sizing a water heater for radiant heat

Mike_B

Is there a rule of thumb for sizing a water heater for a radiant heat system?  OR does anyone know how to size a water heater for a radiant system? For example lets say I need 35,000 btu's for a radiant system and a water temperature of 120 degrees. My delta t is 10 degrees.



Thanks for any help on this



Mike B.

CMadatMe

You anwsered Your Own Question

I size all equipment based on heat loss. If your loss is 35,000 then you need a heating plant that will make 35,000. Personally, I'm not a fan of a water heaters for radiant applications. Heaters are very inefficient, only have a life span of between 6 and 10 years and limit me in controlling a complete heating system. I only need those 35,000 btu's when it is zero outside. I have a question for you.. If you were purchasing a new BMW would you choose the cloth interior? The comfort and efficiency of radiant heat is in my opinion the BMW and the water heater the cloth interior in this case. 

Mike_B

I agree with you

I agree with you.  I have a lot of people calling from the midwest and down south that want to use water heater for heating because that is all they know.  Some of there heating loads are 6,000 btu's and I do not know of any boiler that is that small even a condensing-modulating unit.



Thanks,



Mike B.

NRT_Rob

with water heaters

you need to add DHW demands to the radiant load if the water heater is doing dual use (and note, for some codes it is not legal to do dedicated heating with a water heater that is NOT doing DHW).



So you need 35kBTUs PLUS whatever you need for domestic.



at 35kBTUs/hr you really want something more upscale than a standard water heater.



at 8kBTUs/hr, you don't.

SpeyFitter

Lochinvar Knight WB050?

Well you could use an electric boiler, which have inputs as low as 1000 watts (1 KW). Electric boilers are built more robustly than a water heater and tend to be quite reliable, some with pump relays built right in, and some even have outdoor resets for further savings over a conventional electric hot water tank. Only issue is an electric boiler uses electricity which is often significantly more to heat with. But they are much cheaper to buy and install and would probably only be a blip on the electrical bill if the load is quite small. But for larger loads they are often cost prohibitive compared to a gas boiler (at least right now).

Smaller condensing modulating gas boilers are something that we need more of in North America in my opinion. The smallest unit I know of is the Lochinvar Knight wall mount WB050 which modulates from 10,000 to 50,000 input. (so about 9,000 to 45,000 output roughly).  The next 2 smallest units would be the IBC VFC 15-150 (wall mount) which modulates from 15,000 to 150,000 BTUH, the Triange Tube which modulates from (wall mount) 16,000 to 60,000, and the next largest Lochinvar Knight (wall mount or floor mount) which modulates from 16,000 to 80,000 BTUH (inputs). If someone could come out with a 5,000 or 6,000 that modulates up to say 30,000 it would really tackle a lot of applications from boiler back up to small loads to someone just wanting the redundancy of 2 boilers instead of one larger one with the larger turn down ratio. Even on small loads, you can realize a reasonable payback compared to heating with an electric boiler with the price difference in the utilities.

Mike_B

Thanks

Thank you very much guys this really helped me out.



Thanks again



Mike B.

CMadatMe

Sounds Like You Have

A great opportunity to sell some solar. Find the itch because you have a nich opportunity. With the ability to offer a 30% tax credit and you can do both domestic and the radiant.

jrc

efficiency

Water heater efficiency is calculated weighted by its purpose--dhw; and  considers standby losses, cycling time and stack losses, none of which properly apply to a heater used as a boiler especially when installed in and with its chimney in, heated space, and running constantly.



jc

Mark Eatherton

And YOUR point is..

Just curious :-)



ME

Alan (California Radiant) Forbes

Have not we all

used a water heater for a small radiant job?  Personally, I love the idea of one heat source for both DHW and heating.  And there's no reason to add the two loads together when sizing the water heater.  Just set back the thermostat during peak DHW usage.



And use some common sense.

jrc

Hi Mark

I was responding to Chris who doesn't like water heaters.  One reason is because of their inefficiency as opposed to boilers.  My comment was intended to question his conclusions since there are no numbers out there that define the efficiency of a water heater in a heating application, nor a boiler in a dhw application.  The usual 66% vs. 90% efficiency numbers are just not apples to apples and probably shouldn't be used as evidence in making a decision.



jc

Mark Eatherton

I see...

