Hot water tank sizing, how big is needed?

Conedoctor

So when using a hot water tank for radiant heating is it the BTU I need to look for or does size count as well.

If you have a choice between a 10 gallon and 20 gallon both with the same BTU (or watts) what affect will the tank size have assuming both have 30,000 BTU and I need 10,000BTU.



I am so tight for space if I can use the little one I will be happier.

NRT_Rob

on the heating side

storage won't matter.



if you're doing DHW as well, it might.

Conedoctor

Nope just for heat

Just a little 450 basement, hmm so storage does not matter, there is so much I don't understand.

I need to read more.

So then why do people use such big tanks when they could just use a 20 or 30?

Rob Blair

Because men always think.......

....that bigger is better!!!



Rob

Conedoctor

Cycle time?

So if we have a 20 and 30 gallon HWT both 30000btu can I assume then that the larger one will have to heat for a shorter time due the the extra hot water it has stored?

Gordy

Depends

On what temp the the tank is at, and the supply/return temp of the radiant. It could be equal. Btu used is a Btu needed to be replaced. That said a larger tank will have a little more standby loss.



Gordy

1pipe

water tank size

I missed your question again



You only need what ever BTU's it takes to heat your space.  However I would think a small electric heater would be an expensive way to heat a space.

Below was the answer to the question I though you were asking.



I need to read more carefully.



You can use what ever size tank you think you need for just you hot water demand, you need not count the radiant heat side.  You must use a programmable thermostat so you heater is not heating the floors for and hour before you would need to use your shower on a daily basis. That will give your water heater the time to fully recover.  If you do not there is a very good chance you will not have enough hot water for a satisfactory shower. Most thermostats can be programed to go off and on a coule of times a day.  I have done that a number of times and it works fine.

I wrote whats below before a rereading your question but I though I would just leave it



The rule of thumb I work with is the tank should be as big as you need to fill the largest vessel. IE your largest bath tub. The smaller the boiler the longer it takes to recover.  So the smaller the boiler the bigger the tank.  An average 2300 sq foot home can run nicely with a 80,000 BTU boiler and a 40 gallon indirect water heater with a 5'6"  X 18" bathtub.  A small tank can work only if you are using a 150.000 BTU on demand water heater or bigger, that makes water almost as fast as you are using it.  I only use a small tank when I am installing a hot water return system with a on demand 200,000BTU heater.  I use a 5 gallon electric heater so my on demand does not cycle all day.

Tom

NRT_Rob

people use bigger heaters

because the bigger heaters typically have more storage as well, since they are designed for a bigger domestic load.



Most homes require outputs larger than many regular tank heaters can easily provide.



re: 1pipe,



In most cases, I don't think letting your floors cool for an hour before you hop on them, in the morning, wet and naked is really an ideal solution. but if your water heater is sized for your heat load... and it has to be... then you should be able to turn off the heating system by timer without needing a "recharge" period since the heater was never depleted in the first place. it should only "deplete" if you are over the rating.



and that only works if you want to take a shower at a predictable time, all the time. Most people like to be able to shower when they want, without having to manually shut down their heating system first. In that case you'd need to add DHW load to the heating load.

Conedoctor

I think I have it figured out

Ok so if I understand this now the size of the storage means nothing, the heating power is all that matters.

So here are some made up numbers just to help explain what I think.



I use a 3000watt 5 gallon water heater the 3000watts is enough for the heating job, the 5 gallons will just bring the floor up to temp a bit faster but for every btu I use I will have to replace it stored or not. If I used a 10gallon tank with the same 3000watt heater the floor would come up faster again but I still need to reheat the stored water and again replace the btus I used?

Sort of like my hot tub, it has a 5000watt heater and takes 10 hours to heat up, if I dumped 10 gallons of hot water in the tub it would heat faster but still take the same energy as I did have to heat the hot water I dumped in.



So given the choice would I go for the smaller 5 gallon tank to reduce standby loss or go 10 or 20 gallon for a shorter pump cycle time?

Electric is the way to go on this one as I just can't fit or vent a gas tank right now, I can get a small electric for cheap and fit it in the corner. This is only to heat the basement as well so the forced are will take care of the two upper floors and I still have vents in the basement if need be.

I just need to get it heated before I turn the A/C on and freeze down there LOL



Thanks Matt

1pipe

1 pipe

Matt



I would go with the 10 gallon.  Make sure it can deliver enough  BTU's for the job,  I figure 25 BTU's per sq foot, ie 100 sq feet = 2500 BTU's  1000 sq feet = 25000 BTU's Make sure you insulate between the ground and the slab or you'll be sending a lot of the heat to China.



1 pipe

Conedoctor

Real close on the btu

Well the heater I can get easy and cheap is 3000 watt, this should give me about 10000btu.



If i use the 24 per foot I need about 11250, so I will be short a touch.



So is it unwise to change the 3000watt element to a 3800watt element, I have the power and all that and I can't see this being unsafe. I will call the supplier as well and ask.



Thanks Matt

Nathan_6

Why not do a heatloss???

Why not do a heatloss and know exactly what you need? 

