thanks Simon, I worked out I need a 100kw capacity. I'm thinking I won't fit a TMV, seems pointless to me when you can just adjust the temperature on the cylinder stat to get the right hot water temperature instead, is that practical, or will it just mean that if the flow rate is low, Ill get boiling hot water and when it increases, it'll get less hot?
Convert existing vented cylinder to thermal store and PHE
- Thread starter 1028741
- Start date
thanks Simon, I worked out I need a 100kw capacity. I'm thinking I won't fit a TMV, seems pointless to me when you can just adjust the temperature on the cylinder stat to get the right hot water temperature instead, is that practical, or will it just mean that if the flow rate is low, Ill get boiling hot water and when it increases, it'll get less hot?
Sponsored Links
its much better have a TMV. its a more efficient way of using the energy you have stored and as a result will last longer.
It really depends on how you run the store.
IF you can arrange things that the store is hot at the top, cold at the bottom, with a distinct transition between them, and you take hot water out the top and return it cold to the bottom - then you could get away with controlling the DHW temp from the store temp.
This is much how an open vented cylinder works - you draw hot out the top and put cold in at the bottom to replace it.
But, to do that means having the primary flow through the PHE matched to the DHW flow rate (and inlet temp) at all times - so at a minimum a thermostatic valve and/or electronic pump speed control to regulate the primary flow rate (restrict the flow if the return temp is above some fairly low setpoint). The usual setup I was offered when I was looking at options for the flat has the primary flow rate essentially uncontrolled - which means you will nearly alway be returning "partly used" hot water to the bottom of the store. This means that as you draw off the stored heat, the store is getting cooler - and so would your DHW.
If this is how it's working then you'd need to tart with the store temp above your required DHW temp to give you some headroom - ie so the store can lose some temperature before you notice the DHW going cool.
But rather than a TMV on the outlet of the PHE, you could fit individual ones on each hot tap - which is (I believe) the recommendation or requirement for many situations (public buildings, residential homes, etc, etc). This means the store and pipes can get nice and hot (people who run such buildings are rightly cautious about legionnaires disease), while the water out of the hot tap can never be scalding hot (and you can have different setpoints for different taps).
IF you can arrange things that the store is hot at the top, cold at the bottom, with a distinct transition between them, and you take hot water out the top and return it cold to the bottom - then you could get away with controlling the DHW temp from the store temp.
This is much how an open vented cylinder works - you draw hot out the top and put cold in at the bottom to replace it.
But, to do that means having the primary flow through the PHE matched to the DHW flow rate (and inlet temp) at all times - so at a minimum a thermostatic valve and/or electronic pump speed control to regulate the primary flow rate (restrict the flow if the return temp is above some fairly low setpoint). The usual setup I was offered when I was looking at options for the flat has the primary flow rate essentially uncontrolled - which means you will nearly alway be returning "partly used" hot water to the bottom of the store. This means that as you draw off the stored heat, the store is getting cooler - and so would your DHW.
If this is how it's working then you'd need to tart with the store temp above your required DHW temp to give you some headroom - ie so the store can lose some temperature before you notice the DHW going cool.
But rather than a TMV on the outlet of the PHE, you could fit individual ones on each hot tap - which is (I believe) the recommendation or requirement for many situations (public buildings, residential homes, etc, etc). This means the store and pipes can get nice and hot (people who run such buildings are rightly cautious about legionnaires disease), while the water out of the hot tap can never be scalding hot (and you can have different setpoints for different taps).
Oh I get it, makes sense, so, you store water at a moderately higher temperature than you need it, and fit a TMV with a set maximum temperature for the DHW, so that, as the stored water cools down, it's still hot enough to provide DHW without you noticing the drop in temperature. I'll do this then, although we usually have the boiler's hot water programmer switched on when we're taking baths and showers anyways, so the boiler would probably be replenishing the tank fairly well in that case
Thanks alot for your help, been very useful.
Sponsored Links
Yes, the boiler will assist - provided the system is set up right.
