UVC vs TS ( unvented cylinder vs thermal store ) for hot water.

[Onoff]

Moved by @Nickfromwales and @PeterW to keep things on track...

 

On 10/02/2017 at 12:58, Nickfromwales said:

Ok. In that case, you may be better off with a TS. I do forget ?. Good job you didn't 'rush' out and buy an UVC .

With oil and UFH, there is quite a big conflict as the boiler can't modulate. That is usually best dealt with by means of a buffer tank but a TS would be the buffer AND provide DHW it's just down to losses and how the house is occupied. 

With a TS, you'd typically need it heated according to space heating and DHW consumption, much as with an UVC or your current hot cylinder, but the norm is for it to stay hot pretty much all of the time if the house is occupied other than with 9-5'ers. 

A TS will ebb heat away much quicker than an UVC, mostly due to the extra connective pipework etc, so is less likely to retain useable DHW through the day unless the time clock is left on constant. 

The summer is where the TS may fall largely redundant ( as you don't have PV so this info is case specific to you ) so there is an argument for a smallish buffer + UVC vs TS. You keeping the upstairs rads and going all UFH downstairs?

An open pipe 'combination' TS won't need G3, but a sealed and pressurised one will. 

As usual, no easy solution other than the one that suits you best . 

 

I'll start a TS thread sometime. Need to do a drawing as points of use, boiler etc are some distance from each other so it might serve better to alter their positions anyway. Everywhere there's pipework the easiest rather than best route has been taken.

[reddal]

1 hour ago, JSHarris said:

Somewhere I've got some graphs showing the comparison of two identical capacity tanks, one a thermal store, the other a UVC.  Ther thermal store has a heck of a lot less capacity to deliver hot water than a UVC of the same capacity and at the same temperature, because of the way it cools down.  A decent UVC should be able to deliver most of its capacity as hot water at the right temperature, which is its big advantage.

 

Some graphs in this post that illustrate your point :

- reddal

[jack]

These TS graphs always seem a little over-simplified to me.  

 

Ignoring the full contents of the coil at the full temp of the TS (easily a couple of litres depending on length/internal diameter) cold water coming in at the bottom is heated as it moves along the coil.  The water will gradually gain heat as it moves along the coil, so you end up with a gradient of heat along the coil's length.  Initially, full temp may be reached well before the water reaches the end of the coil.  In that case, the outlet temperature won't drop at all until enough energy has been removed that the water needs the full length of the coil to get to the initial temp of the TS (ignoring losses for this analysis).  The outlet temperature will start to fall at that point.

 

Due to this mechanism, the bottom of the TS is cooled faster than the top, resulting in a temperature gradient along the full height of the TS (ignoring mixing at the moment).  This means that although the average temp of water within the TS will fall at a particular rate given the length of the coil and temp differences, it will be warmer than that average at the top, and cooler at the bottom.  This means that the water at the outlet will be higher than the average temp of the TS at all times.  

 

I don't know how you'd model all of that in practice, but the short version is that TS performance isn't quite as bad as might be concluded based on modelling that doesn't take into account the generation of a temperature gradient as heat is extracted.  I believe this is one of the reasons that tall, narrow TSs are preferred.

[Jeremy Harris]

50 minutes ago, reddal said:

 

Some graphs in this post that illustrate your point :

- reddal

 

 

Thanks, those look similar to the ones I drew up some time ago, when trying to compare the performance of our old thermal store against that of the Sunamp PV.

[reddal]

11 minutes ago, jack said:

These TS graphs always seem a little over-simplified to me. 

 

Yes you are right - its very simplistic. It also doesn't take into account any losses etc.

 

However - I can tell you from experience that the basic idea is correct - that a TS temp will start to drop immediately and a cylinder much less so. Therefore if the TS temp is not much more than the target DHW temp it can only produce a limited amount of water at the target temp. In our initial installation, with the 300l TS at 50c - we couldn't get even a single bath or shower out of it before it wasn't hot enough. Once we added a (smaller) DHW cylinder it was fine.

