DPS thermal store DHW performance

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Thanks for the input/advice guys.

Johnsville - I'll get the TS stats checked out first by my sparkie. Your explanation of the operation of the plate HX is exactly as I was told when I bought the TS. DPS assured me that the HX on the boiler side would mean fast and efficient transfer of heat from boiler to TS primary water. The plate HX on the DHW would mean fast and efficient heat transfer from the hot store water to the incoming cold mains. These 2 combined would mean large quantities of hot, mans water would always be avavilable.

silverback - johnsville has descrbed the same function as DPS when they sold me the tank.

lcgs - The number is CPC-250-ABBDI-IAAB It does not appear in the DPS website. I have emailed them and will be following up with a phone call (but if the are in liquidation I don't fancy my chances...) I will post more details when I get home (away working at the moment).

simond - I'll be looking at the air potential also and will check out the air relief valve. I think it is self bleeding from memory but it could be blocked with something.

Will be concentrating on scaled up HX once I've determined the control side (stats) are functioning correctly. I need to drain down the rad and boiler to replace some TRVs so will take the opportunity to do all the work at the same time...

Will re-post once I have more info and any new developments.

 

[Biggles..]

DPS has indeed vanished but the thermal store bit has been taken over by another company so parts etc are still being produced,weather they'll warrent old DPS stuff i dont know.

What is for sure is that having sent through a spec for a store just last week, the prices have doubled since the take over and i will have seious doubts as to wether its worth using them again.

produt code works on the web site, single external HX14(100KW/30 litre heat ex)
2 grunny pumps.I'll have a look and see how it was designed to work.

Lee

 

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lcgs - I searched on the heatweb site and got the spec sheet. The problem is the spec sheet is not for my unit.... I will try to explain what is different.

1. I have a HX 14 on the boiler input ot the store.
2. I have tappings that supply my UFH from the store.
3. I don't have any immersion heaters 4.

Once I get home I will post a copy of the spec sheet for my unit (I have one in a file somewhere).

I paid 2k for my store and had a hard time justifying htis to the wife (especially when she wanted curtains) - if they have doubled in price you'd have to use an awful lot of water to break even on cost... (unless the cost of gas keeps rising at the same rate).

Hope this helps.

 

[johnsville]

Rotex from Belgium (thanks to Silverback) make a "plastic" thermal store using coils.
http://en.rotex-heating.com/products/hot-water-storage-tank.html

They also make a condensing boiler and thermal store, with the boiler on top.

http://en.rotex-heating.com/products/boiler/gas-condensing.html

 

[silverback]

johnsville, I am sorry I disagree, for what a plate heat exchanger does it would not be placed in a body of water as this would reduce its efficiency to pass its heat to the intended supply on the other side of the plate, as it would be losing heat to the surrounding water, the heat exchangers within the store will be a coil not a plate.

See here http://www.heatweb.com/Xcel.pdf the plate heat exchanger is always shown outside the store

or see here, scroll down to operation, this clearly shows how a Plate heat exchanger works, (Externally) http://www.heatweb.com/products/cylinders/heatbank/heatbank.htm

Silverback, having a plate heat X inside a thermal store cylinder would not reduce its efficiency, it is the reverse. In the case of having one between the boiler and cylinder it is clearly better inside as all heat is pumped into the cylinder with no heat loss outside of the cylinder.

Many makers will put them inside if you want and some do it as standard. I think the UFH company, NUheat have an integral DHW plate as standard. Having them inside makes better packaging reducing the bulk of a thermal store to the size of a normal unvented cylinder. A vented cylinder with a removable lid can have the plates inside for servicing, as some Continental makers do. Who tend to have pressurized thermal stores as standard. It is better all around.

In this case, I assume the reason for the plate between the boiler and the cylinder is to have a cheap pressurized boiler on a vented cylinder, mixing a high pressure circuit with a low pressure circuit. It also keeps the boiler's main heat X clean inside all the time, maximizing boiler efficiency as there is only a few litres of water in the whole boiler and plate heat X. Plate heat X's are highly efficient. If a plate is increased in size on the DHW, you will find the heat extracted from the hot water is unbelievable. The water returning is very cool. This then gives in the bottom of the cylinder a cool body of water sent back to the boiler which ensures it condenses at 95% and above of running time.

Coils in thermal stores have a nasty habit of cracking. They have to be of top quality materials and construction to avoid cracking. The coil can be in 80C of water for hours and then a thermal shock of 5C water from the cold mains comes in. This cracking tends to be more in copper coils, that is one reason why many thermal stores now are moving to stainless steel. This is where plate heat X's are far superior as they can absorb thermal shocks, the heat exchanger surface area is far larger than a coil and they are tiny in size to a coil.

The only drawback of a small plate is that both sides of the plate have to be pumped. Unless you use a large floor standing commercial plate heat X. One of these is the size of a small cylinder. The water content of one side of the plates maybe be many litres ensuring instant DHW at the taps and enough to fill one third to half a bath, with the boiler sized big enough to instantly heat the cold mains water. The primary side is also many litres, making it a small thermal store capable of running CH off it. The problem is that they are "heavy" and very expensive and require some thought on the control side. If a clever manufacturer thought hard about it, one could be made for domestic use packaged with a boiler.