Back to the storage, another way to think about it is :
If you heat the store to (say) 60˚ and by having a perfectly matched HE setup, manage to cool it down to (say) 10˚ - then you have 50˚ worth of stored heat. But when it runs out it will do so quite abruptly.
On the other hand, if you have an unconstrained primary circuit and manage to "mix it all up" while drawing off the heat, then after drawing off just one fifth of the stored heat compared to the first scenario, you'll be down to a tankful of water at 50˚. After another fifth of the heat drawn off, you'll be down to 40˚ - so shile the drop in temperature is not abrupt - you will probably get to below your target temperature a lot sooner.
Back to the boiler. With 24kW available, that means that for 100kW drawoff, you only pull about 3/4 of it from the store. Thus assuming well setup HE, you can draw off around 4/3 of the size of the store before it abruptly runs out. In practice you'll never get perfect stratification like that, so there will be some tailing off - and if you keep the drawoff going, you end up with the equivalent of a 24kW combi.
For this to work, the boiler really needs to be able to do a "top down" reheat of the store at the temperature you need. Since I assume you intend to run this as an indirect system (ie heating via a coil in the cylinder), then this isn't going to happen well. You'll heat the water round the coil, and only if you can get that hot enough will it rise to the top of the store - but it will mix as it goes. And of course, if the load is high enough, the boiler may not be able to maintain a high enough temperature for that to happen.
You'll also be limited by the capacity of the coil - if the manufacturer does quote a figure, then look carefully at the conditions since that's an easy way to provide misleading but "attractive" figures. Eg, it's easy to make a coil with a super high rating if you specify a cold cylinder and a boiler set at some silly high temperature - but with a partly warm store and more sensible boiler setting it may well only be a small fraction of the headline figure.
Back to the storage, another way to think about it is :
If you heat the store to (say) 60˚ and by having a perfectly matched HE setup, manage to cool it down to (say) 10˚ - then you have 50˚ worth of stored heat. But when it runs out it will do so quite abruptly.
On the other hand, if you have an unconstrained primary circuit and manage to "mix it all up" while drawing off the heat, then after drawing off just one fifth of the stored heat compared to the first scenario, you'll be down to a tankful of water at 50˚. After another fifth of the heat drawn off, you'll be down to 40˚ - so shile the drop in temperature is not abrupt - you will probably get to below your target temperature a lot sooner.
Back to the boiler. With 24kW available, that means that for 100kW drawoff, you only pull about 3/4 of it from the store. Thus assuming well setup HE, you can draw off around 4/3 of the size of the store before it abruptly runs out. In practice you'll never get perfect stratification like that, so there will be some tailing off - and if you keep the drawoff going, you end up with the equivalent of a 24kW combi.
For this to work, the boiler really needs to be able to do a "top down" reheat of the store at the temperature you need. Since I assume you intend to run this as an indirect system (ie heating via a coil in the cylinder), then this isn't going to happen well. You'll heat the water round the coil, and only if you can get that hot enough will it rise to the top of the store - but it will mix as it goes. And of course, if the load is high enough, the boiler may not be able to maintain a high enough temperature for that to happen.
You'll also be limited by the capacity of the coil - if the manufacturer does quote a figure, then look carefully at the conditions since that's an easy way to provide misleading but "attractive" figures. Eg, it's easy to make a coil with a super high rating if you specify a cold cylinder and a boiler set at some silly high temperature - but with a partly warm store and more sensible boiler setting it may well only be a small fraction of the headline figure.
I'm going to heat the tank 'directly' (I'm going to cut 2 new flanges into it, making the coil redundant, and getting the full 24kw heating as opposed to just 4kw). Is there a way to make sure I won't run out of hot water as long as the boiler is replenishing the tank at the same time? Do I need to feed the tank from the boiler "top-down" i.e. flow at the top and return at the bottom then? At the moment, the full tank of water is used to fill the bath, so I don't want to end up with less hot water! Also, would this pump be suitable to circulate water around the PHE? http://www.ebay.co.uk/itm/Wilo-Dome...DIY_Materials_Plumbing_MJ&hash=item2c7074814d
Links in this post may contain affiliate links for which DIYnot may be compensated.