 

Thermal stores probably work fine for DHW if the heat source is high temperature (80c say). Be very careful if planning to use a TS for DHW with a heat source at close to the required DHW temp (ie a ASHP or GSHP typically).

[Jeremy Harris]

28 minutes ago, jack said:

These TS graphs always seem a little over-simplified to me.  

 

Ignoring the full contents of the coil at the full temp of the TS (easily a couple of litres depending on length/internal diameter) cold water coming in at the bottom is heated as it moves along the coil.  The water will gradually gain heat as it moves along the coil, so you end up with a gradient of heat along the coil's length.  Initially, full temp may be reached well before the water reaches the end of the coil.  In that case, the outlet temperature won't drop at all until enough energy has been removed that the water needs the full length of the coil to get to the initial temp of the TS (ignoring losses for this analysis).  The outlet temperature will start to fall at that point.

 

Due to this mechanism, the bottom of the TS is cooled faster than the top, resulting in a temperature gradient along the full height of the TS (ignoring mixing at the moment).  This means that although the average temp of water within the TS will fall at a particular rate given the length of the coil and temp differences, it will be warmer than that average at the top, and cooler at the bottom.  This means that the water at the outlet will be higher than the average temp of the TS at all times.  

 

I don't know how you'd model all of that in practice, but the short version is that TS performance isn't quite as bad as might be concluded based on modelling that doesn't take into account the generation of a temperature gradient as heat is extracted.  I believe this is one of the reasons that tall, narrow TSs are preferred.

 

 

Part of the problem is the depth of the heat exchange coil as well, I think.  Most are pretty big to give the required heat output and so take up a fair bit of space in the tank.  A UVC just has a top outlet, so gains pretty much all that coil depth as useful hot water.

[jack]

1 hour ago, reddal said:

However - I can tell you from experience that the basic idea is correct - that a TS temp will start to drop immediately and a cylinder much less so. Therefore if the TS temp is not much more than the target DHW temp it can only produce a limited amount of water at the target temp. In our initial installation, with the 300l TS at 50c - we couldn't get even a single bath or shower out of it before it wasn't hot enough. Once we added a (smaller) DHW cylinder it was fine.

 

Thermal stores probably work fine for DHW if the heat source is high temperature (80c say). Be very careful if planning to use a TS for DHW with a heat source at close to the required DHW temp (ie a ASHP or GSHP typically).

 

Agree with all that.  I don't think anyone's suggesting that a 300L TS is going to be a high performing solution if only heated to just above the required outlet temperature.  My point is really just that when sized and heated appropriately the temperature fall-off isn't as severe as very simple modelling might suggest.

[ProDave]

The main difference is simply the way they work.

 

With an UVC you can set it to say 45 degrees, and it will produce hot water at very close to 45 degrees until it is all used up then it will very quickly go cold.  So in practice, the stored water temperature only needs to be slightly higher than the water temperature you want.

 

But with a thermal store you are not drawing the water that is in it, instead you pass cold water through a coli (or sometimes a plate heat exchanger) so as you run the hot tap, you are extracting energy fairly evenly from the WHOLE tank so the temerature in the tank starts to drop.  So in order to get a decent amount of water at your chosen 45 degrees, the stored water temerature needs to be a LOT higher.

 

That poses two issues, higher standing losses, and not so good to be heated by a heat pump which woks best at lower temperatures. 
 

[reddal]

13 minutes ago, ProDave said:

and not so good to be heated by a heat pump which woks best at lower temperatures

 

Indeed - thats what we found. One solution that was tried with our original install was to use the heat pump to get the TS up to 50c - then an electric immersion heater to boost it to 70c. However - there were also radiators coming off this TS - and we found that the immersion was on all the time keeping the rads hot - and the heat pump never came on at all. This cost a lot of electricity before we worked out what was going on. After much more grief we ended up with a TS and a seperate cylinder for DHW.

