Buffer tank and secondary pumps. Do I need them?

[jimseng]

Hello.

I don't know what more information is needed to guide me on this but I have been trying to learn about my installation before it takes place and after having watched quite a lot on line (dangerous for someone ignorant as I am) I am wondering if I really need a buffer tank and secondary pumps.

I am half way through a new build of a 158 sqM house which is designed to be very efficient (MVHR and masses of Warmcell insulation)
I have been quoted for a Samsung Gen 7 R290 ASHP, the data sheet says a min flow rate of 7 L/m and max flow rate of 48 L/m and the design flow temp for my system is 35 deg.
There is UFH on both floors with a design flow rate of 7.3 L/s (gf) and 12.1 L/s (ff) and I am not planning on doing any zoning.
The ASHP quote specifies a 30L buffer and secondary pumps so I am assuming that is one pump per manifold. I have seen it stated that buffer tanks are not always necessary and I am wondering why this is, why I have been quoted for one and whether I am fussing about nothing. I'm not looking to shave extra pennies off the installation but I am obsessing over getting the most efficiency out of my system and I am averse to "that's the way way always do it" approach.
There is a possibility that the 30l tank is an expansion vessel and not a buffer tank. If so that would leave the question why do I need secondary pumps.

If it is a buffer tank then I am wondering if I should consider secondary pumps with controllable flow rates rather than set and forget.
Happy Christmas by the way.

[JohnMo]

29 minutes ago, jimseng said:

buffer tanks are not always necessary

Generally never required with sensible design.

 

30 minutes ago, jimseng said:

Samsung Gen 7 R290

What size?

 

30 minutes ago, jimseng said:

I am not planning on doing any zoning

Good.

 

So basically the design should look something like this. Two floors if the same design flow temp no mixing should be needed. If one floor needs a higher flow temp some mixing may be required, this should be fine with an electronic mixer not a mechanical one. But if well insulated upstairs should really need much or any heating. So I would be tempted to flow a single flow temp to both floors and balance as needed with flow rates.

 

So next is system capacity. You will need at least 20L per min kW output of the heat pump. If you don't comply with that add a volumiser on the return line prior to tee coming back from cylinder.

 

Then run open loop from ASHP on weather compensation.

 

Alarm bell - if you are bigger that 6kW, find a different installer.

 

Alarm bell installer talks about a buffer, find a different installer.

 

You want an efficient system you want a heat pump that modulates to at least half your heat loss at -3, then the heat pump ticks away all day at 7 Deg outside and maybe above that. Would suggest you heat loss is is in the 2.5 to 3kW range from what you have said. If so a 4kW would-be good.

[Nickfromwales]

Hi.

 

Something’s not right if your design flow temp is as high as 35°! GF UFH, I assume, is going into a slab? If so, 35° into that will cause the house to overheat. You should be mid-high 20’s and certainly under 30°, if well built, airtight, and you have heat recovery too. 

 

Buffer “delete” may be subject to the manufacturer’s installation instructions, and an MCS reg’d installer may refuse to stray from that. On a recent Heat Geek install I witnessed they put either a 25L or a 50L 2-port volumiser in, on the flow which makes more sense to me, as that was stipulated in the MI’s vs what the low heat demand called for. That was a refurb, prob got to EnerPHit levels, and the flow temps at 25° made the place almost unbearable to work in. 

 

@jimseng  As we don’t know much about your particular project we can’t possibly size a heat pump here yet, but 3.5-4kw will be quite slow to reheat a decent sized UVC; your game plan needs thought which considers your DHW needs / frequency / number of occupants.

 

Heating is a doddle to resolve, DHW needs to be considered in isolation and planned for pragmatically. You can, for eg, use an immersion strategically to boost for times of duress, so many ways to ‘make this happen’.

 

Do you have solar PV in the plan?

[jimseng]

It is a 5kw heat pump. PWM flow control.
According to the UFH designs the pipework length is 920m for the whole house. With 16mm pipe / 12mm ID I make that 104L.

The spec for the heat pump states a  flow temp of 15 - 75 deg.

Is it possible the "buffer tank" is in fact an expansion vessel, or does the fact there are secondary pumps in the quote mean it is in fact a buffer tank?