Would this work then? The return from the PHE doesn't go back to the bottom of the tank, but goes to the return on the boiler instead, so, when the boiler and its circulator pump is firing, the cold return from the PHE is 'sucked' through the boiler and back to the top of the tank, and no mixing occurs in the tank, and only when the boiler is off will the water return to the bottom of the tank and mix
In the general case, I'd say not. It would help when the store is well depleted and the demand is heavy - so the boiler is actually running full time, but in general the boiler tends not to hot at the same time as DHW drawoff.
You see, in general, you'll draw off hot water and the boiler won't fire immediately (or at all if the draw off is small enough). At some point, the tank stat will trip and the boiler will fire up - and is likely to keep running after the drawoff stops until the tank is hot enough for the stat to trip off.
In the "filling the bath" scenario - there will be considerably more flow through the cylinder than through the boiler - so it won't make all that much difference.
IMO, the question is really one of "how to ensure the boiler flow is always "sufficiently hot". In realistic terms, for the setup under discussion, I suspect it can be done just by carefully setting the flow rate - and I'd interlock the controls so the CH can't be on at the same time. So set a throttling valve so that under expected store bottom temperatures, the flow from the boiler is "sufficiently hot" to keep the bath filling at an adequate temp. This probably means "bending" the manufacturer's instructions slightly by running at a higher delta-T than they specify.
For example, generally guestimating some numbers. Suppose you decided that the minimum temperature you could cope with was 45˚, and you could throttle the flow such that the boiler had 30˚ delta-T at full power. That would mean that all the time the boiler return (from the store bottom) was above 15˚, the boiler could keep you going with 45˚ water. When the boiler return is above 15˚, the boiler flow will be hotter until it starts to range down to control at it's setpoint.
So you run the bath, initially you use the stored heat, then the boiler kicks in. If it's return is at 15˚ or above, it'll supply water at 45˚ or above. Initially I'd expect the return to be well above 15˚ so the boiler will range down to maintain (say) 60˚. As you consume the stored heat, everything will cool down (from the bottom up), and you should see a decline at the store bottom (and boiler return) temp drops. If the flows are fairly well matched, then the return from the PHE will be quite cool when running at full DHW flow rate.
Since you may be getting cooler water from the boiler than you have stored, you probably want to put the boiler flow into the store part way down - so when it's hot it will thermosyhon up to the top, but when it's cooler it'll fill up underneath the hotter water (or more likely mix with what'll still be hotter water). When you've consumed the hot water from above the boiler flow connection, you'll then get water at the boiler flow temp until you've largely depleted the store. You don't want the boiler flow too low down either - otherwise you'll find that you get a store full of a mix of the partly hot boiler flow and the fairly cool PHE return.
Try and picture (add arrows and numbers to a sketch if it helps) what flows you'll have, and what temperatures. Think about what this will do to the store, and how this will change over time. Perhaps I'm over analysing it - I suspect most just stick it in and work on having a "big enough store" plus "big enough boiler" to not worry about the details
Bear in mind that when drawing anything less than full DHW flow rate, you'll be putting warm water back in at the bottom of the store - the slower the flow, the warmer it'll be. It doesn't really matter in this case, but when trying to extract the most from (eg) solar panels, you try and get the bottom of the store as cold as possible so it'll accept the most heat from the panels on a middling day.
You also want to consider your boiler. If the return temp gets to about 54˚ or above, it'll stop condensing.
Links in this post may contain affiliate links for which DIYnot may be compensated.
In the general case, I'd say not. It would help when the store is well depleted and the demand is heavy - so the boiler is actually running full time, but in general the boiler tends not to hot at the same time as DHW drawoff.
You see, in general, you'll draw off hot water and the boiler won't fire immediately (or at all if the draw off is small enough). At some point, the tank stat will trip and the boiler will fire up - and is likely to keep running after the drawoff stops until the tank is hot enough for the stat to trip off.
