Buffer tank sizing

[vala]

Following on from a thread I previously made I was hoping I could get some help in sizing up a buffer tank for my UFH system.

My boiler is an external oil combi boiler (12/18) and the UFH system is just on the first floor.

I'm also hoping the buffer tank could be fitted in the loft potentially above the UFH manifold.

How do I go about working out what size tank is needed?

[Nickfromwales]

Find out the lowest typical heat load first, eg can the UFH only run when the rads are running? If so, then the system has a degree of energy buffering via the water volume in the rads which would only require as little as 25L buffer volume for worst case scenarios. If the UFH is to be able to run independently, then the requirement increases quite a lot. At that stage you could be into as much as 50-100L of buffering to provide sufficient 'energy run-off' for a scenario where just 1 or 2 UFH zones may need heat, but the boiler is still going full chat as it still has a call for heat signal.

You really should not be fitting the buffer in the cold attic, that will just lose the residual 'wasted' heat from running the buffer. Instead, could you fit it in a cupboard or airing cupboard? It would be far better on the ground floor to use every ounce of waste heat advantageously.

[vala]

There's no radiators in the house. Renovating the whole place and we've stripped the first floor to install UFH up there. Rads downstairs will be replaced next year when that floor gets renovated, however once we make a start on upstairs UFH (end of August), the downstairs ones will come out. Temporarily I'll get some electric ones to do us for the time being.

There is a space under the stairs (which is where the downstairs UFH manifold will be going), and I can get something with a diameter no bigger then 750mm, and height including any pipework of no taller then 1200mm.

However would the water from the buffer need to be pumped to the manifold upstairs? Do they contain pumps or is that something I'd have to fit inline for the first floor UFH manifold?

[Nickfromwales]

Heat source to buffer requires a pump.

Buffer to UFH primary pipework requires a pump.

Each manifold requires its own pump also.

You will have 4 pumps   

[Nickfromwales]

Select 50L model

This will suffice if you have 2 floors of UFH. Dimensions are there, and these wall mount and are very well insulated.

[SteamyTea]

2 hours ago, Nickfromwales said:

You really should not be fitting the buffer in the cold attic, that will just lose the residual 'wasted' heat from running the buffer.

This is an interesting issue.

If the buffer stores 2 kWh of useful energy, that may only be 0.6 kWh of input energy.  Loosing 20% of that will only be 0.12 kWh of energy.  Less than turning on a kettle for 3 minutes.

[Marvin]

8 minutes ago, SteamyTea said:

This is an interesting issue.

If the buffer stores 2 kWh of useful energy, that may only be 0.6 kWh of input energy.  Loosing 20% of that will only be 0.12 kWh of energy.  Less than turning on a kettle for 3 minutes.

Any idea how many times a day that would happen? 

Would it use and lose more energy when it's colder and the ASHP less efficient?

[SteamyTea]

23 minutes ago, Marvin said:

Any idea how many times a day that would happen? 

Would it use and lose more energy when it's colder and the ASHP less efficient?

The number of times a day is irrelevant as it effectively only looses energy when there is no call for heat and a well set up system will be running most of the time.  It will increase the surface area of the whole system slightly, which will increase the power emitted (Nicks point about putting it somewhere useful).

An ASHP is more affected by relative humidity, cold dry air is still good for extracting energy.  Don't use the centigrade scale, use the kelvin one to work out the percentage differences to get the real answer.

I think the losses are too small to worry about, but if it is in the loft, you can build an insulated box around it.

[vala]

45 minutes ago, Nickfromwales said:

Heat source to buffer requires a pump.

Buffer to UFH primary pipework requires a pump.

Each manifold requires its own pump also.

You will have 4 pumps   

To clear this in my head...

heat source to buffer pump is already built into the Worcester external oil combi?

buffer to UFH - can you recommend one?

manifold pumps are already on the wunda manifolds (I've specced the Wilo ones)

[vala]

45 minutes ago, Nickfromwales said:

Select 50L model

This will suffice if you have 2 floors of UFH. Dimensions are there, and these wall mount and are very well insulated.

Is there any point in fitting a larger buffer tank?

Only thing I can think of as a negative is that if its say 100L, the boiler may heat the water in the buffer tank, but this would not get distributed round the UFH loops quick enough, for it to then cool gradually in the buffer tank? Am I right in this thinking, hence why 50L has been recommended?

[PeterW]

Buffer will be 72°C and loops at 40°C it won’t cool that quick. Oil needs a decent burn - find space for a 100 litre tank and be done with it 

[Nickfromwales]

14 hours ago, SteamyTea said:

This is an interesting issue.

If the buffer stores 2 kWh of useful energy, that may only be 0.6 kWh of input energy.  Loosing 20% of that will only be 0.12 kWh of energy.  Less than turning on a kettle for 3 minutes.

Plus all the interconnecting pipework / connections etc. Any waste into a cold attic is bonkers. No constructive maths will change that.

[SteamyTea]

4 minutes ago, Nickfromwales said:

No constructive maths will change that.

Just highlights the relatively small losses though.

Insulation will stop it.

 

Do you have a problem with lofts, you always suggest that no waterwork is out in them.

To a lot of people a loft is dead space.

Edited July 12, 2021 by SteamyTea

[Nickfromwales]

14 hours ago, Marvin said:

Any idea how many times a day that would happen? 

Would it use and lose more energy when it's colder and the ASHP less efficient?

As the attic cools down the delta will rise and the losses will increase.

