Hot water loop

[Jon Maine]

I have fitted a hot water loop to each of the Ground Source heat pumps( integral hw cylinders). Simple idea comes on with the lights or a timer 22mm all the way out reducing to 15mm on the return to the tank with the bronze pump and a pressure valve fitted on the return. Non return valve fitted to stop the return pumping into the cold system (taking care not to isolate the expansion vessel of the pressurised cylinder). The expansion vessel is sized enough to take into account the pipework too. Oh, not to forget to put the pump on low speed so it does not return cold water in such a plume to the base of the tank that you prematurely get cold out of the top of the tank.

I now have a question, one of my loops is running at 3.5 bar as it should, the other 4 to 4.5 bar.

Is it probable that the expansion vessel needs re-pressurising do we think?

That is my obvious reaction.

All ideas gratefully received.

 

[MdB]

Have another go at explaining that and possibly include a diagram. I have just installed a heat pump here so may be able to help.

 

[Jon Maine]

Sorry to confuse.

Domestic hot water loop. Can be used with any pressurised water cylinder irrelevant of heating source.

The idea is that you have a circuit of 22mm pipe running from the top of the cylinder as usual to all of the taps or appliances on the system and then return a 15mm pipe to the base/inlet of the cylinder which has a bronze pump fitted to circulate hotwater so you do not have to draw down a long leg of cold water and waste it when you turn the tap on.

One circuit is running over pressure, not sure why.

 

[MdB]

You will have a pressure reducer feeding the cold water into the tank which will be at 3 bar or there abouts. The pressure should therefore be at this 3 bar when you turn a hot tap off. If the pressure rises slowly to 4.5 bar over a long period of time then this pressure rise is likely to be due to the cold water expanding as it heats up. I'm not sure what increase you would expect if you have a working expansion tank but I would have thought it would be a lot less than a 1.5 bar increase. You can check the pressure of an expansion tank with a normal car tire gauge I believe once you have part drained the tank so that there is no pressure in the tank. If the pressure rises straight away to 4.5 bar then it would look to me as if the circulating pump was causing this issue. One cause could be it pumping in the wrong direction against a NRV.

But then again, I'm not a plumber so could be completely wrong...

DIYnot might be a better place to post this sort of question.

 

[Lime]

Using hot water loops is very uneconomic as the system looses/wastes much more heat than a single pipe.
They went out of fashion as a "bright" idea in the 70s.

 

[MdB]

All the hot water tanks I've been looking at have a return feed so someone must be using them. I thought about it but decided my runs where too short to worry about. In Jon's case, it would only waste heat in the summer when the heating isn't on and if all the pipes are well insulated it might not be too bad.

 

[Jon Maine]

Using hot water loops is very uneconomic as the system looses/wastes much more heat than a single pipe.
They went out of fashion as a "bright" idea in the 70s.

So what if you turn them on with the lights when you enter the room and off when you exit and also run your towel rail on it so you don't have your whole heating system running for that job?

They have a place if managed properly and are well insulated.

They conserve water too.

I would read up about advances before making rash statements.

 

[Penners]

If the pressure rises slowly to 4.5 bar over a long period of time then this pressure rise is likely to be due to the cold water expanding as it heats up. I'm not sure what increase you would expect if you have a working expansion tank but I would have thought it would be a lot less than a 1.5 bar increase.

I'm asking this question out of total ignorance, but wouldn't the pressure rise depend on the volume of water in the system? If you heat up - say - 50 litres of water within a sealed system, won't the pressure increase more than it would if you heated 25 litres by the same amount? There's more water to expand and compress the air in the expansion tank.

Or have I missed a vital point?

 

[MdB]

You are correct Penners but the point you are missing is that expansion tanks come in lots of different sizes and should be sized to match the size of the system they are connecting to. A 50 litre system should have an expansion tank twice the size of a 25 litre system and hence the pressure rise in both should be the same.

With very rough figures, drawing 100 litres of hot water from a tank for a bath will put 100 litres of cold water into the tank. If this is heated by 40 degs then this cold water will expand by 2% as it heats up or an additional 2 litres. With a 20 litre expansion tank, going from empty (2 bar) to half full (4 bar) takes 10 litres so the pressure rise for 2 litres will be something less than 0.5 bar.

 

[Penners]

the point you are missing is that expansion tanks come in lots of different sizes and should be sized to match the size of the system

You're quite right, MdB - I didn't realise that. I'm afraid my limited knowledge stops with old-fashioned indirect, vented systems.

Thanks for the information.

 

[Jon Maine]

My problem is that I have two systems almost exactly the same behaving differently.

I will try to re-pressurise the expansion vessel when I find the bike pump as I can't fathom what else it could be.

 

[Lime]

Using hot water loops is very uneconomic as the system looses/wastes much more heat than a single pipe.
They went out of fashion as a "bright" idea in the 70s.

So what if you turn them on with the lights when you enter the room and off when you exit and also run your towel rail on it so you don't have your whole heating system running for that job?
They have a place if managed properly and are well insulated.
They conserve water too.
I would read up about advances before making rash statements.

