heater sizing spreadhseet

[BeanAnimal]

Heater questions come up quite a bit and the answers tend to vary depending on who you ask. I have authored a small spreadsheet to help people size their heaters.

Simply plug your total water volume in (not tank size, but the amount of water running in the system) and the spreadsheet will show you what kind of temperature gain different standard size heaters will produce.

You should note that the temp gains are absolute and do not take into consideration cooling from evaporation, convection, or other heat loses.

The idea is to use at least 3 heaters on your system (temp controller or not).

With a temperature controller, the heaters should be sized so that 2 of the 3 heaters are capable of maintaining the system the desired temperature. This allows 1 heater to fail and the otehr two to keep up.

Wihtout a temperature controller, the heaters should be sized so that no single heater can cuase the system to overheat in 24 hours (or your choice of times). This means if one of the heaters sticks on, the tank will not be driven into immediate crisis.

Maybe one day I will add a calc for heat loss to the room and evaporation.... maybe.

A lot of small heaters is always better than a big heater.

The standard disclaimers apply.... check the math yourself, I have done this just for fun and can not be responsible for errors or your actions.

temp spreadsheet

[goby1]

I couldn't quite follow your math in the spreadsheet. Shouldn't the T rise asymptote to a constant value?

delta T = (power)(thickness)/(surface area)(thermal conductivity)

G1

[Scuba_Dave]

I like the spreadsheet, I'm just not sure most people will be able to figure out what heater to use based on the spreadsheet

I have approx 240g of water - based on tank size - not actual water volume

I have a 350w heater, and (2) 200w heaters
Usually only 2 of the heaters kick on

Let me see if I understand
According to your spreadsheet the 350w could give me a 14 degree Temp change in 24 hours?

The 200w would only give me an 8.6 degree temp change

So with an ambient room temp of 70, if the 350w stuck on the tank would go to 84 degrees. If the 200w stuck on the temp would go to 78 (maybe higher since the other heater is set to 78/79)

If the ambient room temp was 80, & the 350w stuck on - my tank would go to 94. So at present I am at risk --
This excludes the heat of the MH lamps, and the cooling effect of the water in the basement - 115g - usually 70 max in the basement

I'm actually increasing my water volume to 400g - so that would drop the 350w to a max temp change of 8.6

Am I on the right track?

[BeanAnimal]

Goby... the table is a simple illustration of watts shown in the form of BTUs/hour and how that relates to the volume (weight) of the water. Nothing else has been taken into consideration. No surface area, flow (turnover), evaporation, conduction, convection.. NOTHING. The relationship is 100% linear.

In other words, assume an insulated tank with a heater in it (domestic hat water heater). It's close enough (estimating on the high side) to get an idea of what a heater will do if it sticks on. It is also close enough to be used as a judgement for selecting smaller heaters.

Goby, this works for another reason as well. If we assume a heater is running at say 40% duty cycle, then we can assume that the room and tank are pretty much at a steady state (with some help from the heater). If that heater were to stick on, you would get pretty close to a 60% increase in heat capacity.... so that 1000W heater (for example) could be calculated at a 600W heater (for the overheating effect).

If course the higher the differential between the tank and the room, the faster the tank will give up heat to the room... but again for our purposes this is not really that important.


Dave, the 2 heaters kicking on has nothing to do with demand other than showing that 2 of the heaters are sufficient to meet the tanks heating needs (because the 3rd one does not kick on) and that the set points and hysterisis on each unit are close.

Dave your on the right track.

[crypto]

FWIW, I believe the vaporization of water is about 8115 BTU/Gal. For instance in my tank I replace 2 gal per day which is 8155 * 2 Gal / 24 hours = 676 btu/h for vaporization.

Obviously my lights are a big factor but this number gives you an estimate on the heat loss due to evaporation.

[BeanAnimal]

The latnet heat of evaporation of water is about 1020 BTU/s per pound (depending on temp and humidity of course)... I think if you look it up, they list 970 BTU/lb at standard vapor, standard pressure, and constant temperature. In any case your figure is close.

The amount of evaporation is what is the unknown. This is dependent on so many factors. I just did the spreadsheet as a reality check more than anything.