Jump to content

water heater size for hydronic heat exchange

John Dirks Jr

Recommended Posts

The water heater tag says input rating of 60,000 BTU.

The furnace tag rating is 43,000 BTU.

This confirms that water heater is big enough to supply the furnace, correct?

I'm almost sure it does but the "input" method of rating the WH has me confused. Does the input rating of the WH indicate how much energy it can produce? If so, it would be less confusing to use the word "output", don't you think?

Click to Enlarge

45.96 KB

Click to Enlarge

55.96 KB

Link to comment
Share on other sites

By 'input' is meant the rate at which energy enters the appliance, expressed in BTU/hr. 'Input' is used here instead of 'output' because the manufacturer has no control over the conditions under which the appliance is operated. 'Output' is the product of 'input' and efficiency but the advertised efficiency is valid only when certain conditions are present, such as water supply temperature, exiting water temperature, entering combustion air temps, draft amounts, etc. If the operational conditions which results when the WH is installed and married to the furnace is not the same as the efficiency test conditions then you have a less than ideal union of the two, consequences of which include less than rated efficiency, reduced performance as well as differences-of-opinion, conflicts, arguments, fights and, in extreme mismatches, failure of the marriage.

Since the exiting water from the water-to-air heat exchanger appliance is returned to the WH, it's efficiency approaches 100%. The margin by which the WH capacity exceeds the furnace capacity is good insurance of a good, performing marriage.

Just my humble opinion. I've only been married for 11 years.


EDIT: corrections

Link to comment
Share on other sites

...consequences of which include less than rated efficiency, reduced performance as well as differences-of-opinion, conflicts, arguments, fights and, in extreme mismatches, failure of the marriage.

Then the furnace runs home to her mother and blames herself because she's fat. Meanwhile, the water heater goes out and buys a Harley, gets a tattoo, and cavorts with women half his age. It's sad, really.[:-dev3]

Link to comment
Share on other sites

Ideally, that heater can produce 64 gals of hot water/hour.

It has a recovery rating of 64 gals of hot water/hour.

140 degrees is hot!

How many gallons of water does your system hold? If it holds 16 gallons, you can have a hot floor in 15 mins. Or a warm floor in 12 minutes, agreed?

No, it will depend on the type of setup and lots of variables like area of the room, tube spacing, exposed slab perimeter, perimeter insulation thickness, indoor design temp, maximum effective surface temperature, heating intensity, amount of banded area, horizontal insulation area, perimeter insulation length, unheated floor area, heated floor area, air changes per hour, cubic feet per minute, number of stories, types of floor coverings, etc..

I don't actually know all this stuff, I'm reading it off of one of the assumption reports used by Watts Radiant to provide estimates for one of their systems.

As Marc said, it's re-heating already heated water and since the heated floor water remains warm for a long time once it's heated up there can be more efficiency than if it's simply heating cold water that comes into the tank and is drawn off.

If it were being used simply to heat the domestic hot water, it wouldn't be that difficult to determine what's going to work for the given family size; however, I don't think anyone here who's never been a hydronic floor heating system designer/engineer could every determine exactly what's required of such a system.

You could always assume that the water heater that's installed is the one that was designed for the system and that it's been working fine and recommend that they replace it with one of the same size and BTU rating, but I think that could be risky unless you can know for certain that the homeowner has never tampered with the original design.

If a customer were to ask me that question, I'd simply tell him to contact one of the local radiant heating firms and ask for their help.



Link to comment
Share on other sites

Thanks, Mike. No, I wouldn't endorse any hydronic system beyond, "It seems to be warming the place up, what do you think?" [:)]

Speaking of variable efficiencies, I saw this in a house last year, state of the art boiler and multiple valves with digital sensors and zones everywhere. But the pipes were left hanging loose on clamps between the joists, not in contact with the subfloor anywhere. Big 10 ft tall uninsulated basement. [:-dunce]

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


  • Create New...