Nothing rash about it.
They waste energy compared to a single pipe.
Filling up two cold pipes full of hot water for no matter how long uses more heat that just filling up one.
What happens when the light is turned on but no hot water consumed?
A total waste of energy.
You confuse the issue by talking about reducing water consumption.
A single pipe wastes only the amount of heat required to heat the length of pipe.
The loop method heats up two pipes for however long the loop flow continues.
The argument about well insulated is not valid either as a single pipe would benefit from good insulation too.
Then you confound your argument when you refer to filling up a towel rail with the return hot water.
What even in summer?
so much for good insulation.
Relying on the light being on in broad daylight is not economical either.
Loops are very convenient maybe but don't kid yourself they are as economic as a single pipe.
They are not.

 

[Andrew Bodenham]

All down to personal preference at the end of the day, but it seems like a hugely complex and costly installation to cut down on a delay in getting hot water at the taps. Whilst I can see the convenience argument I can't believe that overall energy use is reduced, and then there's all the embedded energy in the additional pipework, pumps, control systems etc.
(He said, rashly, and without any numerical data to backup his point)

Another way to reduce long 'dead leg' effects is to run the supply in 8 or 10mm microbore, which hugely reduces the volume of water that has to be run off before the hot water arrives at the tap. If plastic pipe was used then there would be an insulation gain too.

For truly remote parts of the empire it may be more fuel efficient to run in a cold supply and use an instantaneous 'point of use' water heater. I'd be interested to hear if anyone has gone down this route as I'm condsidering it for a remote bedroom.

 

[Jon Maine]

I have asked myself all of these types of questions and consider this solution to be an economic use of resources. I have probably spent a week planning the system and weighing up its economics. The cost of installation was reduced thanks to Ebarf. The return plastic pipe was 15mm left over from the underfloor heating, 2 x bronze pump @ £50 and the relay unit to control both £50. The whole point of the system is twofold, 1 to supply instant hot water in the bathroom and 2 to run heated towel rails yes both summer and winter as we wash all year round and need to dry towels each day and actively loose heat through them deliberately to do so. We will loose some heat in the pipes, but that will be offset by the production of hot water economically in terms of energy usage.

I have considered this long and hard as we have 30 metres from the tanks to the furthest point.

Microbore pipe would not enable enough flow at standard pressure to have a shower sadly especially as we have more than one bathroom.

My ground source heat pumps will convert 1kw of electricity to 3 to 4 kw of heat. It is cheaper to use this to heat the towel rails using the hot water loop than putting on an electric rail far outweighing the pumping costs and the heat loss from the return pipe.

Also pouring a gallon or two of cold water down the drain before getting the hot through is an environmental issue not to be dismissed as you have. Infact the water boards will actively encourage you not to do that and in some instances is to be considered illegal to pour unused water down the drain.

Yes you are right I will be able to control the loop independently of the lighting to so we don't have lights on un-necessarily.

 

[Flyfisher]

Also pouring a gallon or two of cold water down the drain . . . . . Infact the water boards will actively encourage you not to do that and in some instances is to be considered illegal to pour unused water down the drain.

Illegal? Undesirable, certainly, but how on earth could its legality be policed?

 

[MdB]

I will try to re-pressurise the expansion vessel when I find the bike pump as I can't fathom what else it could be.

1) Incoming pressure reducing valve set wrong. 2) Circulating pump + NRVs set up wrong. 3)Expansion vessel pressure wrong.

Check 1 by monitoring pressure before and after opening and closing a hot tap. If the same high pressure then the pressure reducing valve is set wrong. Check 2 by switching pump off to see if it makes a difference. Check 3 by running a significant amount of hot water (run a bath) and then monitor the pressure rise as the new cold water in the tank heats up. If it rises by a large amount then the problem is in the expansion vessel. If it does not then the problem isn't in the expansion vessel.

 

[Jon Maine]

Agreed, but knowingly dumping water is a big no no, straight from the Regulations Manager from Bristol Water. In my instance I proposed dumping the contents of a few metres of water pipe on a standby supply every few weeks to stop it going stagnant. Not an option, I would be allowed to use it but not throw it away but as you say how could it be policed?

 

[skier-hughes]

Strange, my water board man told me to ensure I ran water off from each stand pipe on a very regular basis, but made no mention of ensuring we use it. Of course I do, in some way or another, but trees don't need much watering in a cold wet winter!!

 

[Jon Maine]

Strange, my water board man told me to ensure I ran water off from each stand pipe on a very regular basis, but made no mention of ensuring we use it. Of course I do, in some way or another, but trees don't need much watering in a cold wet winter!!

I dont think they like me as I have my own private water supply so they have pulled every rulle thats covered in cobwebs out of the closet to hit me over the head with.

 

[Jon Maine]

I will try to re-pressurise the expansion vessel when I find the bike pump as I can't fathom what else it could be.

1) Incoming pressure reducing valve set wrong. 2) Circulating pump + NRVs set up wrong. 3)Expansion vessel pressure wrong.

Check 1 by monitoring pressure before and after opening and closing a hot tap. If the same high pressure then the pressure reducing valve is set wrong. Check 2 by switching pump off to see if it makes a difference. Check 3 by running a significant amount of hot water (run a bath) and then monitor the pressure rise as the new cold water in the tank heats up. If it rises by a large amount then the problem is in the expansion vessel. If it does not then the problem isn't in the expansion vessel.

Thanks, Someone giving my problem some thought.

1/ Incoming pressure fine.
2/ Pump and nrvs exactly the same on both circuits so no problem there
3/ Probably the problem