Should I be pressurising my central heating system?

ben5053

Hi All, getting confused by plumbers so I'm hoping for some advice from some of you with no skin in the game. 

I currently have a system boiler that does the heating and heats an old vented copper tank in the attic fed by a cold water tank (converted attic so it doesn't get cold). The cold tank has 2 shower pumps (both now broken) off it. My mains pressure is around 18-20 litres per minute. 

1. Currently getting prices to move to an unvented tank so I can get rid of the shower pumps = less noise and future breaks....? is this the right thing to do? we moved into the house 6 months ago and plan to be here long term. 

2. 2/3 plumbers have said they will pressurise our central heating at the same time. stating benefits of less air in the system. The 3rd plumber has warned us against it as potentially causing issues with joint leaks - he doesn't think the benefits are worth it. Thoughts? 

P.s we currently have very noisy pipes when the central heating is warming up and when hot water goes to the main bathroom. Could I assume a pressurised system might help reduce this? 

p.s.s what did you pay for changing from an vented to unvented tank? 

Thanks for the help!

Albermarle

Firstly a modern unvented hot water tank, will be much better insulated, so you will save money that way.
I understood that normally when you move to an unvented system that includes the heating, which is then under mains pressure. However according to one of your plumbers it is possible to have separate systems, which I was not aware of.
When the CH is under mains pressure it might find a weak spot and leak, but normally it is OK ( ours did not leak when we changed, but our mains pressure is not that high)
Depending on the mains pressure you may well find that your showers are weaker than when your shower pumps were working, but stronger than now as the pumps are not working. Assuming the cold taps will also be fed from the mains the flow will be stronger from them as well. 
Fitting an unvented cylinder needs a heating engineer with the correct qualifications. If done wrongly it can be dangerous in certain circumstances.
Regarding the cost, is the hot water tank being relocated and is the cold water tank being removed ?( one advantage of having an unvented/direct mains fed system. 

ben5053

Albermarle said:

Firstly a modern unvented hot water tank, will be much better insulated, so you will save money that way.
I understood that normally when you move to an unvented system that includes the heating, which is then under mains pressure. However according to one of your plumbers it is possible to have separate systems, which I was not aware of.
When the CH is under mains pressure it might find a weak spot and leak, but normally it is OK ( ours did not leak when we changed, but our mains pressure is not that high)
Depending on the mains pressure you may well find that your showers are weaker than when your shower pumps were working, but stronger than now as the pumps are not working. Assuming the cold taps will also be fed from the mains the flow will be stronger from them as well. 
Fitting an unvented cylinder needs a heating engineer with the correct qualifications. If done wrongly it can be dangerous in certain circumstances.
Regarding the cost, is the hot water tank being relocated and is the cold water tank being removed ?( one advantage of having an unvented/direct mains fed system. 

Thanks for the reply. Yes they are all gas safe/HETAS, its just replacing the tank where it is. But it sounds like you don't think we should bother? just replace the pumps? On the separate system - there appears to be a separate header tank feeding the radiators and thats where the change would be - think they mentioned an expansion vessel instead 

FreeBear

ben5053 said: 2. 2/3 plumbers have said they will pressurise our central heating at the same time. stating benefits of less air in the system. The 3rd plumber has warned us against it as potentially causing issues with joint leaks - he doesn't think the benefits are worth it. Thoughts? 

P.s we currently have very noisy pipes when the central heating is warming up and when hot water goes to the main bathroom. Could I assume a pressurised system might help reduce this?

Pressurising the the central heating will not reduce the amount of air in the system. With a vented system, the air has somewhere to escape, whereas a seal system will trap that air. However, over time, the amount of trapped air is reduced as you bleed it - It typically takes me a couple of weeks to get rid of all the air in my sealed system, and only need to bleed a single radiator once every 3 months or so. And even then, it is just a tiny amount of air trapped..

Converting to a sealed system will do nothing to reduce noise from the pipes. Depending on the type of noise, it could be pump speed set too high, poorly routed/secured pipes, or a pipe rubbing on a joist. Used to get quite a bit of noise from my old system due to pipe clips gripping copper tubing too firmly. After replumbing and using different clips, all I get is a quiet ticking from one pipe that runs through a notch in a joist. A thin bit of foam might eliminate that ticking, but CBA to fix it.

A vented heating system is much easier to dose with corrosion inhibitor - Just dump the contents of a bottle in to the header tank, job done. OK, it will take time for the chemicals to mix with water flowing in the system, so draining a bit of water out after dosing will help.

