Buffer Tanks - 2 or 4 port connections

[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.

 

[missplumb]

This is why we don't use the 2 port method. Always have heat and boiler only fires on demand from the buffer

 

[Ermintrude]

2 port is a buffer 4 port accumulator which incorporates buffer. I've just fell off a ****ing wall in the dark. Should bay more attention to where I'm walking the dog at 23.17

 

[Nostrum]

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.

That's how I would have done so prior to this conversation, but if you are being recommended to pipe it in another way to increase efficiency then you've got to look at why?

For me, efficiency is top of the list, after all, that's what it is being sold on. For you perhaps being non domestic, efficiency is lower on the list because of the non domestic RHI returns.

The domestic RHI covers the majority of the upfront cost but for it to really stack up financially, it needs to be as efficient as possible for the long term.

 

[missplumb]

For non domestic rhi, it's more about making the boilers last the 20 years, when we commission and put the fga on, we still have them running at 92-94% efficient, it's what the rest of the system does with that heat.
Ermi, a 2port would be used as an accumulator and the 4 port would be the buffer as it is inline

 

[Nostrum]

Just spoken to the technical team at ZCF. Very interesting and the boilers controller is smarter than I first thought.

The controller can load the buffer and export at the same time. On the P1 and P4 the modulating pump is purely for buffer loading. The boiler uses the internal water ways as Worcester talked about to divert flue gases through various sections of the boiler to achieve the required return temperature, eliminating the need to a return riser. If the buffer temperature drops, the buffer pump will run and the boiler will charge the buffer to the desired set point and provide output for export until it cannot modulate low enough to continue running. When this happens the boiler will switch off, buffer pump goes off and export will come from buffer only by reversing the flow on that section of pipe. So interestingly, the boiler and buffer don't appear to be able to output simultaneously.

The mixing valve ensures water is only circulated through the export with a minimum temperature as well as a set flow temp. The ESBE controller can also be directly controlled through the boiler controller for weather compensation for blended heating flow but maintain the higher set point for the buffer.

 

[missplumb]

Interesting, mind you ive not looked at the Froling pellet boilers as all we do are chip.

 

[Nostrum]

Same setup for chip

 

[Nostrum]

Even upside down as well!

 

[missplumb]

Yes but that has a mixing valve which allows the boiler to fire and run whilst drawing from the buffer

 

[Worcester]

Yep, we're back to the capabilities of the controllers and boilers. With the changes to the RHI, we're finding that the clients are also looking at the most efficient way to control the heating, and using the built in controllers in both ETA and Froling you'll benefit from weather compensated flows to the heating circuits and hence higher efficiencies from the boilers.

If you're not using the built in controllers, then the next most efficient is to use the 4 port buffer.

Next thing is self learning optimised start - we have that built in to our heating circuit controllers that we use with underfloor heating and heat pumps - typically the Heatmiser controls, they offer both comfort levels (i.e different temperatures at different times of day) as well as optimised start. At the moment I'm not aware that either ETA or froling offer optimised start - YET 🙂