 

[Jeremy Harris]

34 minutes ago, reddal said:

 

Indeed - thats what we found. One solution that was tried with our original install was to use the heat pump to get the TS up to 50c - then an electric immersion heater to boost it to 70c. However - there were also radiators coming off this TS - and we found that the immersion was on all the time keeping the rads hot - and the heat pump never came on at all. This cost a lot of electricity before we worked out what was going on. After much more grief we ended up with a TS and a seperate cylinder for DHW.

 

 

That's similar to my experience, although in my case I was using the ASHP to preheat the thermal store and then an immersion run mainly from diverted excess PV generation to boost to 75 deg C, initially.  For heat loss, and resultant room overheating problems, I reduced the thermal store down to 65 deg C.

 

Practically, I've found that the 5 kWh Sunamp PV provides the same, perhaps better, performance than the 210 litre thermal store did, in a package a fraction of the size and weight.  My gut feeling is that a thermal store probably equates to a UVC of around 2/3rds the capacity, or thereabouts.

Edited February 10, 2017 by JSHarris

[bassanclan]

16 hours ago, ProDave said:

The main difference is simply the way they work.

 

With an UVC you can set it to say 45 degrees, and it will produce hot water at very close to 45 degrees until it is all used up then it will very quickly go cold.  So in practice, the stored water temperature only needs to be slightly higher than the water temperature you want

 

in a UVC if you draw off say 100litres of your hot 50c water is that replaced with 100litres of cold immediately or does the tank physically empty first, only ever filling up with hot water if say, your boiler or heat source sends it there?

If so how does this work with solar PV/immersion heaters?

[Nickfromwales]

9 minutes ago, bassanclan said:

 

in a UVC if you draw off say 100litres of your hot 50c water is that replaced with 100litres of cold immediately or does the tank physically empty first, only ever filling up with hot water if say, your boiler or heat source sends it there?

If so how does this work with solar PV/immersion heaters?

Cold mains water is forced into the very bottom of the UVC and pushes the hot water up and out of the hot outlet at the very top of the UVC. The heat source for the UVC is typically a 'wet' central heating water fed coil ( indirect ) which sits in the cold water and that heats the water up. The hot water from the heat source, and the water that comes out of your taps, never meet . 

An UVC which is fed electrically, only, is referred to as 'direct'. 

[Onoff]

Something's got to happen soon!

 

Wife was running a bath last night downstairs so hot from cylinder and cold from CWS. Son decides to have a shower upstairs at the same time. Shower is pumped, hot via Surrey flange in cylinder and own cold feed to pump from CWS. He switches the shower on and bath / shower both stop pretty rapidly. The tiny 25 gallon CWS tank I guess emptied meaning no flow to start the pump switch. At the same time the cylinder now has no head from the CWS. So.....he decides to have a shave.....no hot or cold in the upstairs basin!

 

I boiled him a kettle and went to bed after giving him the whole "Of course, when we were younger and ran out of coal!". To which he replied "FFS it's 2017!"

 

 

 

[Nickfromwales]

Do away with all cylinders etc and just fit an oil combi ? They're exceptionally good at producing DHW and you'll have a single box solution with no G3. 

Youll get a tidy Grant for around £2k

Internal

or chuck it outside in a coal shed

External

Seriously, it's a contender, unless your ever fitting PV. 

 

[Onoff]

1 hour ago, Nickfromwales said:

Do away with all cylinders etc and just fit an oil combi ? They're exceptionally good at producing DHW and you'll have a single box solution with no G3. 

Youll get a tidy Grant for around £2k

Internal

or chuck it outside in a coal shed

External

Seriously, it's a contender, unless your ever fitting PV. 

 

 

Ta. PV is tbh something we want to look at later.

 

Ref the boiler the existing one is seemingly 89% efficient. I'm sure when I very briefly looked at oil combis before they weren't that much better. Appreciate the current one hasn't got the combi advantages.

 

Where do I find out about grants?

 

Cheers

[Nickfromwales]

Grant is the make . 

[Onoff]

1 minute ago, Nickfromwales said:

Grant is the make . 

 

 

Seriously, type in "oil boiler grant".....like I did.