DPS thermal store DHW performance

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Hello All

I have a DPS thermal store (300l) connected to a condensing gas boiler (Worcester 42CDi). The store currently supplies an Invisible heating UFH system (4 circuit) and the DHW.

I have a radiator circuit that comes directly from the boiler which is controlled by a single room stat and timeclock with a zone valve for supplying it when heat is called for.

The DHW heated by the thermal store is continuously on (not a timeclock).

This worked very well when I first installed the sysytem. Recently I have noticed that the availability of DHW is less, even running a standard size bath drains the DHW. The thermal store has a 'quick recovery' system with large plate HX from the boiler. It appears to be cycling more than before, even after running a sink of hot water.

I recently replaced the main pump (it was noisey) from the boiler to thermal store but this has had no impact (good or bad) on the performance.

The UFH works as expected.

Is it possible that there is an airlock in the thermal store or pipework suppying it?

Could faulty thermal store stats be at fault?

Does anyone have any general advice or need further information to help with trouble shooting?

Cheers

 

[simond]

I suspect you have broken it by turning the CH temp control on the 42CDi down.

With a TS it needs to be all the way up.

A 42CDi is a combi, so I am surprised you are using it in this configuration. I suspect you have a 40CDi conventional.

 

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Simond
I may have the boiler model wrong, it is a conventional, condensing boiler.

I will try turning the boiler stat to Max, the stats on the thermal store are at 70 Degrees (as per DPS specificton).

Will update if this improves things.

 

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Turning the boiler to max has had no impact on the availability of DHW (the amount available)

 

[johnsville]

are you saying there is a plate heatX between the boiler and thermal store cylinder? If so that would need two pumps. One on the boiler side and one on the thermal store cylinder side. If this is the case are both pumps working OK? I would think the pump on the cylinder side of the heatX would need to be running faster than the boiler pump to extract it heat from the heatX fast enough.

Thermal stores a "usually" connected up with the boiler heating the cylinder directly.

 

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The plate HX is inside the thermal store, it is used to dump the heat from the boiler into the store. A HX was used over a coil becasue the reheat capabilites and efficiency is better.

The pumps are all working.

 

[silverback]

A plate heat exchanger would not be inside a store, it would be external, only coils are placed within the store
A plate heat exchanger is used to separate water supplies, you would have heating water one side of the plate and the water requiring heating in this case your DHW on the other side of the plate, so the heat exchange is done within the plate, this is why it is placed externally, coils are placed within a store, hot water is passed through the coil which then passes its heat to the body of water surrounding the coil

 

[johnsville]

Some makers are putting them inside to save space and make the system more efficient as there is less heat loss. Is this a stainless cylinder?

A plate between a boiler and a cylinder, fitted inside or not, would need two pumps. The man says the plate is between the boiler and the cylinder, not doing DHW.

The boiler cycling is interesting. If all the heat is being taken out of the boiler then it should not cycle.

This sounds like a pump(s) problem.
Blocked heat X
cylinder stat problem. Some thermal stores have two stats with a relay. Is the relay OK if one fitted?

 

[Agile]

I would vote for a blocked or scaled plate HE.

Since Scotland mostly has very soft water then that would indicate blocked on the primary side rather than scaled. But without steel rads the scope for iron dirt is limited.

It should not be very difficult to identify the problem with a thermal survey.

Tony

 

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There are 2 plate HX inside the store. One transfers heat from the heat source (the boiler) to the thermal store mass of water. The other HX transfers the heat from he thermal store water to my mans cold, therefore giving me mains water at a hot temerature.

The use of HX on both sides is to aid rapid heating and heat transfer, this is more efficient that coils.

I will check the pump functions, the 2 thermal store stats and any relays included. Getting a thermal srevy done may not be so easy but I may know someone with a heat seeking camera.

I did recetly drain down my rads, which are heated from the same boiler. Is it possible that any dirt/scale present there found it's way to the plate HX during drain down and refill?

What is the best method to clean sclaed up (local water is very soft) plate HX?

Thanks

 

[johnsville]

If the store is open vented it may have accumulated some sludge. The steel rads off the cylinder will make the sludge. It may need draining and anti-sludge liquid put in, then a drain down and enough inhibitor which is about 5 or 10% of the water volume, check with makers of liquid.

You may be able to connect up the mains to each side of the plate heat X, assuming it is blocked, to dislodge the sludge with high pressure water.

Fist check the stats and pumps, then make sure enough inhibitor is in the system. If a stat problem and only the top stat is operating then only the top of the cylinder will get hot, not giving enough hot water for the bath.

First determine what the problem is.

best of luck.

 

[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

 

[Biggles..]

On your store they'll be a label just to the side of the wiring Center /time clock arrangement.
Please can you post the really long number which is made of letters/numbers,this is the design spec for your store,if this is pasted into the heatweb site it will give us the schematic of your store.

 

[simond]

I though DPS had gone into liquidation a few months ago?

I would check to see when the thermal store stat clicks by moving it, one really needs to ascertain what the actual store temperature is. Alternatively, as a short term test, you could bridge out the TS thermostat contacts and let the 40CDi keep going until it stops on its own thermostat.

But the idea of air in the system would cause the boiler to cycle and insufficient heat transfer, if the problem occured after a draindown then this is a front runner.

 

[johnsville]

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.