Using an indirect tank as a heat exchanger.

WaltB

I am about to install an indoor wood gasification boiler and I found a good deal locally on an HTP indirect Stainless Steel Heat Exchanger for Pool/Spa Heating.  I will not be using this for a pool but as a heat exchanger right off of the boiler.

 Here is a link to this product. 

<a href="http://www.htproducts.com/superstor-ultra-poolheater.html">http://www.htproducts.com/superstor-ultra-poolheater.html</a>

 

This indirect tank is rated at 200,000 btu output which is more than I need but as I understand it the more output the better.  Just wondering if a product like this can be used in place of a flat plate heat exchanger and if there is any advantage or disadvantage to doing so.

icesailor

Heat Exchanger:

Be sure that you use a properly sized temperature, pressure relief valve, like a Watts 40XL, properly located in the tank. You have no idea what the wood heater can put into the tank. If you ever watch "Mythbusters", they have done a few shows on a demonstration on physics with overheated water heaters.

Wood stoves, coils in contact with the wood fire, and hot water storage tanks can be an interesting physics demonstration.

I would suggest that some codes require the 1" TPR on an indirect like yours that is rated at 200,000 BTU input. Especially when you have no idea of control of how much energy you put through the coil and into the tank.

I personally know someone who went to a wedding and left his house in the morning and forgot to shut off his side arm gas heater. He came home at 2:00 AM and found his house gone. Some of it was in the trees. But they never found the tank.

SWEI

Which gasifier?

Whose gasifier?  Open or closed system?  Most gasifiers require a large buffer thank in order to operate safely at maximum efficiency.

Read this http://www.caleffi.us/en_US/caleffi/Details/Magazines/pdf/idronics_10_us.pdf before you start buying parts.

WaltB

Using an indirect tank as a heat exchanger.

I will be installing an Empyre Elite 100 which produces roughly 125,000 btu's/hr. and is an open boiler.  I am aware of the buffer tank idea and will likely add that in a year or two when I find the cash.

SWEI

why open?

Is there something in particular about this boiler that made you choose it over other designs?  A pressurized boiler will simplify the system and reduce or eliminate chemical requirements.



What kind of radiation do you currently have?  What design temp is/was it sized for?



You really need to work through the entire system design before you purchase anything.  Outdoor reset, zoning, and other factors can change your piping layout; getting it right the first time will save you both time and money.  If you have budget constraints, deciding what to buy now versus later should follow a complete design.

Ironman

Delta T ?

The concept of using the indirect for a heat exchanger is fine; we do it regularly and that's all it really is: a tank with a heat ex.



The point that you're overlooking is the btu rating at the the design Delta T. Let me explain: if the indirect pool heater is rated at 200k btu's, it's rated with about a 90 - 100* temp. difference between the supply water from the boiler (180*) and the pool water (80*). This is also known as the Delta T.



There are two basic rules of thermodynamics in play here:

1. Heat moves toward cold as it gives up its energy to the cooler substance.

2. The greater the temp. difference (Delta T), the faster the movement or heat transfer.



In your application, if you need 170 - 180* water supplied to your emitters, then you're not gonna get any where close to the 200k btu rating of the tank because your Delta T would only be 10* or less.



However, if you have low temp. emitters that only require 90* supply temp, then you'd probably get enough btu's to do the job.



Universal Hydronics Formula: Btu's = (gpm x 500) x Delta T.



The other factor is the surface area of the internal coil. Practically speaking, there's simply not enough surface area to get the rated heat transfer unless the Delta T is near the manufacturer's design.

WaltB

Low temp emitters

Thanks for all the replies and the excellent explanation of the Delta T.  I will be using infloor pex space 6-8" so my emitters should require a relatively low temperature. 

SWEI

low temp emitters

provide the best comfort and more flexibility for solar or other low temp heat sources later.  They really deserve a buffer tank between them and that high temp gasifier.  Have you had a chance to read that Idronics issue yet?