And feel free to correct me if I am wrong, but ANY products theoretical AFUE numbers can never exceed its instantaneous thermal efficiency numbers, no?



So if a water heaters number is 80% instantaneously efficiency, it can't have an AFUE greater than 80%, right?



BTW, I 'm not a big fan either, and I use to own one.



Thanks for responding.



ME

jrc

I have no idea

what the real numbers are nor do I know what instantaneous thermal efficiency is.  I am not a professional.  I only know that the the numbers usually quoted result from different testing procedures and are not good evidence in determining which heating unit would be more economical. 

CMadatMe

Hi Alan

Yes I have used a water heater for a very small radiant load once. Was an basement turned into an apartment. The load was like 10,000 btu's. Used a Nat Gas Power Vent. Used a flat plate for the radiant and the heater for the domestic. It actually worked out great for the application

In this gentlemans post he used 35,000 btu's. To me this is not a small load and this application should not use a water heater. The heat loss of my 2,000 sqft house is just about 35,000 btus. I for one can understand budget constraints but if you can't affored today atleast design and set it up for tomorrow. Water heaters are cheap and I don't see any reason why he couldn't temporarily use it until he can afford a mod con. If his budget is not the issue then he should atleast make a decision based on all the information he can get.



Happy New Year

CMadatMe

JRC

Actually I believe system efficiency is in many ways much more important than the heating plants efficiency. It's great that a piece of equipment makes energy at a given AFUE but what you do with that energy you just paid to make has much more impact on your fuel bill. I don't believe I ever used any ratings in my previous posts. So without using them, here are a few of my reasons.



A water heater is going to make the same amount of btu's every day regardless of the outdoor temperature which changes a structures heat loss on a daily basis. A water heater is always going to maintain a tank temperature regardless if there is a  call for heat or domestic hot water. With a water heater I am stuck with trying to use btu's that I really didn't need to be made in the first place. So, if I can't use the btu's I just paid for then what good was it to make them. I know, to burn some of the money I was trying to save.

jrc

I agree with your points Chris,

 but the first, system design-- puts a huge dent in the last-- standby loss, since the heater loses very little during the cold season if it and the vent are kept in heated space.  Outdoor reset has been a huge pain to me for a long time and my only solution has been manual reset based on weather reports--I have a VW not a BM.



jc

CMadatMe

Reset a Pain?

Reset can only be a pain for 2 reasons. The first being you have the wrong curve set. The second, there is not enough emmitters to get the btu's you need out. I can fathom that your only choice is to manually reset the system yourself. If so, then you must not have that much faith in the contractors in your area or have had some bad experiences and are fed up.

Alan (California Radiant) Forbes

OK

I think we understand each other. 



If a customer comes to me and wants to heat a space, I'm tempted to use a combined water heater with almost anything under 1,000 square feet, especially if there's more than 1 zone.  If it's over that or if the heat loss is high, I switch to a boiler.

digger_2

Wizard is correct

Due to budget I used a 50 gal water heater to heat my 1400 sq ft home for ten years. 35K btu. Now, due to 30% tax credit and a $500 utility rebate, I purchased a Lochinvar WB-050. BTW the boiler, ordered and paid for on 12/17 will not be shipped till 1/15. The ran 3 weeks behind last year too.

The water heater did tend to overheat in mild weather. Especially if there was a lot of solar heat gain. I got pretty good at turning the thermostat down on sunny mornings. I was, however, never cold in those ten years.

SpeyFitter

Let me know about that Lochinvar Knight WB050

Let me know about that small Knight wall mount as to how it works out for you as I want to put the WB050 in my house as it's the smallest Mod-Con out there and the heat loss of my house is about 17,700 BTUH @ 18 Fahrenheit (1000 sq. foot rancher in a suburb of Vancouver, BC). I plan to use it with the Squire 30 Gallon Indirect.

jrc

The only controls

 I have other than balancing valves Chris are the manual temperature adjustment on the heater and a line voltage thermostat on the pump.  If you don't mind, I'll pass on the contractor's question. 

CMadatMe

You Can Do

Whatever you want. It's your customer. I would think that someone of your caliber would offer them choices and your best professional opinion.

CMadatMe

One of My Points

You have very limited control over the system.

jrc

I have complete control Chris,

 it's just not automatic.