Buffer Tanks - 2 or 4 port connections

[Ermintrude]

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[Nostrum]

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As far as I am aware the 3200 controller doesn't have weather comp built in as standard although they will almost definitely have a module you can add to activate it. I've not seen any option for weather comp curves whilst setting up.

The smaller P1 and P4s use a modulating pump controlled by the controller to control the pump speed to maintain the return temperature to the required temp. This may be what is on the drawing.

On the last quote we got a couple of weeks ago they added an export pack which was not on the quote before. I queried what it was (listed as CB mixer) thinking they'd specced two variants for the shunt pump as I couldn't understand how they could spec a pump without asking for hydraulic spec of system.

If they had said it was weather compensated mixing it would have made sense but they didn't, they simply said it was an export pump and mixing valve set. I've a feeling this has all only recently come about.

I've just looked through the controller manual. It would seem as if when you plug a froling room sensor in the weather comp option becomes available

 

[Ermintrude]

That's a very small accumulator in test center...... I did my get as

 

[missplumb]

Looks like a SH 60kw ish log/pellet boiler with a 1800 ltr buffer.

 

[Ermintrude]

Etta

 

[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.

 

[missplumb]

As Worcester said you could use the hot water module on the buffer and pipe the boiler direct to the buffer and come out the other side to your heating. You then add the middle sensor to help monitor the buffer charging status. Difference between the two schematics you've shown is the ETA shows a shunt circuit with mixer valve, the Froling does not.
We have always used the 4 port buffer system, we set top of buffer to 80 min with 5 deg hysteresis and bottom off at 70 deg. We find that the systems are never short of heat as the buffer is always already 70% charged and boiler only fires using buffer sensors only.

 

[Ermintrude]

The buffer works when the boiler isn't even awake.

Demand established :-

Buffer satisfies load whilst boiler gets to 55c and flue up to 100c

Boiler hits :- 55c and flue 100c 3 port and pump wind in ..... Buffer still providing load satisfaction

Boiler hot and 3 port swings in:-

Buffer almost depleted but boiler coming on line.

Load side pumps pull from easiest route which is boiler now that the 3 way is playing ball.

 

[Nostrum]

Won't the boiler and buffer be asking for two separate temperatures though? If the return temperature is coming back in excess of what the boiler is looking for, 20 degrees differential for example, the boiler will modulate to try and match the load?
so unless the buffer is happy to run at 60 also (which is lower than the desired temp at mixer) I can't see how sufficient flow will enter the buffer to raise the temp?

Surely in the example I gave above, you could end up with a buffer at 60 providing no additional input to the heating circuit until the heating mixer pump turns off, at that point the buffer could be loaded by the boiler?

Thats fine for heating, but hot water production is relying on the buffer.

It's a shame they don't explain the controller in a little more detail, maybe most people aren't interested but I'd prefer to know the capabilities of the product in full detail rather than just pipe it up off a schematic as told. It's my companies reputation at stake at the end of the day.

 

[Nostrum]

The buffer works when the boiler isn't even awake.

Demand established :-

Buffer satisfies load whilst boiler gets to 55c and flue up to 100c

Boiler hits :- 55c and flue 100c 3 port and pump wind in ..... Buffer still providing load satisfaction

Boiler hot and 3 port swings in:-

Buffer almost depleted but boiler coming on line.

Load side pumps pull from easiest route which is boiler now that the 3 way is playing ball.

Yep we've gathered how the set up works on start up, but I'd like to know how the controller can maintain the buffer temp whilst under load.

 

[missplumb]

It doesn't, the buffer is only there for peak load demand, once the boiler is up to temp it takes over the best demand and the buffer sits doing not allot as the least circuit of resistance is not through thousand of litres of water in the buffer. As Ermi said, once the demand has gone and heating mixers are diverting back to the boiler, then the buffer is re charged ready for the next peak demand.