it starts to fall.

My only concern is overheating the tank as it can only get to 180*

Do you mean that the tank material itself can only take maximum temp of 180? If so, based on my experience I doubt that would happen, but stratification is significant, and the top of the tank could come close or possibly exceed the 180, while the bottom of the tank is 30* or so less. My storage tank is an LP tank and temperature is not an issue.

 

[woodmaster]

The tank has a max temp of 180 - rubber lined.

I did change the high limit to 195 and this helps but because of stratification it is difficult to get the tank to max temp and am not sure if its actually worthwhile. I still get 4-5 days of DHW above 120* in the tank so I cant complain.

 

[Nofossil]

nofo - checked your numbers and I'm wondering if you actually can achieve this performance. 73,000 btu's will raise 880 gal of water 10 degrees assuming no losses.

Keep in mind that tank temperature is a crude measurement. It varies from top to bottom, and not necessarily in a linear way. I only have three temperature probes in the tank, so I have to make some assumptions when calculating a value for tank temperature. To make matters worse, the other heat loads tend to cycle on and off from time to time, so I rarely get an extended period where I'm only heating the tank. I certainly can raise the top by more than ten degrees per hour, but when that's happening the middle and bottom may not be heating up as quickly.

So maybe it's 8 degrees per hour. Or maybe it's ten if the boiler is operating at an output a bit above it's long term average.

All my measurements are after losses, by the way - especially my 60,000 BTU per hour figure. Actual output is certainly more because it's losing heat into the boiler room and house from the boiler itself and from the plumbing.

It's nice to have people paying attention to the math, though. Thanks for keeping me honest.

Anyone with a pressurized tank? I'm curious whether that can achieve a temperature closer to the boiler.

 

[jebatty]

Anyone with a pressurized tank? I’m curious whether that can achieve a temperature closer to the boiler.

I've shut down for the season, but fully intend to do this next fall. I am very interested to see what I actually can accomplish. The draft fan shuts down at 190 and comes back on at 180, but if heat draw to tank continues and tank return is less than 180, actual boiler temp will drop below 180 before fire is up to full burn and temp starts rising to 190 again. So, my guess is that actual maximum effective tank temp may be around 175-180, or about within 10 degrees of maximum boiler temp, even if hx output to tank is within 3* of boiler output. Next fall will be the maximum heat trial.