Heat Ex-changers - Is there a loss of heat ?

RichinTenn

I am a newbie - I am seeing there is a closed system and an open system that are being used to heat concrete slabs with domestic Water Heaters. Then there is the option of having a heat ex-changer with the source of heat being the water heater for both the house and for the slab heat source. My question is about the heat ex-changer. Is there a LOSS of heat as the hot water passes between the two points? And how big of a Water Heater would be the right size for this type of set-up? I have Natural Gas at the WH. Thanks for the help! (I am sure I will have more questions as I learn about this topic)

EBEBRATT-Ed

There will be some loss of heat but it is negligible.

We would need a lot more information to size the water heater. Most recommend boiler for heating and water heaters for heating water.

It would be better to use a boiler for the heat with an indirect tank connected to the boiler for hot water

Jamie Hall

Water heaters are intended to heat domestic hot water. Boilers are intended to heat water for heating your house or building. Thet are not interchangeable. In general, a water heater -- even one of the really big gas or oil fired ones -- will have a short and miserable life if used for heating the house.

Boilers, on the other hand, are meant to heat the house and will do so for years quite happily. And they can handle the domestic hot water, too, by means of what's called an "indirect", as @EBEBRATT-Ed suggested. Works just fine.

Now... that said, in answer to your questions, there is very little heat loss in a properly designed heat exchanger. and that loss is entirely through the shell of the heat exchanger. Properly insulated, it's minor. You would need one if you were trying to use a domestic hot water heater to heat the slab, as about the very last thing you want to do is to allow the heating water in the slab to mix or come in contact with our domestic water.

You also ask how big a water heater you would need. Aside from the fact that there is no way I can recommend any size water heater for the job, what you need to know is how much heat the building which you are heating with the slab needs to stay comfortable -- that is called doing a heat loss calculation on the house or whatever, and it's not only not difficult, it's essential. Then you size the burner for the hot water heater or boiler based on that number. Otherwise it would just be a guess. For a water heater, though, the gallon capacity of the water heater would be quite irrelevant to the heat required and the size. It's all about the burner.

Zman

The water on the boiler side will be hotter than the system side. This is the nature of heat exchangers and does not represent energy loss.

Rich_49

Some of the most comfortable , efficient projects I have designed , installed used this unit .

https://htproducts.com/literature/mktlit-03.pdf

It is in fact a water heater with programmed controls and a FPHX with a VS circ on the water heater side . It is no longer in production sincE a European manufacturer bought a controlling interest in HTP and determined sales were not large enough to continue production . So , short answer is yes , it can quite easily be done .

There is a small penalty for every heat exchange to answer your original question

EdTheHeaterMan

File this under "Exception to every rule:"

Sometimes a water heater is less expensive than a space heating boiler. I have purchased the lowest cost Gas water heater I could find and connected a basement slab radiant tubing system to it with an expansion tank and circulator pump with acceptable results. Water heater is dedicated to heat only. The DHW was existing and was not part of the equation. Line voltage thermostat to operate the pump and set the water temperature to 110° for the radiant tubing. Works great. I would not recommend this for a whole house system. Would not recommend this on higher temperature radiators, like baseboard or cast iron standing radiators. Water heater is not meant to operate at the higher temperatures needed for conventional heating units.

I know that my heat loss calculation for the basement slab was less than 30,000 BTU and the water heater had a 40,000 BTU burner (probably operating between 68% and 72% efficiency) but water heaters are measured with a different scale than space heating boilers.

RichinTenn

Thank everyone. What I did not make clear is that I do not intend to heat the whole house with a hydronic radiant system. This will be a new house build with a walk-out basement with a concrete floor. The radiant heat system will not be the primary heat for the house NOR for the 28x40 square foot basement, as there will also be a 90+ Forced Air Gas Furnace system for the home a wood stove in the basement and a fireplace in the great room on the first floor. Mainly the purpose is to warm the floor of the basement slab. The use of the under-slab hydronic system would be limited in use, and seasonal at best since the home will be in TN in a moderate temperature environment. The use of a heat exchanger seems to make good sense in this application to separate the domestic water from the hydronic loops, Again any help will be greatly acknowledged and appreciated!

RichinTenn

Rich_49 said:

Some of the most comfortable , efficient projects I have designed , installed used this unit .

https://htproducts.com/literature/mktlit-03.pdf

It is in fact a water heater with programmed controls and a FPHX with a VS circ on the water heater side . It is no longer in production sincE a European manufacturer bought a controlling interest in HTP and determined sales were not large enough to continue production . So , short answer is yes , it can quite easily be done .

There is a small penalty for every heat exchange to answer your original question

The link you provided for HTP does not take me to any page. PS: I have been to your website for your business - very impressive.

Jamie Hall

Ah. Now you're in a very different application! I was misled.

RichinTenn

Jamie Hall said:

Ah. Now you're in a very different application! I was misled.

Jamie I apologize for not sharing the details in the initial post... As I said I am a newbie, and was trying my best to simplify the query. Yes, the scope of the use is much different than trying to heat the whole house with a radiant heat system using a domestic water heater.

I was also sent to the HTP site by one of the commentators here -- While the HTP systems (and dual heaters) look to be designed with the radiant heat and domestic water functions combined into a single instant heat appliance, I have no idea of the cost of such a unit as I could not even find a price advertised anywhere. Perhaps some of you plumbing contractors and shop owners can give me an idea on the cost of those HTP water heater/aka combination systems.

