Effects of Pressure on DHW

Todd_27

I know that the flow rate is critical for heat transfer to take place effectively, but I'm sure that I read somewhere that pressure palys a role as well.

Todd_27

high pressure reduces DHW production

Anyone know where I can read about the affect of water pressure on DHW production? I know I saw it somewhere, but I can't seem to find it now.

ALH_4

Pressure

Water pressure shouldn't matter. Though, the pressure drop of the coil is important. Some tanks have terrible pressure drop across the coil.

Mike T., Swampeast MO

Unless you're using pressure to increase the boiling point beyond that in the atmosphere--e.g. keeping the water from turning to steam--pressure itself is irrelevant to heat transfer in any reasonable closed system.

Todd_27

Pressure

The situation I have in mind is for an oil fired boiler with a tankless coil. The guy I'm working with feels that the incoming pressure is too high and this is causing a lack of heat transfer. I am thinking that it is a flow issue that he's describing as pressure, so I want to read up before we looked at this application.
I may be thinking about steam, since water will boil at room temp under a vacuum and at a higher temp under pressure.

jp_2

true but....

water will 'boil' at different pressures. remember, boiling doesn't mean 212F.


pressure has no effect on the temperature.

[Deleted User]

Some tankless coils have a restrictor built it. Some come with an external one. In my area many homes with city water pressure around 100 psig don't have a pressure reducing valve.

Todd_27

True but

I saw water boil with my own eyes in refrigeration class. This was in a clear graduated cylinder with a vacuum pump hooked up. I don't remember if it was "hot" but it did boil. Isn't this the same premise as a vacuum steam system?

jp_2

exactly

you saw water 'boiling' at room temp! it too was at room temp.

Todd_27

Okay

But isn't the idea of a vacuum system to give off sensible heat at a lower temp?

Dick Charland

Pressure does matter

I assume you are asking what effect incoming water pressure would have on the DHW output whether in a tankless coil or a direct/indirect heater? Tankless coil ratings are based on an incoming pressure of 50 psi, increasing that pressure dramatically reduces the output of the tankless coil. Similar reduction in output also occurs in water heaters and indirect heaters. A flow restrictor does not address this problem, you need to use a pressure reducing valve and thermal expansion tank as you have now setup a closed system. This is something that should be checked ahead of time because the usual response after you have installed the equipment is , " It worked fine until you touched it.".

Todd_27

Pressure

Thanks ****, Do you know where I could read about this?

ALH_4

Pressure vs. Flow

I think I misunderstood your question. A higher supply pressure obviously increases the flow rate at the fixtures. With a tankless coil in the boiler, this means the boiler has to be hotter to achieve the same temperature rise at the increased flow rate.

Each gallon per minute requires approximately 30,000 btu for a temperature rise of 60°F. Knowing just how much the flow rate will increase at a given fixture at different pressures is dependent on the flow characteristics of that fixture and the piping it is connected to.

Mike T., Swampeast MO

I wasn't thinking in terms of "tankless" or "instant" DHW production where excessive pressure can result in excessive flow and reduced DHW temperature. While it's possible for the flow rate to be so high that it does affect heat transfer ability, it's fundamentally a flow problem--not a heat transfer problem.

Dick Charland

Todd, off the top of my head I'd recommend George Lanthier's book on hot water, should be available at his site Firedragonent.com I believe.

hr

Seems like flow rate or GPM

would be a more accurate way of determining DHW production? I've never seen DHW output tables listed by pressure.

hot rod

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Todd_27

Pressure

I did read through Firedragons book before I posted here, I didn't see it mentioned. It seems plausible that these two are related. Maybe not enough that it would be measureable on a residential setup. I'm going to address the flow first and take it from there. Thanks for all the help and comments!

Glenn Sossin_2

Boyle

Don't think he would agree with you

Glenn Sossin_2

Pressure on tank

My $.02 No one has mentioned the effects of changing pressure on the wall of the indirect. Alot of these units are made of a thin walled stainless. It is my understanding that the expansion/contraction/pressure change of the heating cycles causes the welds to fail prematurely.

I believe this was an issue years ago for the Phase III tank when used with well systems where there could be a 20 - 30 psi differential between the two tanks because of the pressure switch on the pump.

We've all seen the number of leakers in the beginning versions of these products. What was causing all these weld breaks? Could have been changes in pressure?

Thoughts ??

jp_2

depends

on the content of the conversation.......

in this case boyle would agree.

surely if you increase pressure in a system is will heat up, but thats not really what was asked here.