DIY Heatbank - fine tuning of system (incl CH)

Yup, that is what I meant by "every half hour indefinitely, or faster for a limited time" ;-)

Although looking again at the figures - you probably can't draw that much energy out of the store without it impacting the final water temp a little toward the end...

that 33.5MJ would represent a 26.5 degree store temperature drop without any replenishment. With 20kW going back in that would be a 22 degree drop. So the last bit of the bath fill would start to cool very slightly. If you were sticking 35kW back in then you would only see a 16 degree drop in the store and output temp would be unaffected.

Well if you allow a couple of degrees from loss on the PHE, then that

18 degree difference represents (so 300 * 4200 * 18 = 22.7MJ) of energy. Once you have drawn that and without any replenishment then the output temp from the PHE will start to fall below 60. That equates to about 100l of drawn water.

Not quite - you can't usefully get all of the energy out of the store unless you can accept a lower final water temperature. If you say that the minimum useful temperature of the store water is 50 degrees (to allow draw off at say 48 deg), and the maximum store temp is 80, then you have 30 x 4200 x 300 = 37.8 MJ for water heating with a 300l store.

With ground water at 5 degs (i.e. worst case in the winter) and a bath at 45 final temperature, that gives you no more than 300 / 40 x 30 = 225 l of hot water without replenishment. So with the boiler running, you can probably just squeeze two deep baths in quick succession out of the store. You would also need to adjust taps from time to time to allow for the falling hot water temperature.

Reply to
John Rumm
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But surely as the boiler is supplying to the top of the cylinder rather than further down, (and perhaps boiler is better connected using a surrey flange at top of cylinder), the output of the boiler is mainly going straight to the PHE. Therefore, although flow rate may have to be adjusted, you could continue to fill baths infinitely. This would act like a combi so would just take a bit longer to fill.

On the point of........ "Depending on the sophistication of the boiler and control systems you may get better results with the rads driven directly rather than from the store"

This is something I'm struggling with as boiler is new condensing.

Reply to
ian

Chav, please be quiet and do not comment on things you know sweet FA about.

Reply to
Doctor Drivel

Not a problem. The bottom of the store will be quite cool and condensing will occur most of boiler run time. Most boilers only modulate to about 7 to 8 kW minimum. Lower than that and boioer cycling occurs. Taking the CH off the store means that only 0.5 kW can be drawn off the store and no boiler cycling. Then no auto-by -passes that can affect the boiler, centralised room stats that cut out the boiler when some rooms may need heat, etc.

When a boiler is heating rads directly with all rads except one with TRVs and an auto by-pass fitted, when all or most TRVs close up the auto by-pass opens up and hot water flows back to the boiler, then the boioer efficiency drops like a stone.

Reply to
Doctor Drivel

Of a sort, yes. However the drop in performance from a 115kW combi to a (say) a 20kW one will make a dramatic difference in performance: 30 lpm of 60 degree water down to 9 lpm of 40 degree water. So you could fill that last bath slowly, but someone expecting shower at the same time is in for a bit of a shock.

I note on your schematic you had a blending valve on the boilers return. Presumably to maintain a minimum return temperature. While this is common practice where one wants to prevent an older design of boiler from seeing excessive periods of low return temperatures, it would be counter productive on a condensing boiler since there is no requirement to maintain a minimum return temperature. Artificially raising the apparent return temperature when cooler water is available would just lower the boilers condensing efficiency. It also adds an extra tapping to the top of the tank and associated pipework.

If you have a high end "smart" boiler with analogue room temp sensing or external weather compensation, then you would loose the ability to run very low flow temperatures through the rads during the milder parts of the year.

The presence of a big energy store between the boiler and rads can also affect the performance of the boilers control system since it introduces a big damping and delaying effect - possibly leading to unexpected fluctuation of the store temp in response to changing heating loads, and hence reduction in performance on hot water delivery. You also incur the cost of the two extra pumps for these circuits.

You need to balance the cost and complexity of all the hardware against what gains you get (both in performance terms and also running and maintenance costs). You may well find a decent boiler with a simpler S Plan+ system driving the rads directly and the store as another zone would work equally well or better.

Reply to
John Rumm

LOL!, one could suggest you do the same. You have demonstrated many times that your lack of grasp of control theory, surpasses even your famously poor grasp of basic physics.

Reply to
John Rumm

Chav, the man has come here for advice. There are people here who know this field backwards and their experience does not entail fitting one combi. Please be quiet and do not comment on things you know sweet FA about.

Reply to
Doctor Drivel

Chav, it is to maintain a high flow temperature into the thermal store for DHW purposes. The CH section of the cylinder need not have the return blending valve on the flow/return. It also keeps the boiler within its delta T ensuring correct operation and longevity of the heat exchanger..

There is a requirement to maintain a Deta T on all boilers

DHW has to be....well hot. And there is no getting away from the fact that the return temp will rise to create DHW. The Blending valve may be set to

50C with a boiler giving a 25C temperature rise. Many stores may run at 70C comfortably as plate Heat Xs are so efficient. That is a return temp of 45C, which is very efficient.