 

13 hours ago, PeterW said:

Buffer will be 72°C and loops at 40°C it won’t cool that quick. Oil needs a decent burn - find space for a 100 litre tank and be done with it 

50L can suffice if the system is not too small. Remember that the buffer can be installed as an energy buffer rather than an intermediate heat bank, so basically it becomes an energy balloon, inflated an deflated in line with downstream heat demand fluctuations. When the call for heat goes off, the buffer no longer gets serviced ( heated ). Use of single check ( swing check ) non return aka anti-gravity valves will ( can ) reduce heat convection away from the buffer during the 'cool' cycles of any heating cycle. 

[Nickfromwales]

4 minutes ago, SteamyTea said:

Just highlights the relatively small losses though.

Insulation will stop it.

 

Do you have a problem with lofts, you always suggest that no waterwork is out in them.

Not me guv'.

Remember I am a fan of boilers in attics, where space is constrained, and I've put many an UVC in an attic, again for the same reasons. If I was asked ""is that the best place?", I would have always replied "nope" followed by "it is what it is"....  If there is somewhere within the heated envelope for this stuff, then that is where it always goes. 

[Nickfromwales]

7 minutes ago, SteamyTea said:

Insulation will stop it.

Agreed. It can be mummified in dense rockwool and the losses nigh-on lost. Or a PIR box etc.

[SteamyTea]

1 minute ago, Nickfromwales said:

Remember I am a fan of boilers in attics

So it only blows the roof off, rather than take out a wall, followed by the next one.

 

In a small house like mine, putting all the gubbins in the loft would be the sensible thing to do. An extra square metre of storage would help. And there would be plenty of room to add half a metre of, easily removable, insulation around it. Has to be cheaper than an airing cupboard.

[Nickfromwales]

1 minute ago, SteamyTea said:

So it only blows the roof off, rather than take out a wall, followed by the next one.

Depends on whether we have one leaky gas joint or a few........

Neighbour called me to say they couldn't light the gas fire any more. I went into the alleyway dividing the two terraced houses and removed the soot plate to find that rainwater had been dripping onto the copper sleeve of the 8mm gas pipe for years, and it had eventually got through the sleeve and had made a hole in the 8mm pipe, worsening year by year. When I got to it, the gas was following the convection airflow from the living room, up the chimney to atmosphere at a rate of knots. As they were pensioners on a fixed costs tariff, they never got alerted to the huge amount of gas leaving the pipe and whooshing up the chimney for what must have been years!!  

Christ only knows how they never smelt the gas when the wind changed direction, but the house was at a constant 25oC, as they were freezers, and the convection out of the living room up the chimney was like a hurricane.

[vala]

7 hours ago, Nickfromwales said:

50L can suffice if the system is not too small. Remember that the buffer can be installed as an energy buffer rather than an intermediate heat bank, so basically it becomes an energy balloon, inflated an deflated in line with downstream heat demand fluctuations. When the call for heat goes off, the buffer no longer gets serviced ( heated ). Use of single check ( swing check ) non return aka anti-gravity valves will ( can ) reduce heat convection away from the buffer during the 'cool' cycles of any heating cycle. 

 

The loop lengths on the first floor equal approximately 304m. I expect downstairs to be slightly bigger so maybe 660m in total.

50L still ok? Or go with a bigger tank? As mentioned I can get something no more then 1200mm high and with a diameter of no greater then 750mm under the staircase (same location for the downstairs UFH manifold.

[CotswoldDoItUpper]

I’m looking for the same answer, will eventually have about got about 800m pipe downstairs and rads upstairs. Should I go for 50,60,100 or 120L buffer? 
currently an LPG boiler but hoping to get a reasonable ASHP quote soon. 

[Nickfromwales]

3 hours ago, CotswoldDoItUpper said:

I’m looking for the same answer, will eventually have about got about 800m pipe downstairs and rads upstairs. Should I go for 50,60,100 or 120L buffer? 
currently an LPG boiler but hoping to get a reasonable ASHP quote soon. 

Both gas and ASHP will modulate if selected to do so, therefore reducing the need for a large buffer. 100L would be ample.

[Nickfromwales]

On 12/07/2021 at 20:25, vala said:

 

The loop lengths on the first floor equal approximately 304m. I expect downstairs to be slightly bigger so maybe 660m in total.

50L still ok? Or go with a bigger tank? As mentioned I can get something no more then 1200mm high and with a diameter of no greater then 750mm under the staircase (same location for the downstairs UFH manifold.

You can have 3000m2 as that is what you size the heat source to, it's the smallest heat load that causes issue; eg if the study was the only room needing heat at a particular time, with every other room stat turned off / satisfied, then that would be too small a load to stop short cycling of the heat source. The buffer adds to the smallest load to allow the heat source to dissipate what it has produced in any single heat ( burn ) event.

[CotswoldDoItUpper]

17 hours ago, Nickfromwales said:

Both gas and ASHP will modulate if selected to do so, therefore reducing the need for a large buffer. 100L would be ample.

Thanks! 
 

sorry to ask annoying questions, but what size expansion tank do you think I will need? 

[Nickfromwales]

14 hours ago, CotswoldDoItUpper said:

Thanks! 
 

sorry to ask annoying questions, but what size expansion tank do you think I will need? 

18L should suffice, but I’d prob go larger just to reduce the frequency of ‘topping up’. You can never have too much expansion imho, so I’d prob go around 24L. 

[CotswoldDoItUpper]

On 23/08/2021 at 01:39, Nickfromwales said:

18L should suffice, but I’d prob go larger just to reduce the frequency of ‘topping up’. You can never have too much expansion imho, so I’d prob go around 24L. 

Thanks @Nickfromwales for your help, much appreciated! 
 

this buffer also contains 20L expansion. Seems perfect to me! Any issues you can see with more experience?

https://coolenergyshop.com/collections/buffer-tanks-cylinders/products/cool-energy-120l-stainless-buffer-tank-ce-b120