Buffer tank explanation

[Super_Paulie]

Evening. 

Looking for an easy to understand explanation here. 

 

I've got a 2 port buffer in the loft (along with my combi) on the return to reduce short cycling on my downstairs UFH. It works. 

 

When trying to explain it to a friend, I said it was there to add volume to the system so that it always has enough water engaged to not have a too hot return and shut the boiler down. That's correct right? 

 

My mate then asked why did I bother lagging the tank if my intention was to get the return temperature down. I had no answer for him, which leads me here to enquire as I've seemingly built something which works but I don't fully understand.

 

Can anyone sum this up for me?

[Nickfromwales]

How quickly can you walk through an empty train, vs a packed one?

 

A bit like, "What is the sound of one hand clapping" to the untrained ear I'm afraid.

[SteamyTea]

It is to stop uncontrolled thermal losses.

Ideally the temperature as it leaves the UFH pipework will be at the ideal temperature for efficiency.  Adding a buffer/volumizer just adds more water, it is not there to control temperature, the boiler/heat pump does that, it just reduces cycling.

So insulating the buffer/volumizer keeps the return temperature at, or very close to, the ideal temperature without losses (or at least reduced losses).

[JohnMo]

I've always looked at it this way. Uninsulated pipes lead to heat losses mostly where you don't want them, heating and area unnecessarily.

 

A buffer or volumiser in your case is a little like filling a kettle, put a cups worth of water in it, heats in about a minute, you have a an empty kettle after your cup of tea - no energy left.  You fill the kettle it takes 4 mins to heat up.  You take your cup out and there is still a lot of hot water left, the wife comes in she makes a tea etc.

 

So for a boiler, the single cup situation occurs when the system doesn't have enough volume. Boiler starts heats, stops quickly as up to temperature really quickly, the water volume is low so heat is lost quickly, boiler tries to start (may have 10 mins lock out), starts, stops soon after, lock out etc. 

 

The full kettle situation, boiler starts heats the full volume of water. The contents of the buffer/volumiser get distributed around the heat system by the boiler pump, until the return temperature is low enough to enable the boiler to start. So now it's heating starts, runs for a longer period (more water to heat), stops, has a break (uses the energy in the volumiser to heat the house), starts runs stops, takes a break.

 

Having the correct minimum volume keeps things running in a controlled manner. Same is true for all heat sources.

[Super_Paulie]

Gentlemen, thank you, I'm now on board.

 

For what it's worth, the UFH you guys helped me build works an absolute treat. Few little snags to iron out (reverse circulation from the bypass rad) but other than that I am well pleased with how it's turned out and it seemingly costs sod all to run compared to my previous rad only system.

[JohnMo]

14 minutes ago, Super_Paulie said:

reverse circulation from the bypass rad)

Could you just add a check valve so it cannot reverse flow?

[Super_Paulie]

Probably, i still have access to the plumbing upstairs.

I have the UFH re-joining the main return as the very last tee and the bypass rad the tee before. So I guess the order, from boiler down is UFH, bypass rad, rads. Therein lies the problem I guess, so I could re-jig the above order to get the bypass in as the last in the system if that sorts it.

[MikeSharp01]

I am being told I may need one for the defrost cycle but as far as I can see it's only about ensuring that there is enough water no matter what the secondary pipework volume is depending on settings IE in my case when the DHW coil is shut - OR the UFH is closed so it's only needed because the ASHP's are not designed to take command of those valves to ensure enough volume is engaged for defrost. In the case of rads I guess it's that for ASHP systems and for boilers the  cycling problem to be sure any uncontrolled TRV rads don't limit the water volume too much. 

[JohnMo]

Defrost just needs water. The 4 way valve reverses, the circulation pump continues running.  The ASHP is now running in cooling mode, taking heat from the circulation water to defrost the condenser.

 

As soon as you add hydraulic separation 4 port buffer or low loss header, the primary volume is heat pump and buffer - heating system is now excluded from defrost activities.

 

Two things the system needs to meet, volume flow, and min engaged volume.

 

Best way to have the system configured is one or two big zones using the ASHP circulation pump. You need each zone to meet min volume and allow enough circulation flow.

 

If you meet min flow required but not the volume bit add a volumiser.

 

If you cannot meet either min requirements, then configure a two or three port buffer with hydraulic separation. Only the water that needs to go to the buffer flows there all other water bypasses it. But doing that your control system needs to control charge temp on the buffer as well as the room stats.

 

Don't install a 4 port buffer as these just add inefficiency. Unless you pay lots for a well constructed unit - bit like hens teeth.