500L buffer Vs Unvented HW

[Marty92]

Having a chat with colleagues on this one to decide on best route.

Existing 500L megaflo which does not provide sufficiant HW.

Cold water entering cools the hot way too quickly and consequently the 500L is good for about half that.

Coil is 54kw boiler is 80kw

Cylinder options…

The cylinder has 6 physical connections.

1 & 2 coil (heat input)
3, Cold feed
4, Hot Out
5, Secondary return
6, T&P



Option 1 - virtually normal setup

Heat tank water via coil
Use hot out & SR port for plate flow
Cold for fill and expansion 500l
Prv for relief

Pros

Limits heating system water for expansion

Keeps primary plate cleaner

Cons

Very limited use as a heating buffer via 12kw Immersion heater

Unvented controls still required


Option 2 - Buffer.

Cylinder water is heating system water.
Plate flow is on Hot out and return in to cold, NRV to return only to boiler with bypass in place for cylinder ZV Off or omit NRV for simplicity.
Heating Flow is in to PRV port
Heating Return is out from Cold


Secondary return is in with cold main on plate secondary via double check valve. Similar setup to Solar Thermal with thermal store.

Pro’s
500 L of stored water also available for heating system via 12kw immersion backup

Coil free for heat pump/solar

No need for additional Unvented controls separate from boiler.

Cons’s

Increased heating system volume expansion needs accommodating, around 650L

We cannot change the cylinder,

Your thoughts gents?

 

[ShaunCorbs]

How long is the timer on for hot water only ?

Also what’s the flow rate / usage rate in lpm ?

 

[Marty92]

Has timer controls but is set to 24/7, control was via cylinder stat.

Max flow rate 105l/m (unlikely scenario though)

 

[snowhead]

Is this in a Care Home or Hotel or similar?
How many people / rooms is it serving?

 

[ShaunCorbs]

Has timer controls but is set to 24/7, control was via cylinder stat.

Max flow rate 105l/m (unlikely scenario though)

What’s the average usage rate as the hot water has to be leaving at the same rate as the cold incoming so basically your running out of hot water

 

[SJB060685]

What’s the average usage rate as the hot water has to be leaving at the same rate as the cold incoming so basically your running out of hot water

I have heard of but never known of HW draw off to be greater than incoming cold in some scenarios. That’s where an anti vacuum valve is used to stop a cylinder collapsing. This is obviously not the case in this situation and I agree with you.

 

[Scott_D]

Can you post some pictures?
Have you checked the temp it gets to when not in use? Cylinder star could be faulty, coil scaled up, destratificatin pump needed.

 

[Marty92]

in

Can you post some pictures?
Have you checked the temp it gets to when not in use? Cylinder star could be faulty, coil scaled up, destratificatin pump needed.

All is fine with cylinder, you just cannot hope to get 500L of hot water from a 500L cylinder… cold water rushing in at the same rate as your using the hot water cools a cylinder massively and quickly.

The question is more about weather to heat the volume directly as system water or to utilise the existing coil… we wont be using the cylinder for direct DHW, just too inefficient.

 

[Marty92]

What’s the average usage rate as the hot water has to be leaving at the same rate as the cold incoming so basically your running out of hot water

Yes, running out of Hot was the issue… the way around this is a plate to limit direct mixing of mains cold with the heated volume.

Usage is sporadic, 6 showers is the simultaneous limit.

Question not really about the DHW, more the method of heating the water in the cylinder, coil vs direct and the resultant pros & cons…

 

[ShaunCorbs]

Plate load it then and use the 500l cylinder as a buffer / store and a destrat pump from the 400kw plate to the cold feed (ish)