Buffer Tank.... Pro's and Cons

[Dave10]

Hi,

New to the forum and my knowledge of ASHP's isn't vast, to say the least.... So please go easy on me..

 A bit about what has been installed...

 

A new build house fitted with a Riello Nexpolar 15k/W ASHP which is connected to UFH (approx 300 m2 ) and a 300 Litre DHW tank.

The UFH has 8 zones, each controlled by its own thermostat.

 

The original design plan was to heat the UFH and DHW with a Biomass boiler, but before it got installed we were given the option of ASHP instead.  After a couple of nights reading up on them, we decided to go with it.

 

Our setup probably isn't ideal.... We also had an annexe built and all the gubbins regarding the heating are in it, so the HP and DHW tank etc are about 40 - 50 metres from the house.

 

No buffer tank has been installed, but I have recently been reading about them and spoke to another heating company who highly recommends one for my setup.  He quoted me for a 200 Litre tank (size based on 10 - 14 Litres per k/W of the HP) and said the big advantage would be when only 1 or 2 zones in the house are requiring heat.  I have since spoken to the engineer who originally planned the heating setup for the house and he said I wouldn't really gain anything..... So to say I'm a bit confused is an understatement.

 

Your thoughts and comments most welcome.

 

Dave

[joe90]

I have a buffer tank, mainly to stop short cycling and keep the antifreeze in the ASHP only. If you have that many loops then the amount of  water you will have will be much larger than mine so maybe not required in youre case. Some here have no buffer, horses for courses.

[SteamyTea]

3 hours ago, Dave10 said:

have since spoken to the engineer who originally planned the heating setup for the house and he said I wouldn't really gain anything.

What makes them think that?

Would it be lack of insulation under the UFH that was relying on the higher temperature supply from a wood burner?

I posted up a link to a report about ASHP and stuff that includes buffers.

 

 

report about heating.pdf

Edited December 11, 2021 by SteamyTea

[JohnMo]

30 minutes ago, SteamyTea said:

What makes them think that?

Would it be lack of insulation under the UFH that was relying on the higher temperature supply from a wood burner?

I posted up a link to a report about ASHP and stuff that includes buffers.

 

 

report about heating.pdf 639.19 kB · 2 downloads

Link is about heat exchangers?

[SteamyTea]

Whoops, try this one.

 

hot_water_cylinders_buffer_tanks_heat_pumps.pdf

[ProDave]

3 hours ago, Dave10 said:

so the HP and DHW tank etc are about 40 - 50 metres from the house.

That bit got my eye.  WHY would you put the DHW tank so far away?  You are going to have stupidly long hot water runs, you will need a hot water circulating system which will have heat losses or it will be unworkable.  You want the HW tank in the house as close to the points of use as possible.

 

Distance to HP is not as bad but you will still have losses in the pipes.

 

Is the system actually working properly?  Are you thinking of a buffer because someone on the internet says you need one?  Or to solve an issue you are having?

[Dave10]

6 minutes ago, ProDave said:

That bit got my eye.  WHY would you put the DHW tank so far away?  You are going to have stupidly long hot water runs, you will need a hot water circulating system which will have heat losses or it will be unworkable.  You want the HW tank in the house as close to the points of use as possible.

 

 

Forgot to add in my first post I live in Portugal and tradesmen here have some weird ways, to say the least.  We asked about the having the HW tank in the house, but was told it's not possible..... for whatever reason..... Lost in translation.  Yes, we do have a hot water circulating system and it does seem to work well.

 

10 minutes ago, ProDave said:

Is the system actually working properly?  Are you thinking of a buffer because someone on the internet says you need one?  Or to solve an issue you are having?

 

System seems ok. Another heating company suggested the idea while they were fitting PV panels.  Was my thought that perhaps I could save on electric because when temps here go below 3 degrees the HP gets very thirsty.

[Dave10]

2 hours ago, SteamyTea said:

Whoops, try this one.

 

hot_water_cylinders_buffer_tanks_heat_pumps.pdf 2.58 MB · 3 downloads

 

Thanks... Made good reading.

 

Basically, I'm reading it as I don't need one. My reasons being:-

 

 Buffer tanks are unlikely to be required when the heat pump can modulate (i.e. if the 
heat pump is not fixed speed).

 

 Buffer tanks should not be installed in unheated spaces.

 

My HP is not fixed speed.

 

The only place I can install one is in my annexe.... Which is not heated...

[SteamyTea]

1 hour ago, Dave10 said:

 Buffer tanks are unlikely to be required when the heat pump can modulate (i.e. if the 
heat pump is not fixed speed).

 

 Buffer tanks should not be installed in unheated spaces.

Depends on how low the output can go and you can insulate a buffer, and the structure it is is in.

Just a case of doing the sums.

[J1mbo]

In this scenario the issue is going to be how much water there is circulating when only one zone is calling, and the liklihood of that scenario. Even with a modulating ASHP, especially mid-season the range isn't going to be that much and it will need 10 litres per kW circulating to avoid short-cycling. The other issue is defrosting, if all the zones are closed?

[Marvin]

On 11/12/2021 at 13:03, SteamyTea said:

I posted up a link to a report about ASHP and stuff that includes buffers.

 

 

report about heating.pdf 639.19 kB · 12 downloads

Any one thinking of installing HEAT PUMPS should read from page 62 onward of this report. 

[SteamyTea]

On 11/12/2021 at 18:24, J1mbo said:

will need 10 litres per kW circulating to avoid short-cycling

That is only 0.0116 kWh/K.

Or, if you raise the temperature by 15 K, 0.23 kWh, which at 3 kW will be up to temperature in 4.64 minutes.

Think you really need more that that.

 

[SteamyTea]

12 minutes ago, Marvin said:

On 11/12/2021 at 13:03, SteamyTea said:

I posted up a link to a report about ASHP and stuff that includes buffers.

report about heating.pdf 639.19 kB · 12 downloads

Any one thinking of installing HEAT PUMPS should read from page 62 onward of this report. 

Not that report, it is the one I posted up by mistake.

On 11/12/2021 at 13:39, SteamyTea said:

Whoops, try this one.

hot_water_cylinders_buffer_tanks_heat_pumps.pdf 2.58 MB · 17 downloads

This is the one to read.

Maybe a mod can come and edit it to put it right.

[Marvin]

9 minutes ago, SteamyTea said:

That is only 0.0116 kWh/K.

Or, if you raise the temperature by 15 K, 0.23 kWh, which at 3 kW will be up to temperature in 4.64 minutes.

Think you really need more that that.

 

All that conversation is in the report you loaded up!  Also minimum 10 minutes ASHP on per cycle, and so on... 

[SteamyTea]

1 minute ago, Marvin said:

All that conversation is in the report you loaded up!  Also minimum 10 minutes ASHP on per cycle, and so on... 

My work is done.

 

Till next time.