Buffer Tanks - 2 or 4 port connections

[missplumb]

Yeah ETA SH

 

[Nostrum]

Looking at the schematic a few times today I can see where this may differ to a 4 port buffer.

In this configuration the return temperature will fluctuate in line with the heating load. In a 4 port buffer configuration the temperature difference between flow and return will be relatively constant throughout the heating cycle until the buffer set point is reached, regardless of heating load.

In this configuration, the boiler cannot heat the buffer as the flow is reversed. The boiler will receive a return directly from the heating load which I presume will be a much lower water volume and from mixed circuits.

I don't quite understand how the flow from boiler and flow from buffer is balanced to feed the heating circuit ? Any ideas?

Only once the heating circuit is satisfied will the boiler reheat the buffer which would probably mean the boiler ramps up assuming the boiler modulates depending on return temperature rise. EG: a slower rise will cause the boiler to increase output to achieve set point.

Thoughts?

 

[missplumb]

The flow from the boiler and buffer would be created by the export pump, but that is what I don't quite get as the boiler pump would only run when the buffer needs heat. I've been to several systems piped like this and because the heat loads have not been calculated, the size of the boiler has been well undersized. This means the boiler is constantly running and buffers sit there cold.

 

[Nostrum]

Yes and what is stopping cold water being pulled through the boiler in reverse until the buffer temp drops (which will then trigger boiler to fire)

Thinking about it the system is self balancing, any short fall from the boiler will be made up through the buffer.

The buffer will provide instant heat when heating is called on without the boiler firing until the buffer temp drops, the only pump running would be the mixer pump.

At the end of the heating demand the mixer pump will stop and the buffer will be loaded as normal using the boiler pump.

 

[missplumb]

Yes providing the boiler is sized correctly.

 

[Nostrum]

Yes providing the boiler is sized correctly.

That should go without saying and would apply to any system :biggrin5:

I'm wondering, there must be the need for the buffer/boiler set point to be higher than the heating flow temp in order for the mixing set to work properly and allow enough time for the boiler to fire.

 

[missplumb]

I don't see how the system without the mixing valve would work properly as you couldn't have the boiler running and the system being serviced by the buffer. If you have the mixer valve, that provides the mechanical separation between the system and buffer, until the boiler has reached its operating temp.
I think I'll stick to simple boiler - buffer - heatload.

 

[Nostrum]

Once the heating pump is running the boiler and buffer are effectively working in cascade. The buffer temp and boiler output should presumably equalise although I can't quite get my head around how as the only input to the buffer is the return temp. Surely the buffer would theoretically become exhausted at some point? Or will the fixed flow rate from the boiler pump mean that the only flow through the buffer is excess to what the boiler pump can provide?

If that's the case then the buffer really isn't used much during operation in most circumstances (providing good design)

This would make sense as the buffer only becomes active to satisfy the boilers run time requirements and to provide heat when the boiler is starting up, or to make up any shortfall in boiler output for short periods of time.

 

[Nostrum]

Which will actually work in the way I was suggesting at the beginning of the thread. It just uses basic principles to achieve it as opposed to any controlled intervention.

Quite a clever set up if I'm correct, if I'm not then I've just wasted half my day thinking up a load of tosh. Where's Worcester for his opinion?

If you could control the flow to buffer depending on flow rate as opposed to temperature, it would make more sense.

 

[Worcester]

Worcester's been having a day with family

I think we're there, and having gone round in circles as we've gotten our heads around it a combination of MissPlumb's and Nostrum's responses answer it.

i.e assuming the heating circuits use a mixing valve (room stat, external temp sensor, weather comp) and are managed by the boilers inbuilt systems, then the buffer provides instant heat the boiler then fires up only working at the level need to maintain heat demand, it also monitors the buffer, so if there is also a hot water demand or if the buffer tank is cooling - part of the heating return has to go that way if flow is coming from it, the boiler will ramp up and then after heating demand ceases, it will continue to heat the buffer up until it's replenished.