No boiler modulates very low and they all cycle. He can fit a weather compensator to the CH section of the cylinder and lower and raise that water body to maintain outside weather influence and this will result in very low return boiler temperatures being created.

Reply to
Doctor Drivel

Par for the course...

Reply to
John Rumm

Blending valve added as it was suggested on the original thread that the temperature difference between the boiler flow and return should be around the 20 deg mark to allow optimum condensing. To be fair, when I asked DPS for a quote, they included a blending valve too on their spec. In fact they said it should be a 28mm version for boilers of >20KW as the internal parts on a 22mm were too small.

It was considered better to use pumps rather than zoning valves due to reliability.

In all honesty, I just want a good mains presure system that will supply enough hot water for 2 showers running at once, where I don't have to wait 30min+ to refill a bath and that ideally gives me 2 zones(upstairs and downstairs). I have a new 28KW condensing system boiler and now need a cylinder. Don't want to pay =A31300+ from DPS for same design as you see here (and that's kit form and no metal casing round the copper cylinder to make it look pretty either !) and don't want an unvented. Had liked the idea of CH off the store but really only due to the fact that the contents of the store could be dumped into the rads for instant heat.If I could get something for around =A3800 I think I'd be happy!!!

Reply to
ian

Yup, that I what I thought it looked like they were trying to do... It makes good sense on a fixed output boiler, and especially on a non condenser. Not so sure it adds much with a condenser that will modulate to lower return temperatures, and will give best condensing performance with the lowest return temperature available....

As an aside, have you considered using a indirect store with a fast recovery coil carrying the primary heating water? That would allow the use of a sealed system primary to drive the rads and the store. Would do away with the header tank as well. You could then have a quite simple system that would also provide the hot water performance you want without needing an unvented cylinder.

Simple two port valves seem quite reliable if exercised from time to time during the summer...

Yup, makes sense. Since you have the modern boiler, and I take it you have enough cold mains flow rate available, I would be inclined to leave the rads driven directly by the boiler (in their own zones). With a prog stat per zone and TMVs on the rads you are going to end up with pretty good efficiency and performance anyway on the space heating. Even if the boiler does cycle a little when the house is warmed up, that is far less of an issue with modern low water content boilers.

Its easy to end up adding more and more complexity to these solutions and before long the parts bill alone is scary (something you have to watch with Dr. Dribble's designs - they soon end up with half the contents of the BES catalogue used in there somewhere!)

Reply to
John Rumm

I have considered using another PHE to heat the store(particularly as my boiler is ideally supposed to work on sealled systems but will apparently work on vented if pressure sensor is disconnected). Thought this better than coil as can be cleaned easily if necessary. Also I assume order of efficiency is coil,PHE and best is heat store direct.

Wouldn't I still need the tank as a) stored water will still expand b) stored water will need topping up from time to time?

Reply to
ian

Chav, you really haven't a clue, even after an explanation. It guarantees the highest temperature set so only very hot water enters the store at the top of the cylinder ready for use to give useful DHW, even after 2 minutes of running from cold a sink can be filled. It also guarantees the boiler is operating within its delta T and maximises thermal expansion. It guarantees top down heating of the store and the water is heater one pass of the boiler quickening the re-heat. It also increases boiler longevity as the boiler is operating in an ideal hydraulic environment.

Again the ignorance abounds. Having the two pumps means the CH zones are fully independent (one has no effect on the other which is not the case when using one pumps and zone valves) of each other as they both go back to the neutral point - the store - which means superior balancing of rads and that two smart pumps may be used using TRVs on "all" rads. Also the pumps can inject even 0.25 kW into their respective CH zones, increasing comfort.

That is because you know sweet FA about thermal storage and heating systems.

The boiler still cycles, and the auto by-pass will eventually restrict flow through the boiler as it wears or open up too early and elinminates condensing as a direct short cut is created.

Having the CH zones off the store is a great bonus. Having the boiler only heat the heat bank cylinder is great bonus.

The store is a great neutral point. Each function of the system can be isolated and optimised and one does not influence the other. The boiler is de-coupled and optimised for performance. As are the CH zones, as is the DHW take off too. The boiler can be optimised for condensing efficiency - heat bank setpoint of 70C and return temperature of 45C. This means the boiler condenses the vast majority of run time and operates in an ideal hydraulic environment.

Forget all this tripe that a modulating boiler is the best option coupled to rads. It may be if it modulates from 0kW to 27kW and no TRVs on the rads. As no boiler goes down to 0kW that will not happen. The auto by-pass screws up efficiency performance and the cycling reduces reliability putting heavier demand on controls. The auto by-pass can screw up the heat exchanger is not enough flow is allowed through the rad.

Most of the time a boiler is on part load, and this means a lot of the time the auto by-pass valve is open eliminating condensing competely with efficiency dropping like a stone.

Reply to
Doctor Drivel

I assume you mean the low pressure sensor. See this:

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is a pressurised store running at 2.5 bar, so the boiler will operate fine. Can be DIYed using plate heat Xs. They may make one to order without the coil and insert the tapping where you want.