Since it is a basement you might find 17 btu's per square foot is all that is needed.  With a heatloss at your outdoor design temp will tell you exactly how many BTU's are needed and then you won't be guessing and possibly buying more BTU's then you need or possibly installing a unit that is to small.

We always do heatloss calculations when sizing boilers and heat emitters

Conedoctor

Good idea

Im sure I can find an online calculator.



Here is the setup in case anyone has any further ideas.

550 sq/ft basement.

3 forced air vents in the ceiling

Walls are 8" thick with 2x4 framing and R-12 top to bottom, ceiling is 8 feet and floor is about 6 feet below grade.

The floor is 4" thick with 1/2 pex covered by another 1.5" of gypcrete, so no insulation under the slab or in the footing just gravel. Retrofit so we had no choice and from what I have seen here in Calgary not many do the insulation, yes I would have liked to but the floor was going to get too high.

I will also have one wall with radiant heat, I have a 55" Plasma TV to go on the wall

Alan (California Radiant) Forbes

"......and from what I have seen here in Calgary not many do the insulation, "

OK.........that makes it tough.



What is your design temperature way up there in Calgary?  This is the coldest average temperature you see.

Conedoctor

Temps

I would say -15C to -20C, we do see -35+ as well but never for more than a few days.



I need to go back and do some math, I started this so long ago I have forgotten most of the numbers I used.



Thanks again for everyones help so far.

CMadatMe

Tank Sizing

Size of the tank is based on the gpm it needs to deliver based on the heat loss. For instance. Your basement from reading all the post has a radiant heat loss of approx 10,000- 12,000 btus give or take. Let's take the 12,000.

In a radiant slab application we are running a 10 degree delta-t so your system radiant flow rate needed would be 2.4gpm.





8.33x60x10 divided by 12,000 = 2.4gpm





To calculate how many btu's I need for my water heater to make 2.4gpm I use the following formula





2.4 x Temp Rise x 500 = btu's needed.

2.4gpm is the flowrate of the radiant





Temp rise is the difference between your cold incoming water temp to the heater and your desired water temp output of the heater.





500 =  8.33 x 60 (weight of 1 gal of water x minutes).





Here in my area we use 50 degrees incoming water temp. Let's say the radiant needs 100 degree water at design temp giving you a 50 degree rise. Every part of the country is different so you need to calculate this based on your area.

2.4 x 50 x 500 = 60,000 btu's





To meet my heats flow rate needed,  I need a water heater that will produce 60,000 btu's. Could you do it with a smaller heater. Probably, but that meter outside will be spinning like a frisbe.





The math is what it is...So in real terms your heating plant is about 25% efficient!

Fortunat

math is what it is...

Chris,



the math is what it is....which in this case appears to be 'wrong'.



Your analysis seems to assume that we start with cold water, heat it in the tank to the required distribution temp, run it out through the floor and then dump it in the drain. I doubt this is the set up here.



I think if you think about it for a while you'll see that in a closed loop for heat the tank's element needs only to supply exactly as much heat as the room needs, no more, no less. So if the room heat loss is 12000 BTU/h, the tank needs to provide 12,000 BTU/h.



The tank volume has no effect at all, except acting as a small buffer which might temporarily mask that the element is undersized. But the extra 10 G won't hide the fact for long.



To the original poster, I'd say get the smallest water heater that will do the job. Better yet, since you don't actually want or need the storage....get a small on demand electric water heater or boiler like a Seisco or Thermolec and get rid of the standby loss all together (and save some space while you are at it)



~Fortunat

www.revisionenergy.com

CMadatMe

So in October

At the beginning of the heating season I have no substantial rise? That mass will have the meter spinning like it was yesterday with all the cold water that is in the slab. How about Nov, Dec, March April when the system at times will be off all day and needs to be satisfied at night. You still have a rise to take into consideration.  



Maybe the rise is not as substantiall as my previous post but there is rise. That was my point to properly size a heater. I design around worst case and it is possible that there is a substantial temp rise and it needs to be taken into consideration during sizing. I think you are wrong to suggest that he needs to match the btu of the heater to the heat loss. This isn't a boiler we are talking about that has a definitive btu output.



You are saying that a water heater is 100 percent efficiency so if it makes 12,000 btu's you get 12,000 btu's? 

Fortunat

yes. that is what I'm saying...

>You are saying that a water heater is 100 percent efficiency so if it

makes 12,000 >btu's you get 12,000 btu's?



other than the tank heat loss, YES. And if the tank is in the space being heated, then even that small heat loss is not lost. Pretty much every electric heater is 100% efficient; think about it for a second...how could it not be? where would the lost energy be?



I'll grant you that there may be times when it makes sense to size the heat source larger than the design day heat loss (i.e. if quick recovery from setback is a requirement), but most of the time, it would be a waste of capacity. Unlike a combustion boiler there is really no penalty in oversizing an electric heat source, it just costs more and is a pain in terms of wire sizing and load center feeder calcs etc...