Rich_49

They are discontinued as the bean counters at Ariston did not see a need because of lower sales of these type units . You can however build your own , See page 19 . I know some folks believe this should not be done . But it has been done for quite some time , successfully .

https://htproducts.com/literature/lp-179_0921.pdf

RichinTenn

Rich_49 said:

They are discontinued as the bean counters at Ariston did not see a need because of lower sales of these type units . You can however build your own , See page 19 . I know some folks believe this should not be done . But it has been done for quite some time , successfully .

https://htproducts.com/literature/lp-179_0921.pdf

Rich -- Thank you for this! Of course this opens up an avenue for more questions, but you seem to be very knowledgeable and are willing to tolerate me asking questions that I am sure some already know here.

Looking at the schematic from page 19 of the HTP manual you linked me to, I guess I will have to ask #1) what the advantage will it be to have the Phoenix Series Water Heating Unit over say an A.O. Smith Heater?

#2) Are we talking "Heat on Demand" here with the HTP Phoenix WH?
Question #3) That schematic drawing represents what HTP would consider to be the piping configurations for serving as a radiant heating unit, correct?
#4) Can you describe from the schematic where the cold water line is coming in - is that called "make-up water" on the dwg?
#5) If that is the incoming water source, where on the schematic is the domestic heated water line? (what I see is a closed loop system with the cold water flowing and being circulated through the radiant heat tubes/coils, then traveling back to the WH Tank at the "lower auxiliary port" is this correct?
#6) I see what is called out to be a "brazed plate" is this a "heat exchanger"?
#7) Would I be correct to assume that if on the domestic Hot Water usage, (as you open a hot water faucet) that it would be having cold water come in to refill the WH tank, (thus "makeup" water), and the water would move through the radiant tubing and be circulating in the closed loop that would bring in Hot water for the radiant system to heat the floor?

#8) FINALLY -- Am I missing anything here? I would also assume that this system is what you have been referring to that does the same thing as the system that you say has been discontinued and that you are still doing similar installations of this schematic for floors and other radiant applications?

Appreciate your time in answering my Newbie Questions -- I have been in construction over 55 years (I may be old, but I can still be taught a few things -- If you keep it simple for me! LOL)

jumper

Zman said:

The water on the boiler side will be hotter than the system side. This is the nature of heat exchangers and does not represent energy loss.

Yes but temperature loss (entropy generation). Theoretically a water heater with recirculated water can last longer thanks to limited scale & oxygen. But water heaters are supposed to receive cold water. So run output through heating system and then through exchanger for DHW? Easier to order a water heater with burner smaller than heater is rated for.

Rich_49

Rich -- Thank you for this! Of course this opens up an avenue for more questions, but you seem to be very knowledgeable and are willing to tolerate me asking questions that I am sure some already know here.

Looking at the schematic from page 19 of the HTP manual you linked me to, I guess I will have to ask #1) what the advantage will it be to have the Phoenix Series Water Heating Unit over say an A.O. Smith Heater?

The biggest advantage I could think of is the fact that Phoenix modulates whereas I don't believe AO Smith units do . Unless of course you get a BTH but then you'll realize that AO Smith is bt far overpriced . Any water heater which the manufacturer staes can be used for space heating also would probably do fine .

#2) Are we talking "Heat on Demand" here with the HTP Phoenix WH?

Certainly , would anything else do ?

Question #3) That schematic drawing represents what HTP would consider to be the piping configurations for serving as a radiant heating unit, correct?

HTP as well as anyone commenting here or another manufacturer . Anyone that would oppose this would , in my opinion not be worth listening to . This would be the correct way to do it . I might add an Outdoor reset mixing valve however on the space heating side .

#4) Can you describe from the schematic where the cold water line is coming in - is that called "make-up water" on the dwg?

This depiction shows only a space heating application connected to auxilliart ports on the unit . Cold water for DHW would be on opposite side and shown in the IOM also . Shown here except showing an air handler ,


#5) If that is the incoming water source, where on the schematic is the domestic heated water line? (what I see is a closed loop system with the cold water flowing and being circulated through the radiant heat tubes/coils, then traveling back to the WH Tank at the "lower auxiliary port" is this correct?

Certainly is

#6) I see what is called out to be a "brazed plate" is this a "heat exchanger"?

Correct , flat plate heat exchanger .

#7) Would I be correct to assume that if on the domestic Hot Water usage, (as you open a hot water faucet) that it would be having cold water come in to refill the WH tank, (thus "makeup" water), and the water would move through the radiant tubing and be circulating in the closed loop that would bring in Hot water for the radiant system to heat the floor?

The flat plate makes this 2 distinct systems . When I have done this , I store in the Tank at 160* and use a DHW mixing valve . The radiant circuits and circulators should only come on during a call for heat .

#8) FINALLY -- Am I missing anything here? I would also assume that this system is what you have been referring to that does the same thing as the system that you say has been discontinued and that you are still doing similar installations of this schematic for floors and other radiant applications?

That would be accurate .

Appreciate your time in answering my Newbie Questions -- I have been in construction over 55 years (I may be old, but I can still be taught a few things -- If you keep it simple for me! LOL)

Would you happen to know a Gentleman that contacted me from Tennessee a week or so ago ?

hot_rod

Here is some good reading on heat exchangers, piping info, etc
https://www.caleffi.com/sites/default/files/file/idronics_29_na.pdf