Needs a good boiler controller, so won't work properly with some of the simpler boilers out there.

ETA show basically the same configuration, so it works when the boiler has a comprehensive control system - this schematic presumes using the inbuilt eta controls.

If you're NOT using the built in controls and / or the boiler has simplistic management, or a cheap buffer that doesn't stratify well/properly, then I can see possible problems with plumbing it the 2 port way: increased cycling, buffer tanks not heated properly, poor performance of the heating sytem (client perception).

So what do Windhager / HDG recommend? - I know that Okofen reckon that they don't need a buffer on smaller systems perhaps for similar reasons.

So:
Using Froling/ETA built in heating controls, using indoor and outdoor stats, mixing valves on the heating circuits and their weather comp system, the boilers should run more efficiently. Reducing the flow through the buffer also helps to maintain good stratification.
If you're not using their control systems then a 4 port buffer is a 'safer' configuration as far as ensuring heat is always available, though may trigger greater full power cycling as opposed to modulation; subject to: programming the correct buffer tank control, by using multiple stats - as a minimum top and bottom stats, and now looking at the hysteresis programming of those stats. Once again the "simpler" boilers may not be able to handle those types of inputs and modulate properly.

I think we've finally answered my original question!

For the simpler heat pumps we have developed our own buffer controllers specifically to address this using two stats and hysteresis management. - The controller is less than £100, needed a bit of lateral thinking to find it though

So next up is how do you configure the buffer tank control on / off points, what temperatures, what hysteresis?

 

[Nostrum]

My thoughts would be that the buffer set point would have to be at least 10 degrees higher than the design flow temp and the hysterisis would be maybe 5 degrees below set point to prevent any short cycling on the buffer reheat.

Realistically though it will depend on a lot if you want to be spot on, buffer size, boiler output, heating load.

Here's another thought, I'm contemplating using a buffer with hot water coil in one of the upcoming jobs so that we can use the space of the existing oil boiler and cylinder to house the buffer internally, rather than externally and have to export heat through underground pipe work constantly which will increase losses.

In the 2 port configuration, with heating on in depths of winter and customer runs a bath or has a long shower. Buffer temp will drop fairly quickly, heating pump will be on so flow to buffer will be reversed, how's the buffer going to remain charged?

 

[Worcester]

In the 2 port configuration, with heating on in depths of winter and customer runs a bath or has a long shower. Buffer temp will drop fairly quickly, heating pump will be on so flow to buffer will be reversed, how's the buffer going to remain charged?

Same as with any 'small' buffer that could suffer depletion, in the 2 port case you would effectively have heating priority as opposed to in the 4 port case the possibility of reduced temperature in buffer, hence reduced flow temp to both buffer and dhw.

Could always use one of the strap on DHW heat exchanger units - both Froling and ETA offer them, flows up to 40l/min. "Virtual Combi". There again as soon as the Buiffer tank is being deleted the boiler output should raise to match demand up to it's maximum. Should work fine IF using the built in controllers.

 

[Nostrum]

But how's the boiler able to up any flow to the buffer if the heating pump is on?

The hydraulics don't allow flow to heating and buffer simultaneously from what I can see. You'd have to turn off the heating pump.

 

[Nostrum]

Just looked at the schematic for this set up and it seems they use an additional buffer sensor half way down the tank, maybe this tells the controller to stop heating if it reaches a trigger point?

 

[Worcester]

Unless heat load = boiler output, there will be some spare capacity to heat the buffer. Hence you'll need to design boiler output in this case to cope full heat load plus an acceptable buffer recovery - actually that's no different from normal.

Practice says boiler size = heat load + 200W per person for DHW, if you had a property demanding full heat load, you'll still have a problem heating the DHW cylinder - it'll take a whiie. Plus boilers come in discreet sizes, not actual heat load size, so there should always be some spare capacity.

As you suggest it could also be charging to the middle sensor to ensure capacity for DHW.