ASHP, buffer & storage tanks

[MattT2]

Hi Folks,

 

I'm new here so be gentle with me. Our project is a 125m2 extension to a derelict 75m2 Victorian lodge in the Scottish highlands. We have no heating currently. We are not on the gas grid. We are on the electricity grid. We are considering ASHP (versus tanked LPG or Oil). We will have a total of 150m2 of UFH. The Bosch 7001iAW looks suitable, along with the AWE indoor unit.

 

I'm trying to understand buffer tanks & storage tanks.

 

From what I have read, my understanding is this: The purpose of a buffer tank is to prevent short-cycling of the ASHP - which we don't because our UFH will have so much water in it that it will achieve the same outcome (it is effectively a buffer tank). The temp in a buffer tank is the same as the output temp of the ASHP - which should be as cool as possible for the ASHP to run efficiently (in the range 35 deg -> 55 deg, depending on season). Although the ASHP can run hotter, it gets * very * inefficient if you do. So... a buffer tank is NOT for DHW. A storage tank is for DHW - it gets the water from the ASHP (in the range 35 deg -> 55 deg) and then elevates it for DHW (it is also an immersion heater). 

 

Is this basically correct? If so, my conclusion is that we don't need a buffer tank, but we do need a storage tank (for DHW).

 

All advice / education gratefully received.

 

Thanks,

 

MattT

 

 

[joe90]

Yes I think you are correct, I ran my DHW at 48’ and found it fine for all usage but I had a larger DHW tank as blending with cold water was much less. I had a buffer but borderline on being a requirement, however it gave me the chance to instal an immersion for the UFH in case the ASHP or pump etc failed as a short term measure. Siting the DHW tank as central as possible and shortest pipe runs and some in 10mm pipe means less wastage of water and time (=money).

[IanR]

Hi MattT, and Welcome

 

47 minutes ago, MattT2 said:

The purpose of a buffer tank is to prevent short-cycling of the ASHP - which we don't because our UFH will have so much water in it that it will achieve the same outcome (it is effectively a buffer tank).

Maybe, but you'll need to have sufficient volume of the UFH always "open". The HP will have a minimum volume of water it needs to avoid short cycling so if you don't have a buffer then you need some/all of the UFH to always be open to the HP to meet that minimum volume. 

 

There's not one solution that suits all properties. If you have a number of other heat sources that effect the property, ie. solar gain, WBS, cooking, occupation levels then you may need a control over the UFH that can adjust to the dynamic nature of those other sources and in those circumstances you may want all loops of the UFH to be able to be closed and then you will require a buffer or volumiser.

 

In those circumstances a volumiser is the simpler option, ie. 2 port tank in the flow or return line (it's generally thought the return is the better option). A 3 or 4 port buffer has a specific use case where you have other heat emitters that use much less energy than the HP can modulate down to ie. a wet duct heater on an MVHR system or perhaps a fan coil heater, and you want a store of water at as close to flow temp as possible for those heating options.

 

Yes, HP efficiency drops off the higher the flow temp it is run at, so for space heating and DHW you want to run each as low as you can. In a well insulated house you may be able to get your space heating flow temp below 30°C for much of the year, increasing it only for the coldest nights. Weather Compensation curves allow this to be automated.

 

47 minutes ago, MattT2 said:

 So... a buffer tank is NOT for DHW. A storage tank is for DHW - it gets the water from the ASHP (in the range 35 deg -> 55 deg) and then elevates it for DHW (it is also an immersion heater). 

Not quite sure what you are saying here. With an ASHP you do require a water cylinder for DHW. Most would go with a UVC. To keep the ASHP flow temp at a reasonable level the UVC should be sized to supply sufficient hot water for the house at a stored water temp of, say, 50°C. Yes the UVC will have an immersion, but this is not used to generally lift the stored water temp to a higher level, except for a fortnightly legionella prevention cycle when the water is lifted to 60°C

Edited January 20 by IanR

[JohnMo]

46 minutes ago, MattT2 said:

understanding is this: The purpose of a buffer tank is to prevent short-cycling of the ASHP - which we don't because our UFH will have so much water

Two reasons, short cycling and defrost. Both require a minimum volume of water available.

 

If you zone the UFH your volume will reduce very quickly. Volume is smallest zone capacity plus piping to that zone.

48 minutes ago, MattT2 said:

ASHP to run efficiently (in the range 35 deg -> 55 deg, depending on season

You need to design for no more than about 40 at your lowest temperature to good efficiency. Then a lot of the time you will flow closer to 30.

 

There are buffers and volumisers. Generally a buffer will provide hydraulic seperation between heat pump and central heating. If done well can be ok, but most times it isn't and you end having to run the heat pump hotter than it needs to be due to mixing. A volumiser does not provide hydraulic seperation, it adds volume and is more likely to an ok addition to the system.

[MattT2]

Many thanks for the replies & advice. I wonder which of these I need:

 

* Storage tank (aka cylinder)

* Buffer tank

* Volumizer tank

* Expansion tank

 

Looking at the ASHP documentation for the Bosch AW/AWE, the https://www.worcester-bosch.co.uk/professional/products/cylinders/green-storage-wb-range-cylinders looks like the conventional choice - for DHW. 

 

IanR's response intrigued me. If the output of the ASHP is, say 35 deg, what component in the system raises this temp for DHW? (isn't it the immersion in the Storage tank?)

 

A couple of more general notes; I don't see an issue with having a significant volume of the UFH 'always open' - especially if it means I can avoid a separate buffer tank. I see in other threads that there are ways to calculate what a reasonable volume is - and that 10-15l for each kW (ASHP rating) was a rule of thumb. 

 

We are space (& budget constrained). More tanks = more space, cost & complexity. We are also considering having no Storage tank, and just a Buffer tank - and using electric taps / showers for DHW.

 

Thanks again. This forum is amazing. I got 3 replies within an hour of posting. I hope I can contribute in the future (rather than just being the one asking dumb questions).

 

MattT 

 

 

 

 

 

[JohnMo]

Basics are

 

Heat pump generates heat for central heating at one temp or a temperature based on weather compensation.

When there is a demand to heat DHW the heat pump moves a diverter valve from central heating to DHW cylinder heating. The heat slowly increase output temperature until your cylinder is hot enough. It then reverts to central heating.

 

You are better to have a heat pump cylinder and no buffer. But you need to understand the size of heat pump required etc. if you are going for a grant you have to heat DHW via the heat pump!

[IanR]

2 hours ago, MattT2 said:

IanR's response intrigued me. If the output of the ASHP is, say 35 deg, what component in the system raises this temp for DHW? (isn't it the immersion in the Storage tank?)

 

The HP will have two different flow temps, the lower one for Space Heating, which could be in the 35°C degree range, and a higher one for DHW which would be to achieve a stored water temp of the 50°C I mentioned, so would peak at 55°C.

 

2 hours ago, MattT2 said:

We are also considering having no Storage tank, and just a Buffer tank - and using electric taps / showers for DHW.

If you are budget constrained, simplify the system and avoid a Buffer, accepting you will have at least a portion of the UFH always open, but electric resistance heating for taps and showers is a false economy, you'll be paying around 3 times as much to heat your hot water and have a poorer hot water service.

Edited January 20 by IanR

[MattT2]

Thanks again guys. You have given me some stuff to research / think about. The fog is lifting!

 

MattT2