Help with Replacement Boiler! - Oil boiler costing a fortune!

[Nickfromwales]

22 hours ago, Dazza said:

Sorry if this sounds like a silly question, but how do you maintain pressure if all 4 showers were running at the same time?

No silly questions here mate  The more questions you ask, the less things will have a chance to go wrong. 
You use a cold mains accumulator. Size of that is critical otherwise it won’t suffice. 
An accumulator is basically a big rubber balloon inside a cylinder which uses the force from the static cold mains to ‘charge’. The acc will typically hold 50%, or less, of its stated max volume so you need to double the size of the acc to get the required stored energy ( water held against its will at pressure ). 
I’ve installed loads of these with fantastic results, and there’s no pumps / mechanical / electrical parts to go wrong. The rubber ‘bladder’ does deteriorate over time but that rate varies massively dependant on how the bladder is cycled. Design of pipework / sizing of acc is everything, but relatively easy to estimate tbh. 
It’ll be very easy to put them in the basement, and for what you’re asking you will need 2x 500L most prob. 
A survey of the cold mains would be the first port of call, to see how much flow and static pressure you have available before any more discernible advice could be given.

Pulps and break tanks would be for a much larger installation like a nursing home / small hotel / B&B etc IMO. 

[fezster]

4 hours ago, Nickfromwales said:

No silly questions here mate  The more questions you ask, the less things will have a chance to go wrong. 
You use a cold mains accumulator. Size of that is critical otherwise it won’t suffice. 
An accumulator is basically a big rubber balloon inside a cylinder which uses the force from the static cold mains to ‘charge’. The acc will typically hold 50%, or less, of its stated max volume so you need to double the size of the acc to get the required stored energy ( water held against its will at pressure ). 
I’ve installed loads of these with fantastic results, and there’s no pumps / mechanical / electrical parts to go wrong. The rubber ‘bladder’ does deteriorate over time but that rate varies massively dependant on how the bladder is cycled. Design of pipework / sizing of acc is everything, but relatively easy to estimate tbh. 
It’ll be very easy to put them in the basement, and for what you’re asking you will need 2x 500L most prob. 
A survey of the cold mains would be the first port of call, to see how much flow and static pressure you have available before any more discernible advice could be given.

Pulps and break tanks would be for a much larger installation like a nursing home / small hotel / B&B etc IMO. 

 

Nick, would you be able to advise on my setup as I'm unable to get a definitive answer on how / whether an accumulator would solve my issue.

 

Static incoming main is 4-6 bar (depending on time of day).

Megaflo unvented cylinder with PRV set to 3 bar.

32mm incoming MDPE with a flow rate of >40L/min off peak. I haven't measured, but I suspect this drops significantly to about 20L/min during peak time.

28mm pipework to unvented and balanced cold. This then reduces to 22mm polypipe running to both showers.

 

2 showers in the house:

- 1 shower being used is great, with working pressure of ~2.2 bar at PRV.

- 2 showers being used are either just ok or not very good (depending on time of day) - working pressure drops to ~1.4 bar at PRV.

 

If I have a 450L accumulator installed with a pre-charge of 1.5 bar, what kind of performance gain can I expect and will it allow both showers to operate at full blast until the accumulator depletes? Am I right in thinking the accumulator will increase the working pressure to 3 bar, which will then gradually reduce to 2.2 bar as the accumulator discharges?

 

 

[Nickfromwales]

1 hour ago, fezster said:

If I have a 450L accumulator installed with a pre-charge of 1.5 bar, what kind of performance gain can I expect and will it allow both showers to operate at full blast until the accumulator depletes? Am I right in thinking the accumulator will increase the working pressure to 3 bar, which will then gradually reduce to 2.2 bar as the accumulator discharges?

In a nutshell, yes. Performance improvements with as little as 200 / 300L acc's are significant. With your stats this should perform extremely well, taking into consideration the preparatory works you've done with correct ( over sized ) pipework etc which already puts you in the best position to integrate one of these.

You need a PRedV on the incoming cold mains and have that set to 4bar. Then have a 28mm 1/4 turn lever ball valve ( full bore ) to isolate the accumulator. Install a drain off cock in between the acc and the 1/4 turn valve so you can service the acc without turning off the water to the house.