Personally I prefer to use plate heat exchangers as they were designed to be used, these companies trying to use it in different ways is oftened flawed in my opinion, coils belong in stores imo

 

[johnsville]

A plate is another form of heat exchanger. Having them inside a cylinder makes a lot of sense for reasons outlined. Having one inside a cylinder with a removable lid makes even more sense as you can get at them.

 

[silverback]

If the plate heat exchanger is in the body of water, then I can't see the benefit, you may as well use a coil, as it now requires the addition of a pump to transfer the heat

 

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Silverback - does larger surface area (of the plate HX against coil) not mean better heat transfer?
The cost of running the pump isn't that high - in my case anyhow.

 

[SimonH2]

Silverback - does larger surface area (of the plate HX against coil) not mean better heat transfer?
The cost of running the pump isn't that high - in my case anyhow.

Yes and no. Yes, you are right that it's easier to build a small PHX than a small coil of the same capacity - but saying one has a higher capacity than the other doesn't really make sense as any type can by built for a specific capacity. It's more a case of "for a given physical size" a PHX has a higher capacity.

The thing (or rather one of the things) that put me off the DPS store was the pumped primary for the DHW circuit. At full DHW flow rate, and assuming the correct sizing of exchanger and setting of pump, the return into the bottom of the tank will indeed be cold. But at lower DHW flow rates, the return temperature will be higher. If your DHW flow tends to be at low rates (as might be the case with a shower with a system sized for running baths) then you'll put a lot of "nearly hot" water back in at the bottom and destratify the store. If you've got any plans for solar panels, then this would eliminate solar gain for a lot of the time.

I did raise this with DPS when I was looking at options, and they offered (over the phone) two different options - neither of which turned out to be on offer when I enquired further.

I have in fact doodled an arrangement to deal with this, it involves two PHXs, a pump, and a TMV. I've been rather busy lately and so haven't been able to refine my model, but I think it should be able to put water back in the store bottom at very close to mains water temperature until the flow rate is such that you reach the capacity of the PHXs. I just need to start looking out for 'salvage' PHXs (I reckon 2 or 3 from decent capacity combis should do) and I'll be set for some tinkering

In my flat I ended up fitting a Gledhill Torrent - I guess time will tell if I got it wrong !

 

[ChrisR]

You don't exactly need a thermal imaging camera...
This from BES works well. There are cheaper plain-ended thermometers.
You'll be able to work out where the blockage is, but it still could be air or something harder:

 

[johnsville]

Silverback - does larger surface area (of the plate HX against coil) not mean better heat transfer?
The cost of running the pump isn't that high - in my case anyhow.

Yes and no. Yes, you are right that it's easier to build a small PHX than a small coil of the same capacity - but saying one has a higher capacity than the other doesn't really make sense as any type can by built for a specific capacity. It's more a case of "for a given physical size" a PHX has a higher capacity.

That is correct. Additionally, PHX's resist scale as the stainless plates flex. Coils have to be massive for the same performance, meaning a larger cylinder. The coil has to be high quality manufacture so as not to crack. PHXs are cheaper overall. A PHX inside a cylinder loses no heat to the atmosphere. Having PHX's inside a cylinder makes it all so much better reducing the size.

PHX's are so efficient that they have revolutionized the combi boiler. There are high output combis that can do two baths all because of plate heat exchangers. Combis now outsell regular boilers.

The thing (or rather one of the things) that put me off the DPS store was the pumped primary for the DHW circuit. At full DHW flow rate, and assuming the correct sizing of exchanger and setting of pump, the return into the bottom of the tank will indeed be cold. But at lower DHW flow rates, the return temperature will be higher. If your DHW flow tends to be at low rates (as might be the case with a shower with a system sized for running baths) then you'll put a lot of "nearly hot" water back in at the bottom and destratify the store.

Danfoss, Oventrop, etc, make TMVs with remote temperature sensors that can be configured as diverter valves. They sense the DHW outlet temperature and have the modulating valve body on the outlet of the PHX. One port to the bottom of the cylinder,the other to the inlet of the PHX. The pump is located between the PHX and the valve body. It circulates on itself through the PHX. To maintain the DHW outlet temperature the valve opens slowly sending water to the bottom of the cylinder. This way it only draws from the cylinder the right amount of hot water. Some makers modulated the pumps to do the same thing. Gledhill did that. You could have bought one of their pcb boards and the three sensors and the right Grundfoss modulating pump. I do not know the cost of the pcb.

 

[johnsville]

If the plate heat exchanger is in the body of water, then I can't see the benefit, you may as well use a coil, as it now requires the addition of a pump to transfer the heat

To reemphasize, a plate heat X inside a cylinder: uses far less space making cylinder smaller, reduces heat loss, resists scale due to the flexible plates.

 

[SimonH2]

Additionally, PHX's resist scale as the stainless plates flex. Coils have to be massive for the same performance, meaning a larger cylinder. The coil has to be high quality manufacture so as not to crack. PHXs are cheaper overall. A PHX inside a cylinder loses no heat to the atmosphere. Having PHX's inside a cylinder makes it all so much better reducing the size.

