Horizontal Unvented Cylinder

330d

I am planning on moving my boiler and unvented cylinder into the loft. Due to space I have been recommended a horizontal cylinder.

But looking online, a few people have mentioned that the horizontal cylinder are not as good as the vertical one's. But no one has actually said why. Is this correct, if so what is the reason why horizontal are not as good as vertical?

ComicGeek

The bigger the volume, the bigger the issue with mixing and temperature stratification.  The hot water rises to the top, the cold water goes in at the bottom - that just works so much better with a vertical cylinder than a horizontal cylinder.

We've used the smaller 150/180l Joule horizontal cylinders on a few jobs with difficult spaces, and they've worked ok - I probably wouldn't use a large horizontal cylinder for the above reasons though.

330d

Thank you. 

I have heard the word stratification used a few times when referring to horizontal tanks. Do you mind me asking what exactly this is?

Interesting point about cylinder size, I was look at 250/300l cylinders! 

Ectophile

Most people know that hot air rises, and cold air falls.  The same is true of water.  Hot water is slightly less dense than cold, so it floats on top.

Water tanks are plumbed so that the hot water goes out of the top, and cold water comes in at the bottom.  So even if you have used some of the hot water, and it's been replaced by cold, then the remaining hot water should stay floating on top.

In reality, it never works perfectly.  After a few hours, they gradually mix.  But a vertical tank has a smaller area where the hot and cold meet, compared with a horizontal tank.

330d

Ok thanks. Is one option to use a re-circulating stratification pump to solve this issue?

ComicGeek

Stratification in water cylinders means that water is not evenly warm throughout the vessel but rather arranged in layers of cold/warm water, given the different density of cold and hot water.

In a vertical cylinder, with the heating coil near the centre of the cylinder, there isn't much stratification. As the cylinder gets much bigger then we have to put in circulation pumps to mix it better, but not normally in domestic use. 

With horizontal cylinders the cold water inlet, coil and hot water outlet are normally at the same end (like the Joule cylinder) or all along the top (like the Telford cylinder). It's trying to work against some basic principles of fluid thermal dynamics (ie hot water rises), and I definitely wouldn't pick a horizontal cylinder unless I had no other choice. 

The coil position of the Telford cylinder is better https://www.heatandplumb.com/acatalog/telford-unvented-horizontal-indirect-stainless-cylinder-tsmi300h as it's more central, rather than at just one end.

330d

ComicGeek said:

Stratification in water cylinders means that water is not evenly warm throughout the vessel but rather arranged in layers of cold/warm water, given the different density of cold and hot water.

In a vertical cylinder, with the heating coil near the centre of the cylinder, there isn't much stratification. As the cylinder gets much bigger then we have to put in circulation pumps to mix it better, but not normally in domestic use. 

With horizontal cylinders the cold water inlet, coil and hot water outlet are normally at the same end (like the Joule cylinder) or all along the top (like the Telford cylinder). It's trying to work against some basic principles of fluid thermal dynamics (ie hot water rises), and I definitely wouldn't pick a horizontal cylinder unless I had no other choice. 

The coil position of the Telford cylinder is better https://www.heatandplumb.com/acatalog/telford-unvented-horizontal-indirect-stainless-cylinder-tsmi300h as it's more central, rather than at just one end.

Thank you, great info.

As it will be going in a loft, only a horizontal works for me.

Regarding the Telford being the better one out there, looking at there installation manual, it mentions that a circulating pump must be fitted for best performance.

Ectophile

If you have a circulating pump, you won't have any stratification.  If you use half a tank of hot water, you'll then have a whole tank of warm water until you heat it up again.

330d

Ectophile said:

If you have a circulating pump, you won't have any stratification.  If you use half a tank of how water, you'll then have a whole tank of warm water until you heat it up again.

Sorry I'm not quite with you, apologies for the confusion.

So a circulating pump will or will not solve the issue with stratification?

ComicGeek

The circulating pump is on the hot water pipework to the showers, taps etc. It pumps the hot water around the house in a loop and then back to the cylinder - the idea is that in very large houses/buildings it means that it doesn't take long to get hot water from any tap, and also helps to maintain water temperature for legionella protection etc. Technically it's called domestic hot water secondary return.

The unused hot water then comes back into the cylinder (in between the heating coil and cold water inlet) - the benefit in terms of stratification is that it constantly keeps the water moving within the cylinder so helps with mixing. The downside is that you always lose some heat from the water loop while pumping around the house and it is more expensive to install. Not normally required for most houses, it's just trying to get around the key design flaw of horizontal cylinders.

We put a lot of vertical cylinders in lofts, and they fit most times - I would really check that you can't get them to fit first, I would put in a standard 200l vertical tank instead of a 300l horizontal tank if it fitted.

Horizontal tanks are also normally made to order, whereas vertical tanks are off shelf items - so lead-in times at the moment are probably quite long.

330d

ComicGeek said:

The circulating pump is on the hot water pipework to the showers, taps etc. It pumps the hot water around the house in a loop and then back to the cylinder - the idea is that in very large houses/buildings it means that it doesn't take long to get hot water from any tap, and also helps to maintain water temperature for legionella protection etc. Technically it's called domestic hot water secondary return.

The unused hot water then comes back into the cylinder (in between the heating coil and cold water inlet) - the benefit in terms of stratification is that it constantly keeps the water moving within the cylinder so helps with mixing. The downside is that you always lose some heat from the water loop while pumping around the house and it is more expensive to install. Not normally required for most houses, it's just trying to get around the key design flaw of horizontal cylinders.

We put a lot of vertical cylinders in lofts, and they fit most times - I would really check that you can't get them to fit first, I would put in a standard 200l vertical tank instead of a 300l horizontal tank if it fitted.

Horizontal tanks are also normally made to order, whereas vertical tanks are off shelf items - so lead-in times at the moment are probably quite long.

Thank you, great reply and very good info. 

So by the sounds of it a pump can be used to help but then again the pump has its own disadvantages as you mentioned. 

Interesting point about having a 200l vertical vs 300 horizontal. The horizontals must be bad!

I have not measured the height I have available in the loft but do have a photo of it which I have attached. As you can see, the gable wall is roughly 6 blocks high. So if easy block has a height of 200mm, I have around 1.2m height to play with.

Looking at online retailers, even the 200l tanks have a height of around 1.4-1.5m. 

The 'best' I found was this 250l cylinder which only has a height of 1.33m which is short for a 250l cylinder - https://www.plumbnation.co.uk/site/telford-tempest-stainless-steel-indirect-250l-unvented-cylinder/

But even then I am not sure if that will fit.

Any suggestions please?