Personally, I'd wait for the day comes when you have to replace the boiler. At that point, give serious consideration to a heat pump along with bigger radiators all round. With that in mind, if you are changing the DHW tank for a sealed one, it would be prudent to fit a heat pump ready one - Tanks for heat pumps have bigger heat exchangers inside, so with your existing boiler, you'll get a faster reheat time. And if/when you do fit a heat pump, you'll save a good chunk of money on installation costs.

sheenas

I have had both system and would definitely get rid of the pumps in favour of the pressurised system. The pumps use quite a bit electricity. The benefits would be equal pressure on the hot and cold water systems. The point around getting air in the heating system, I would agree that this would not be solved by have a closed system, but you can have a valve installed that automatically removes air, so maybe thats what they meant.

ComicGeek

sheenas said:

I have had both system and would definitely get rid of the pumps in favour of the pressurised system. The pumps use quite a bit electricity. The benefits would be equal pressure on the hot and cold water systems. The point around getting air in the heating system, I would agree that this would not be solved by have a closed system, but you can have a valve installed that automatically removes air, so maybe thats what they meant.

Small pumps use extremely small amounts of electricity, and also only when they run which is an extremely small amount of time. It's likely to be less than £5/yr even if you take long showers every day.

Broken pumps save even more energy...

sheenas

I was think that a small 1.5bar would use around 500w which when you mention the run time I guess its not huge, but still something to go wrong.

Albermarle

sheenas said:

I have had both system and would definitely get rid of the pumps in favour of the pressurised system. The pumps use quite a bit electricity. The benefits would be equal pressure on the hot and cold water systems. The point around getting air in the heating system, I would agree that this would not be solved by have a closed system, but you can have a valve installed that automatically removes air, so maybe thats what they meant.

I have a closed/unvented system and never have any problems with air in the CH. 
I might bleed the radiators in the loft once a year, but usually no air comes out, or if it does for about a second.
Perhaps I am lucky !

nofoollikeold

Before you do anything, get the incoming cold flow rate and pressure measured.  For an unvented hot water system you need:
1. A dynamic water pressure of around 2 bar.  Dynamic means pressure measured with at least one other mains connected outlet open.  Typically measured at an outside tap with the kitchen cold running.
2. A flow rate of around 20 litres per minute.  This should be measured at a mains outlet with no restriction.  Not anything with flexi pipe connections or an isolation valve (unless it is a full bore isolation valve) before it.
If your system can't meet these requirement, you won't get the benefits of an unvented hot water system.

One advantage of a sealed central heating system is that the system water is not exposed to the air, which reduces the risk of radiators rusting from the inside out.  

FreeBear

nofoollikeold said: One advantage of a sealed central heating system is that the system water is not exposed to the air, which reduces the risk of radiators rusting from the inside out.  

Lets see what Heat Geek has to say on the subject...

In an open-vented system, the water rapidly reduces its 'dissolved oxygen' content as the system heats up above 80°c. The fact it's open to the atmosphere means this oxygen can leave the system and so after this temperature the corrosion rate begins to drop rapidly.

In a sealed system, however, the fact it's slightly pressurised increases this oxygen saturation temperature and so the oxygen stays within the water. The fact that the system is sealed also gives nowhere for the oxygen to go, so the corrosion rate steadily increases.

As you go above ~75°C, an open vented system has a reduced corrosion rate. Below that, an open vented system still has a slight advantage, all be it negligible. In either system, dosing on a regular interval with corrosion inhibitor will keep rusting down.

Albermarle

nofoollikeold said:

Before you do anything, get the incoming cold flow rate and pressure measured.  For an unvented hot water system you need:
1. A dynamic water pressure of around 2 bar.  Dynamic means pressure measured with at least one other mains connected outlet open.  Typically measured at an outside tap with the kitchen cold running.
2. A flow rate of around 20 litres per minute.  This should be measured at a mains outlet with no restriction.  Not anything with flexi pipe connections or an isolation valve (unless it is a full bore isolation valve) before it.
If your system can't meet these requirement, you won't get the benefits of an unvented hot water system.

One advantage of a sealed central heating system is that the system water is not exposed to the air, which reduces the risk of radiators rusting from the inside out.  

This is very true.
As part of a loft conversion we converted to a unvented system. It was also sold on the basis that with a 4 bed/4 person family home it would be better as it could support full pressure/flow of cold and hot water at more than one outlet at a time.
However it never has been able to do that, which I recently discovered was because our mains water pressure is not high enough at 1.5 bar. If you are showering and someone uses another outlet, the shower gets weak.

On the other hand without a loft, we had nowhere really to put a cold water tank anyway.