In the "filling the bath" scenario - there will be considerably more flow through the cylinder than through the boiler - so it won't make all that much difference.
IMO, the question is really one of "how to ensure the boiler flow is always "sufficiently hot". In realistic terms, for the setup under discussion, I suspect it can be done just by carefully setting the flow rate - and I'd interlock the controls so the CH can't be on at the same time. So set a throttling valve so that under expected store bottom temperatures, the flow from the boiler is "sufficiently hot" to keep the bath filling at an adequate temp. This probably means "bending" the manufacturer's instructions slightly by running at a higher delta-T than they specify.
For example, generally guestimating some numbers. Suppose you decided that the minimum temperature you could cope with was 45˚, and you could throttle the flow such that the boiler had 30˚ delta-T at full power. That would mean that all the time the boiler return (from the store bottom) was above 15˚, the boiler could keep you going with 45˚ water. When the boiler return is above 15˚, the boiler flow will be hotter until it starts to range down to control at it's setpoint.
So you run the bath, initially you use the stored heat, then the boiler kicks in. If it's return is at 15˚ or above, it'll supply water at 45˚ or above. Initially I'd expect the return to be well above 15˚ so the boiler will range down to maintain (say) 60˚. As you consume the stored heat, everything will cool down (from the bottom up), and you should see a decline at the store bottom (and boiler return) temp drops. If the flows are fairly well matched, then the return from the PHE will be quite cool when running at full DHW flow rate.
Since you may be getting cooler water from the boiler than you have stored, you probably want to put the boiler flow into the store part way down - so when it's hot it will thermosyhon up to the top, but when it's cooler it'll fill up underneath the hotter water (or more likely mix with what'll still be hotter water). When you've consumed the hot water from above the boiler flow connection, you'll then get water at the boiler flow temp until you've largely depleted the store. You don't want the boiler flow too low down either - otherwise you'll find that you get a store full of a mix of the partly hot boiler flow and the fairly cool PHE return.
Try and picture (add arrows and numbers to a sketch if it helps) what flows you'll have, and what temperatures. Think about what this will do to the store, and how this will change over time. Perhaps I'm over analysing it - I suspect most just stick it in and work on having a "big enough store" plus "big enough boiler" to not worry about the details
Bear in mind that when drawing anything less than full DHW flow rate, you'll be putting warm water back in at the bottom of the store - the slower the flow, the warmer it'll be. It doesn't really matter in this case, but when trying to extract the most from (eg) solar panels, you try and get the bottom of the store as cold as possible so it'll accept the most heat from the panels on a middling day.
You also want to consider your boiler. If the return temp gets to about 54˚ or above, it'll stop condensing.
ok, thanks for all this advice, I hope you're getting something from it btw and I'm not just draining your knowledge base lol. The only reason I thought of returning the PHE to the boiler's return is so that WHEN the hot water valve is switched on (usually the case when taking baths), IF the tank stat temperature goes below say 55-60 degrees whilst running a bath, the cold return from the PHE will stop returning to the tank, cooling it further, and go straight back to the boiler for heating instead, that's my thinking. Otherwise, the boiler will be trying to heat all the cold water in the tank instead of just providing all the heat to the top of the tank and keeping my bath water hot. Catch my drift? Plus that would mean I only have to cut one new flange into the top of the tank, instead of 2
Links in this post may contain affiliate links for which DIYnot may be compensated.
BTW - please don't quote entire posts. In situations like this where you are "just replying" then nothing needs to be quoted. Where you're replying to specific points, then quote enough to set the context.
<clink>Penny drops
<original comment deleted>
I see now what you're getting at. Once the boiler kicks in, if it's flow temp is lower than that through the store, then your flow to the PHE will be a mix of that and what's coming from the store - you won't be mixing within the store itself. But you'd want to tee the boiler flow and PHE flow together at the top of the tank as well (so no new flange required at all )
One issue I can see is that if you tee stuff together and share a flange, there is the rick of drawing water through other circuits - eg the PHE loop pump would draw some water through the boiler even when the boiler isn't running. A simple check valve may be enough to stop that (the pressure drop through the flanges being insufficient to lift the plunger off it's seat).