~Fortunat

www.revisionenergy.com

NRT_Rob

slab start up

first, the meter spins based on output. if you have a low output unit it will spin longer but not faster.



if you want to design for a specific slab startup time, then decide how fast you want to heat up the slab, how many BTUs that will take, and match your loop field and heat source to that output rather than design day.



otherwise, just match the heat source to the load and call it a day. they probably aren't starting up the slab during design conditions, so you'll have some extra capacity to work with.

BobbyG

what Constantin said

I try to explain it like this: (not sure if I heard it in one of Dan's articles are not....)



Think of the slab like those old fashion dunking booths with a leak (at a specific rate).  Instead of heating it up - we have to fill the booth with water (think heat).  We could fill it up with a fire hose and it probably wouldn't take it long, then it would have to slowly leak out enough to turn on the monstrous hose.

If we used a garden hose that was close to the same rate as the leak, it would always run, but only at the rate at which it was being lost.

The proverbial leak wouldn't be as bad during the fall when you are turning the system on if you sized the "garden hose" for design conditions...

I also try to explain (after a slab has been brought up to temp. before) if you have a 10-gallon tank and the circulator moves the 2.4 gallons, in approx. 4.2 minutes the entire tank would lose 10-degrees, in another 4.2 another 10-degrees until it reaches equilibrium.  Then it all falls back on the "firepower" slowly raising the temp.  So a larger tank will only lengthen the time it takes for the "firepower" to be the main factor.



Hope that helps

Conedoctor

What about this?

http://www.pexsupply.com/Eemax-EX48T-EX48T-Thermostatic-Electric-Tankless-Water-Heater-12252000-p



It is cheap and small, but with no tank the response time of the floor may not be awesome but I don't think the floor will ever no be heated with how cold it gets with the A/C on so I will always have a bit of heat to take the chill off the floor.



I understand what you guys are saying and I guess I can pay now or pay later, I will not use more energy with a small tank as I have to replace what I used no matter what but response suffers.

I think I will end up with a 10 gallon tank with 4500 watt element in it, $250 is not a bad price and I can mount it up on the walll a few feet from the manifold.

zacmobile

eemax

I put one of those in that a customer had bought once, I was skeptical at first but it seems to work well. You would do well to pipe it in primary/secondary though as they have mad pressure drop through the heat exchanger, if you don't have enough flow they won't run @ capacity.

Conedoctor

Install will be next weekend!

Looking forward to getting the system up and running, I will post how it all goes.



I will have a pressure temp gauge on the return and supply manifold so I can see what my pressure and temp drop is.



One last set stupid newbie question, keep in mind this is a point of use heater with only two connections out the side.

I draw water from the hot connection and return to the cold?

Expansion tank tee'd in to the return side?

Air eliminator tee'd into return line and at highest point?



Thanks very much everybody.

1pipe

purging and expansion

30lb relief valve on water heater not your standard 150 lb.  Small expansion tank 2.5 gallon and a shut-off valve and hose bib for purging the system right before you go back into the water heater.



Tom

Conedoctor

Does it matter where parts go in the system?

So I want to come out of the hot on the top side of the tank with the pump then with a cross ( + ) put the air eliminator up and hang the expansion tank on the bottom side then continue on to the inlet manifold, the return would go come right back and in.



I will of course have all sorts of valves for filling and purging.

BobbyG

yes it matters - greatly

if I am following correctly...you want to have the circulator "pumping away" from expansion tank/air separator.   Also, not sure what you mean by "cross" by putting the air eliminator up and tank down.  Usually the air eliminator has inlet and outlet on the horizontal and a smaller tap on bottom for expansion tank and sometimes a tap for a automatic air vent (if it's not built-in).



Make sure you don't have air eliminator confused with automatic air vent. The vent is a way to get rid of air that happens to make it to the vent.  The air separator "pulls" the air out water and sends it to the auto air vent.

Conedoctor

Sort of like this

Ok so I did some reading and this is what I want to do.

I will come off the hot side of the heater with a VJR075 Spirovent and mount the expansion tank on the bottom port of the Spirovent then my pump and out to the manifold, the fill and drain valves will be in there too.

Mitch_4

I agree..somewhat

I agree that the initial operation of the tank to heating temp (50- 100°F), meanining your 60,000 is accurate. But not required. Once the system has heated, you are no longer operating at a 50° delta T, more likely 20. so 20-25000 btu would be sufficient to maintain, and if the tank is set up at a higher temp with a mixing valve to temper it down, likely a smaller capacity tank will work fine.



60000 btu will heat eh slab faster, but a 15000 @ a higher temp and mixing valve should work fine so long as the system isn't started in January. Once heated, its a matter of maintaining the slab temp.



Mitch

BobbyG

good job.

You have done some good research.  To make things easier for filling and purging air out, you may look at installing a tee for a boiler drain between the spirovent and water heater.  I'm sorry it should go like this....water heater, tee, ball valve, spirovent, pump, manifold.

You close the ball valve and open the boiler drain to help make it easier for the water to travel through the supply pipe and the air to "escape" out the boiler drain.

For more information on "power purging air on filling the system", the configuration is a little different but the concept is the same:  http://www.bellgossett.com/literature/files/6646.pdf

Your on the right track.