# Gas fired water heater - sizing

## Main Question or Discussion Point

Hi Guys,

I am aware how to calculate flow rate Q (c x dt), but when sizing a direct gas fired water heater there is no dt but of course need allow for temp rise, but what parts of information can be known and unknown and how would i even start to put a formula together? Ultimately i would like to be able to size the flow rate required?

Any help would be really appreciated!

Regards
Lewis

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Bystander
Homework Helper
Gold Member
Flow rate of water? Of gas? Without a temperature rise? For what purpose? Household hot water? Restaurant dishwashers?

Hi Bystander

Thanks for the reply to my question, i think i have moved on a little since my post, but would appreciate any feedback.

I have mocked up a scenario where a gas fired water heater is being used in a hospital for 75 people.

From cold to heat the hot water the following has been considered.

35 litres per person (based on a hospital) x people 75 = 2625 litre capacity requirement

2625 x spc (specific heat capacity) of 4.2
x 50 (temp rise) = 551250/3600 (seconds to hour) = 153 kw input to heat 2625 litres of water.

Do you feel i could be missing anything from this? Or am i right in thinking that this would be enough info to size accordingly?

Bystander
Homework Helper
Gold Member
If I'm reading this correctly, you're heating 35 liters/person/hour. That much hot water consumption for a hospital? Or is it per day? Which would cut the heating load somewhat.

Without having alot of experience in sizing/thermodynamics i would have thought that my investigation would be based on peak demand? I have also done a calculation on re-heat over an hour and a temp rise of 30 needing an kw input of 35kw?

russ_watters
Mentor
Is this for a real application, homework or just curiosity? For real applications, domestic water flows are determined by codes.

Good afternoon Russ,

I work in sales and would look like to further my understanding as i also find it very interesting!

I have identified average flow rates from a building services book i brought, (i am also waiting to take delivery of a physics thermodynamics book too!), anyways the flw rates ive obtained are as follows:

Basin 0.08 l/s
sink 0.15 l/s
bath 0.15 l/s
shower 0.6 l/s

Would you agree or is this not what you are referring to in the sense of codes??

russ_watters
Mentor
Good afternoon Russ,

I work in sales and would look like to further my understanding as i also find it very interesting!
Ok, so a hypothetical...
I have identified average flow rates from a building services book i brought, (i am also waiting to take delivery of a physics thermodynamics book too!), anyways the flw rates ive obtained are as follows:

Basin 0.08 l/s
sink 0.15 l/s
bath 0.15 l/s
shower 0.6 l/s

Would you agree or is this not what you are referring to in the sense of codes??
Yes, but they also specify diversity, which is how many you can expect/assume to be in use at once, expressed as a percent. Obviously, a person wouldn't tend to use his sink and shower at the same time and not everyone will be using one or the other at the same time either.

In any case, just for the thermodynamics:
35 litres per person (based on a hospital) x people 75 = 2625 litre capacity requirement

2625 x spc (specific heat capacity) of 4.2
x 50 (temp rise) = 551250/3600 (seconds to hour) = 153 kw input to heat 2625 litres of water.
You didn't specity the time -- Bystander asked. Since you divided by 3600 seconds, that implies you meant 35 liters per person per hour, but that doesn't come anywhere close to any of the usage point flow rates, so I have no idea where you got that number. However, yes, 153 kW will heat 2625 liters by 50 C in one hour.

Hi Russ,

Appreciate your time and and feedback!

So does this mean rather then me being so rigid with my calculations i should base them in worst case scenario?

In the eye of design and not hyperthetical, what should it be based on?

My understanding was a gas water heater was sized on heating a volume of water from cold (in an hour) and also a re-heat (in an hour). But yes from what you have stated.... what does happen if that water is being used gradually?? Would the insulation of the tank stop the water from cooling too much, so therefore avoiding heatloss and the energy,time and money to re-heat all the time? Or is a gradual reheat throughout the day better than a full one?

What would you suggest in a way of formula to achieve the correct method?

I honestly would be grateful for your help, hope you dont feel bombarded with all this, i have tried to help myself but just cant get it right!

russ_watters
Mentor
So does this mean rather then me being so rigid with my calculations i should base them in worst case scenario?

In the eye of design and not hyperthetical, what should it be based on?
To be real, it should be based on whatever the code says, but since it is hypothetical, it is probably fine the way it is.
My understanding was a gas water heater was sized on heating a volume of water from cold (in an hour) and also a re-heat (in an hour).
Re-heat? You mean from re-circulation? It may or may not have re-circulation, but if so, sure. But the re-circulation will be a small flow rate and low delta-T.  Or, if you mean keeping a tank warm, that's a pretty small number too. I don't think that is taken into account into the calculations because it is a lot smaller than any safety factor would be.
But yes from what you have stated.... what does happen if that water is being used gradually?? Would the insulation of the tank stop the water from cooling too much, so therefore avoiding heatloss and the energy,time and money to re-heat all the time? Or is a gradual reheat throughout the day better than a full one?
For a hospital, I'm not sure you need a tank at all. Larger buildings tend not to have them, though 75 is a pretty small hospital.
What would you suggest in a way of formula to achieve the correct method?
The formula you used worked fine, it is just the method of selecting the flow rates that is suspect. The solution to that is to find a copy of the international plumbing code.
I honestly would be grateful for your help, hope you dont feel bombarded with all this, i have tried to help myself but just cant get it right!
You're welcome.