Hot water low pressure effect on temperature.

[John Brown]

There was a mention in a recent thread of lower boiler pressure causing a drop in HW temperature. Could anyone kindly explain how or why this happens?

 

[Cabinetman]

Take this with a pinch of salt as I don’t know a great deal about central heating boilers, but reading up on an error code this morning it’s possible that to prevent the boiler overheating due to not enough water going through it and being heated that the boiler has cut back on heating the water to prevent itself overheating.

 

[Spectric]

Yes @Cabinetman is on the right tracks, these boilers have many built in safety features, the days when the heat exchanger was a great lump of cast iron and almost bomb proof are long gone so the heat exchangers these days need protection and firing the burners when they have no water is not good for them.

 

[John Brown]

There's loads of water - it's simply at a lower pressure.

 

[Sideways]

No doubt unrelated but water boils at what, 70C (?) at the top of mount Everest...
(Where the air pressure is much lower than it is down here near sea level)

 

[Distinterior]

No doubt unrelated but water boils at what, 70C (?) at the top of mount Everest...

Close!......68 degrees.

 

[John Brown]

No doubt unrelated but water boils at what, 70C (?) at the top of mount Everest...
(Where the air pressure is much lower than it is down here near sea level)

Good point, but I believe the return temp adjustment on my boiler stops well short of 100C, and I'm not that high up.

 

[Distinterior]

........ and I'm not that high up.

 

[Yojevol]

There was a mention in a recent thread of lower boiler pressure causing a drop in HW temperature. Could anyone kindly explain how or why this happens?

Are you referring to boiler circulating pressure or domestic water mains pressure?

There's loads of water - it's simply at a lower pressure.

Sounds like you're referring to the latter.
Loads of water at low pressure ain't going to move very fast.
Low flow rate between boiler and tap means longer transit times.
Longer transit time means more heat loss.
More heat loss means lower temperature at the tap.

Well it's a scenario, perhaps not very convincing but it does beg the question - why low pressure?
Brian

 

[Pedronicus]

TBH we don't know exactly what type of boiler the OP is talking about. Combi? System? Open vented? Are we talking about the pressure and delivery of water from an outlet (tap) or are we talking about the system pressure indicated by the gauge on the boiler?

"There's loads of water - it's simply at a lower pressure." is a bit of a vague comment without further details.

Very few people (including most installers nowadays) know the difference/importance or take notice of the flow rate and delivery pressure or even bother to measure these parameters.

 

[John Brown]

Are you referring to boiler circulating pressure or domestic water mains pressure?

Sounds like you're referring to the latter.
Loads of water at low pressure ain't going to move very fast.
Low flow rate between boiler and tap means longer transit times.
Longer transit time means more heat loss.
More heat loss means lower temperature at the tap.

Well it's a scenario, perhaps not very convincing but it does beg the question - why low pressure?
Brian

I'm talking about the former, the pressure in the sealed system.
Why the low pressure? The expansion vessel has failed, and I'm waiting for a convenient time to drain down the system and replace it. Ideally I'd like to change a few radiator valves at the same time, so a convenient time might be when the grandkids are their mum are on holiday.
But I thought I'd noticed a slower heating rate of DHW, and wondered about how this might happen, owing to lower system pressure.

 

[Spectric]

No doubt unrelated but water boils at what, 70C (?) at the top of mount Everest...
(Where the air pressure is much lower than it is down here near sea level)

Another example is your car, not electric ! The cooling system is run at a higher pressure so the water runs hotter without boiling to increase thermal efficiency. Take the rad cap off and you get a cloud of steam because water cannot exist at the elevated temperature at the lower pressure as water, it becomes steam.

 

[Fergie 307]

Another example is your car, not electric ! The cooling system is run at a higher pressure so the water runs hotter without boiling to increase thermal efficiency. Take the rad cap off and you get a cloud of steam because water cannot exist at the elevated temperature at the lower pressure as water, it becomes steam.

Not a bad analogy as if you have a sealed system as most are now, the pressure is about the same in your car rad and your boiler, ballpark 1 bar.

 

[John Brown]

I thank you all for your interest, but nobody seems to have answered my question. I know it takes longer to boil an egg at the top of Everest, and I know car cooling systems are pressurised, but I still don't know why my system boiler should heat the tankful of water less efficiently if the system pressure is low. The tank is right next to the boiler, and there's a whole first floor with seven radiators above it, so I doubt there's any air in the relevant part of the system.

 

[Spectric]

but I still don't know why my system boiler should heat the tankful of water less efficiently if the system pressure is low.

Maybe it is to do with heat transfer, another benefit of a pressurised cooling system apart from it can run hotter is that it can help reduce aeration which can lead to hot spots within cylinder heads around the valves and combustion chambers.