I think you mean in reverse order. The coil is way less efficient than a plate heat X. Direct is the ultimate.

Reply to
Doctor Drivel

["Chav" is a stupid and judgemental term and I do not wish to be asscoiated with it.] On the other hand, DD's explanation for the blending valve seems plain common sense and was part of my assumption. Also these thermal store systems only work well (high hot water flow) if the primary side of the PHE (and hence the store itself) is at a very high temperature (75-90degC). This can only be achieved by subtracting the boiler's maximum delta-T from the desired temperature and ensuring the answer is used as the set point for the return water. The condensing efficiency argument is irrelevant as the whole system won't work without the higher than normal return temperature.
Reply to
Bob Mannix

Another PHE would do the trick quite nicely. While these are not cheap, you would not need such a large one for this application since the boiler power is the limiting factor here. You may even save equal or more money by not needing an expensive cylinder with fast recovery coil. (any direct tank could be used). Water to water PHEs are quite efficient so you would not loose much if anything over pumping the primary through a direct store.

Your system boiler will already include an expansion vessel designed to cope with the expansion of the primary circuit. The water in the store itself will also expand and contract, but this does not need to be hermetically sealed and can have a small expansion gap at the top (assuming your pumped PHE connections are via side tappings on Essex flanges).

It can be filled from a hose (or suitably positioned fixed pipework and tap of some sort). The only route of water loss from it will be via evaporation.

Reply to
John Rumm

A Chav is a Chav and he is one. And like you I want nothing to do with them. All those tattoos and heavy jewellery!

Nope. Plate heat exchangers are so efficient (and some are better than others, like the Danfoss two-pass and the Swep used by Gledhill), that a store temperature of 65C can be set and they will give more than adequate DHW. The higher the temperature the more energy stored and the smaller the cylinder, the lower the temp the larger the water store.

More old wives tales. A longish cylinder ensures that the bottom of the cylinder has "very" low temperatures - superior stratification. Plate heat Xs are so efficient at extracting heat the return temperature from them will be 30C and below.

Thermal stores using an immersed coil DHW take off require higher temperatures because the coils are so inefficient to a plate heat OX.

A well designed CH & DHW heat bank knocks cobs off a direct to the rads boiler CH system. They are very efficient and highly reliable.

The design presented is a good one. If the points I suggested to improve are implemented and the CH section heated direct not via a return blending valve (a 3-way diverter valve is needed) a weather compensator controlling in the water temperature in the CH section of the store the efficiency will be "very" high.

Heat bank - plate heat X DHW thermals store = immersed coil DHW

Reply to
Doctor Drivel

It is cheaper, far more efficient in economy, and far quicker using a plate heat X and bronze pump heating up a large direct cylinder. 95% plus of the boilers heat is extracted via the plate and put into the fresh water in the cylinder.

Here is an off-the-shelf model:

You are describing a DPS Pandora. They have a few safety controls on that and a deflated expansion vessel on the top of the cylinder.

Reply to
Doctor Drivel

The blending valve will indeed keep the boiler flow hotter (at least in the early recovery stages after heavy demands have been made on the store, and assuming you have very well controlled stratification in the store), but there are many interacting variables and trade-offs here. These are at their most complex with a directly heated store since you can't control the heating load presented to the boiler as easily. It is worth bearing in mind that this design is being proposed for use with a modern modulating, condensing boiler that supports wide temperature deltas. The whole "thermal store with everything hung off it" design concept is very well suited to older boiler designs where long burns, reduced cycling, high flow temps (and maintained above a threshold return temps) are all "good things". Hence you have a nice "rule of thumb" solution to a number of problems. The whole situation is much less clear cut here.

Dribble is right that running rads from the store will let them run at low powers, which was a big gain when the alternative was 20kW or nothing and a mechanical stat overseeing the outer loop control. When the choice is anything from 7[1] to 28kW or nothing and tighter outer loop control, you are not going to notice much difference in a typical house with a few kW of heat losses.

([1] number picked at random - I don't know how low the OPs boiler will modulate).

Higher store temps are obviously good from an overall energy density point of view, but less good from a condensing efficiency one. The temperature you set for your hot water take off also has a big impact since it dictates how much headroom you have under the store temperature, and when falling store temperature will start to impact on the HW temperature. Opting for water at a temperature close to that of final use, and with less mixing at the point of use, will give more consistent water temperature output form the store with less worries about maintaining high and well stratified store temperatures.

With the modern boiler, the most efficient way to get the high flow temp is to take advantage of a bigger delta T. If your boiler supports analogue control, then even better. You can select higher temps for replenishing the store, and much lower ones for running the heating (which is where you will probably spend most of your money - and so efficiency is of greatest importance).

Reply to
John Rumm

This also is a two-stage DHW delivery too. If he cylinder is exhausted of heat it reverts to what the boiler gives like a combi. If no draw-off it just re-heats the cylinder. So, even better performance from using a plate rather than a coil.

Reply to
Doctor Drivel

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