Personally, I'd be putting a flange in (say) about 1/3 to 1/2 of the way up and running my CH from it with a modulating pump. Works well in the flat.
<clink>Penny drops
<original comment deleted>
I see now what you're getting at. Once the boiler kicks in, if it's flow temp is lower than that through the store, then your flow to the PHE will be a mix of that and what's coming from the store - you won't be mixing within the store itself. But you'd want to tee the boiler flow and PHE flow together at the top of the tank as well (so no new flange required at all
)One issue I can see is that if you tee stuff together and share a flange, there is the rick of drawing water through other circuits - eg the PHE loop pump would draw some water through the boiler even when the boiler isn't running. A simple check valve may be enough to stop that (the pressure drop through the flanges being insufficient to lift the plunger off it's seat).
Personally, I'd be putting a flange in (say) about 1/3 to 1/2 of the way up and running my CH from it with a modulating pump. Works well in the flat.
Sorry about that.
okeydoke, I think I'll tee the returns at the bottom, so that when the boiler's running, very little spent cold water from the PHE is being returned to the bottom of the tank and instead going back to the boiler for heating and returned to the tank hot. I think I'll still use an extra flange at the top for hot supply from the boiler around 1/4-1/3 the way down. Reason being, the tank is then the predominant source of heat for the PHE (even when the boiler is on), and therefore the tank and all its stored heat isn't being 'byassed' straight through the boiler loop in effect whilst the boiler's running. I don't follow how water could be drawn through the boiler loop when it's not firing though (because the motorised valves close off when the boiler's not running, preventing flow and diverting the PHE loop around the cylinder only)
okeydoke, I think I'll tee the returns at the bottom, so that when the boiler's running, very little spent cold water from the PHE is being returned to the bottom of the tank and instead going back to the boiler for heating and returned to the tank hot. I think I'll still use an extra flange at the top for hot supply from the boiler around 1/4-1/3 the way down. Reason being, the tank is then the predominant source of heat for the PHE (even when the boiler is on), and therefore the tank and all its stored heat isn't being 'byassed' straight through the boiler loop in effect whilst the boiler's running. I don't follow how water could be drawn through the boiler loop when it's not firing though (because the motorised valves close off when the boiler's not running, preventing flow and diverting the PHE loop around the cylinder only)
Ah yes, in your case that's true.
For my install, there's not a single motorised valve as I run the CH from the store - though with the store in the basement, there's a slight issue of thermosyphoning round the CH loop.
Simon, thanks for all your help. Big learning curve. I will be hacking into my airing cupboard shortly. Going to attempt to put all the pipes in a nice neat series, supported by unistrut channel and munsen rings something like this
lol. I'm a bit obsessively neat. wish me luck.
I think that layout has some serviceability issues - for example, you appear to have pumps hidden behind pipes.
Can you ease your constraints by using more of the height of the wall ? If you make the shelves easily removable, then access shouldn't be a problem - it "just" means moving some towels/whatever and lifting the shelf out. In our airing cupboard, Dad's made a shelf unit that fits into the space alongside the cylinder (it's shaped to follow (partially) the shape of the cylinder) and in front of the boiler - which just wheels out if we need access to anything.
Can you ease your constraints by using more of the height of the wall ? If you make the shelves easily removable, then access shouldn't be a problem - it "just" means moving some towels/whatever and lifting the shelf out. In our airing cupboard, Dad's made a shelf unit that fits into the space alongside the cylinder (it's shaped to follow (partially) the shape of the cylinder) and in front of the boiler - which just wheels out if we need access to anything.
DIYnot Local
Staff member
If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Are you a trade or supplier? You can create your listing free at DIYnot Local
Sponsored Links