I'm going to go back to the installer and get more information, this is all useful to me. So the main point of the question is do I need a buffer? The answer seems to be no.

The second is do I need secondary pumps if I don't have a buffer. Are they specified because of the buffer?
The UFH on the ground floor is already installed and the screed is down. (150mm insulation, 75mm screed)

The FF heating is 22m routed boards and 16mm pipes, that won't be going in for some months.

The DHW is possibly for a different conversation although FYI I have opted for a 300l cylinder, despite me being a single person with no kids. It is a house suitable for a family. Yes solar, yes immersion strategy.
 

[SimonD]

2 minutes ago, jimseng said:

Is it possible the "buffer tank" is in fact an expansion vessel, or does the fact there are secondary pumps in the quote mean it is in fact a buffer tank?

 

Yes the addition of the secondary pumps indicates that it is a buffer. I'd be curious to understand what the designer's pressure loss calcs are for the pipework index circuit (the circuit with the most pressure loss)and whether they're worried the heat pump has enough residual head, whether they've specified the buffer because of different flow rates between gf and ff, or whether they haven't thought much about it at all and/or are compelled by some manufacturer requirements. 

 

Although 102l open volume is okay, I would personally look to add in a 50l volumiser on your system at least.

[JohnMo]

14 minutes ago, jimseng said:

Is it possible the "buffer tank" is in fact an expansion vessel, or does the fact there are secondary pumps in the quote mean it is in fact a buffer tank

If they are quoting a buffer it isn't an expansion vessel.

 

A buffer requires secondary pumps, a volumiser doesn't. A buffer provides hydraulic seperation and therefore needs a pump(s) for the heating circuit.

 

Samsung Gen 7 schematic don't compel the use of a buffer, if your are doing an open system. They show a volumiser on the flow to heating circuit.

 

14 minutes ago, jimseng said:

300l cylinder

Cylinder size is basically worked out as follows, 45L per bedroom, plus 40L.

 

Reheat time assuming 300L and a 5kW output will be around 1hr and 20 mins to 50 degs, assuming bulk average temperature is about 30 degs.

 

 

[jimseng]

Quote

I'd be curious to understand what the designer's pressure loss calcs are for the pipework index circuit (the circuit with the most pressure loss)and whether they're worried the heat pump has enough residual head

This is probably my fault but after going through various options I elected to have the UFH designed and installed by a different company to the ASHP supplier. But this is where I am, it's too late to change that now. But I can get the ASHP design right with the right knowlege.

Quote

or whether they haven't thought much about it at all

This is the bit that concerns me. The worst thing I find is those who say "That's the way we always do it". 
So to summarize: I am gleaning from this thread that, given that I want to achieve the most efficient system, I don't need or shouldn't have a buffer or secondary pumps? To me this leaves the flow rate up to the heat pump control system which, from the little I know as of now seems like the best option. I'm going to go back to the installer in the new year and query this and, to be fair,  they are responsive, if a little dismissive of my technical questions. I need to be in a reasonable position to argue the case if I am to change the system design. 

As for the DWH cylinder. I chose a large one and have not had any push-back on it being over-sized for a 3 bedroom house. Yes it is big but to me, and my need for deep, long baths I would be furious if I installed one too small. I would never get over it.

[SimonD]

48 minutes ago, jimseng said:

This is probably my fault but after going through various options I elected to have the UFH designed and installed by a different company to the ASHP supplier. But this is where I am, it's too late to change that now. But I can get the ASHP design right with the right knowlege.

 

No, not your fault at all. The ASHP supplier should be doing at least a basic calculation of the index circuit of the UFH as that will help them to design the ASHP system properly. But I assume from what you say that the people who installed the UFH have installed just the UFH manifolds without mixers and pumps on the manifold?

 

51 minutes ago, jimseng said:

So to summarize: I am gleaning from this thread that, given that I want to achieve the most efficient system, I don't need or shouldn't have a buffer or secondary pumps? To me this leaves the flow rate up to the heat pump control system which, from the little I know as of now seems like the best option.

 

Yes, absolutely. The simpler the better, but for a 3 bed house, there should be no need for any hydraulic separation or additional pumps with a system that runs low flow temps straight into the UFH.