Of course, if you have the cylinder as a store, then reducing the size might not be what you want. I can certainly see the benefits of a small store where it's being used to de-couple the various circuits and the small amount of storage is a secondary consideration though.

PHX's are so efficient that they have revolutionized the combi boiler. There are high output combis that can do two baths all because of plate heat exchangers. Combis now outsell regular boilers.

Personally, I don't think it's due to performance that combis outsell conventional boilers. I think the hard sell by every man and his dog who think that dispensing with any form of backup whatsoever is a good idea if it means you can build a rabbit hutch with 4 sq foot less floor space and/or save yourself the hassle of doing any work. But that's just my prejudice showing through after getting the earache from disgruntled tenants when the combi breaks down as they inevitably do. Having tested the shower in the flat, quite frankly I wouldn't have put up with that myself - constantly fluctuating temperatures although I admit combis have improved over the years. Now the store is fitted, the shower is loverly and constant - and doesn't go cold even if another hot tap is opened.

Danfoss, Oventrop, etc, make TMVs with remote temperature sensors. Sense the DHW outlet temperature and have the modulating valve body on the outlet of the PHX. One port to the bottom of the cylinder,the other to the inlet of the PHX. The pump is located between the PHX and the valve body. It circulates on itself through the PHX. To maintain the DHW outlet temperature the valve opens slowly sending water to the bottom of the cylinder. This way it only draws from the cylinder the right amount of hot water. Some makers modulated the pumps to do the same thing. I think Gledhill did that. You could have bought one of their pcb boards and the sensors and the right modulating pump. I do not know the cost of the pcb.

Ah, useful information - didn't know about the modulating controller, but I guessed the valves might exist. My cunning plan was to combine exactly that sort of arrangement with a second PHX. The second PHX goes between the primary and the bottom of the store/cold mains in. The idea is that the hot outflow from the primary exchanger is chilled by the incoming mains so it's completely cold before going in the bottom of the store. Without it, although the flow rate is directly proportional to DHW flow rate, it's still a hot return to the bottom of the store.

To reemphasize, a plate heat X inside a cylinder: uses far less space making cylinder smaller, reduces heat loss, resists scale due to the flexible plates.

But does require access inside if it's going to be serviceable. Though I suppose the same applies to a coil.

Got any pointers to cylinders with removable tops AND which are suitable for about 25 foot of head ? I am intrigued by the possibilities.

 

[johnsville]

PHX's are so efficient that they have revolutionized the combi boiler. There are high output combis that can do two baths all because of plate heat exchangers. Combis now outsell regular boilers.

Personally, I don't think it's due to performance that combis outsell conventional boilers.

It clearly is in IMO. The old coils and bubble heat exchangers gave poor performance, giving the older combis a poor reputation. It is true that a combi has no integral electric backup, although a simple cheap means can be fitted in the DHW draw off pipe after the combi. There is no real reason why electric backup cannot be incorporated into a modern combi. Modern good combis do not fluctuate the DHW temperatures.

Ah, useful information - didn't know about the modulating controller, but I guessed the valves might exist. My cunning plan was to combine exactly that sort of arrangement with a second PHX. The second PHX goes between the primary and the bottom of the store/cold mains in. The idea is that the hot outflow from the primary exchanger is chilled by the incoming mains so it's completely cold before going in the bottom of the store. Without it, although the flow rate is directly proportional to DHW flow rate, it's still a hot return to the bottom of the store.

A simple two port valve body can be used. There needs to be a by-pass around it using a screwed in-line adjusting valve to ensure the pump does not pump against nothing.

A good ploy of using a second PHX to ensure the water entering the bottom of the cylinder is cool (not new and used in commercial setups), however that can be achieved by using a much larger PHX in a simple pump/TMV setup. But once the DHW flow is too low the return will be hot.

A TMV can be setup as a diverter valve. Using one of these on the pipe returning to the cylinder may send cool water to the bottom and warmer half way up the cylinder. It may need tweaking. Not perfect, but will achieve some results.

Got any pointers to cylinders with removable tops AND which are suitable for about 25 foot of head ? I am intrigued by the possibilities.

Domestic? Not that I know of. Although a number of makers have lower pressure thermal stores with lift off lids. I have seen high pressure custom commercial units with removable tops. What does it entail in a domestic unit? A SS cylinder with a removable lid with a good replaceable seal, secured by about 10 bolt lockable clips. It will only be running at 3 bar at most. There are some flange topped lids on some domestic cylinders that may be used a thermal stores. A rather custom job by the installer.

British makers are still way behind the Continent in domestic units. They are slowly catching up. The cheap copper jobs are waste of time. They are so shoddy.

The ways of avoiding the dreaded sludge in thermal stores is, have a pressured cylinder, or have the CH rads taken off a coil to avoid any magnetite entering the cylinder store.

 

[DP]

Since Scotland mostly has very soft water then that would indicate blocked on the primary side rather than scaled.

Tony where did you get this nugget? What would be the pH of this very soft water?