 

[Alli]

Extract from Warranty People:​

Why is Boiler Pressure so Important? from​

Maintaining a constant boiler pressure in your system is essential for it to work efficiently. Your boiler and heating system require a steady and stable pressure to continue to heat your home and supply hot water. If your boiler pressure drops below the minimum safe pressure, your boiler and central heating will not function efficiently. It will instead spend more energy trying to heat your house with low pressure.

Low boiler pressure will not necessarily damage your boiler or central heating system. However, it will affect their performance. Lower pressure means your system has to work harder to heat your home. The harder it works, the more energy it consumes, and the higher your costs go. Obviously, inefficient heating will also take longer for your rooms to warm up. If efficiency and heat are a concern for you, you may want to pay attention to your boiler pressure. Low boiler pressure can indicate if there is a drop in the amount of water circulating in your system. The lower the pressure, the more energy your boiler spends in heating water and circulating it.

https://www.warrantypeople.co.uk/what-causes-low-boiler-pressure-how-to-fix-that/#:~:text=Low boiler pressure will not,the higher your costs go.

If your rads are heating up okay but not your tank, If I remember correctly, there is more resistance pushing water through your tank than through the rads, So If your system is struggling because of low pressure, your tank will suffer more.

 

[John Brown]

Once again, thanks, but it doesn't explain why. I suppose I'm really looking for the physics of the matter.

 

[Sideways]

I doubt it is anything to do with physics as such.
A sealed system at +1 bar pressure above atmospheric : water has a boiling point of about 120C
At +2 bar above atmospheric it's around 134C
On my own boiler the instructions are to keep a minimum pressure, cold, of +1 bar. The green operating zone is up to +2 and the max is +3.
Flow temperatures measured at the boiler outlet are adjustable - we are trying to keep these as low as possible because low flow temp means lower return temp and the boiler should spend more time condensing properly = greater efficiency. We aim for mid 60's C flow temp because we want the hot tank to get up to 60C.
But in the core of the heat exchanger, temperature is bound to be a fair bit higher than at the outlet pipe and if the system isn't pressurised the system water is far more likely to boil as it goes through. That adds bubbles into the system as well as noise, and I doubt everything recombines perfectly, so the system is going to need venting somewhere.
Boiling water into steam requires energy - latent heat of evaporation - but that should be given back when the bubbles condense, so I don't see that having any meaningful effect.
Your boiler won't have a flow temp set above 100C so apart from the risk of boiling in the heat exchanger the system should work even at atmospheric pressure.

The amount of energy that can be carried by water is related to it's specific heat.
Specific heat (C) is the amount of heat required to change the temperature of a mass unit of a substance by one degree.
Specific heat is different depending on whether you let the water expand or not. In a sealed system at constant pressure it's called Isobaric Specific Heat(Cp)
At 90 degrees Cp of water is 4.21 kJ/(kg*K)
At 120 degrees Cp is higher, so theoretically it can shift more energy for each degree temperature rise, but only by 1% which I doubt anyone would be able to measure.

So apart from a risk of kettling in the boiler at low pressure, I can't see physics explaining a drop in temperature and I'd guess is it's something to do with the boiler safety systems.

HTH

 

[John Brown]

I doubt it is anything to do with physics as such.
A sealed system at +1 bar pressure above atmospheric : water has a boiling point of about 120C
At +2 bar above atmospheric it's around 134C
On my own boiler the instructions are to keep a minimum pressure, cold, of +1 bar. The green operating zone is up to +2 and the max is +3.
Flow temperatures measured at the boiler outlet are adjustable - we are trying to keep these as low as possible because low flow temp means lower return temp and the boiler should spend more time condensing properly = greater efficiency. We aim for mid 60's C flow temp because we want the hot tank to get up to 60C.
But in the core of the heat exchanger, temperature is bound to be a fair bit higher than at the outlet pipe and if the system isn't pressurised the system water is far more likely to boil as it goes through. That adds bubbles into the system as well as noise, and I doubt everything recombines perfectly, so the system is going to need venting somewhere.
Boiling water into steam requires energy - latent heat of evaporation - but that should be given back when the bubbles condense, so I don't see that having any meaningful effect.
Your boiler won't have a flow temp set above 100C so apart from the risk of boiling in the heat exchanger the system should work even at atmospheric pressure.

The amount of energy that can be carried by water is related to it's specific heat.
Specific heat (C) is the amount of heat required to change the temperature of a mass unit of a substance by one degree.
Specific heat is different depending on whether you let the water expand or not. In a sealed system at constant pressure it's called Isobaric Specific Heat(Cp)
At 90 degrees Cp of water is 4.21 kJ/(kg*K)
At 120 degrees Cp is higher, so theoretically it can shift more energy for each degree temperature rise, but only by 1% which I doubt anyone would be able to measure.

So apart from a risk of kettling in the boiler at low pressure, I can't see physics explaining a drop in temperature and I'd guess is it's something to do with the boiler safety systems.

HTH

Thanks. That's pretty much what I thought, if somebody hadn't mentioned it in the "F22" thread, it would never have occurred to me.