Set the pre charge to 2.5bar and allow the system to equalise with the 1/4 turn valve fully open. Once equalised, shut the 1/4 turn off and fit a hose to the drain off cock. Have a 10L container or similar vessel on the end of the hose so that you can keep filling that to record the amount of stored water, discharging the acc until empty. For a 450L acc you should be getting a minimum of 200L of water stored in it.

 

"will it allow both showers to operate at full blast until the accumulator depletes?" Yes it will. Deffo. Plus it will also support any other cold mains dependant outlets simultaneously; WC's flushing, appliances filling, etc.

 

Biggest mistake folk ( and some 'plumbers' ) make is having the outside tap come off the acc associated cold network. Somebody washes the car, showers conk out.The OS tap 1000% must be before the single check NRV at the incoming cold mains, so it gets fed off the supply aka 'rising' cold mains and NOT the 'artificial' mains you have created inside the house with the acc.

[fezster]

25 minutes ago, Nickfromwales said:

In a nutshell, yes. Performance improvements with as little as 200 / 300L acc's are significant. With your stats this should perform extremely well, taking into consideration the preparatory works you've done with correct ( over sized ) pipework etc which already puts you in the best position to integrate one of these.

You need a PRedV on the incoming cold mains and have that set to 4bar. Then have a 28mm 1/4 turn lever ball valve ( full bore ) to isolate the accumulator. Install a drain off cock in between the acc and the 1/4 turn valve so you can service the acc without turning off the water to the house.

Set the pre charge to 2.5bar and allow the system to equalise with the 1/4 turn valve fully open. Once equalised, shut the 1/4 turn off and fit a hose to the drain off cock. Have a 10L container or similar vessel on the end of the hose so that you can keep filling that to record the amount of stored water, discharging the acc until empty. For a 450L acc you should be getting a minimum of 200L of water stored in it.

 

"will it allow both showers to operate at full blast until the accumulator depletes?" Yes it will. Deffo. Plus it will also support any other cold mains dependant outlets simultaneously; WC's flushing, appliances filling, etc.

 

Biggest mistake folk ( and some 'plumbers' ) make is having the outside tap come off the acc associated cold network. Somebody washes the car, showers conk out.The OS tap 1000% must be before the single check NRV at the incoming cold mains, so it gets fed off the supply aka 'rising' cold mains and NOT the 'artificial' mains you have created inside the house with the acc.

 

That's awesome - thank you.

 

I did consider using 2.5bar/4bar - though this would mean adding a second PRedV (one for the incoming main and another for the unvented at 3 bar).

 

I used this formula, taken from a forum years ago, to try and calculate the usable capacity:

 

Using Precharge = lowest dynamic pressure,
Vessel volume V and
Static Mains pressure M, (all pressures as measured on gauge)
Stored volume is given by V(1 - (P+1)/(M+1))

 

which for a 450L accumulator gives a usable capacity of 135L (30%).

 

Even at 1.5bar/3bar, usable capacity is only 37.5%.

 

Could be the formula is wrong.

 

[Nickfromwales]

Formulas are “not my thing”. 
I got these based on the manufacturers recommendations and a bit of practical knowledge from being on the tools for the last 25+ years or so.

The acc will acclimatise to the peak static pressure which is attained around 03:00-04:00 hrs, and then decrease in effectiveness throughout the day. A second ‘peak’ will be late morning which should see you storing a very useful amount of free energy for use at typical bathing times eg mornings and evenings.

I start out with an elevated Pre-charge pressure and then drain / refill / adjust / repeat until I get satisfactory results ( sufficient volumes of stored water ). 
I gave away my scientific calculator many moons ago, to someone who would get more use out of it   Everything I’ve fitted to date either met or exceeded expectations so I’m happy to assume I know enough to offer useful advice here. With as good a cold mains as 4-6bar static, 30% would be due to an incorrect design / sizing etc. You could likely get up to 60%+ but I wouldn’t recommend cycling the bladder more than is required so as to promote longevity. 
 

2 hours ago, fezster said:

I did consider using 2.5bar/4bar - though this would mean adding a second PRedV (one for the incoming main and another for the unvented at 3 bar).

Yes. At 3 bar the flow rate will be strangled slightly, so the second PRedV is absolutely essential to allow max potential at source ( to get the big L/p/m flow rates whilst capping the incoming ( pre balanced ) pressure to a constant manageable level. 
Not fitting that is not an option afaic.  

[fezster]

Thanks again, appreciate the expert opinion, and it's definitely made me confident to splash the cash on one now ?