Efficient heating of DHW cylinder with Vaillant ecoTEC

[UpgradeME]

After having sucessfully managed to optimise my Vaillant ecoTEC 418 for CH, I'm now looking to do the same for the HW now we're having this great weather and the heating is no longer on. (see my other thread for CH optimisation)

I have a 117 litre (vented) hot water cylinder, a Vaillant VRT392, VR65 and VR10 NTC for controling the HW temperature to give around 55C at the tap. After adjusting the balancing valve, I currently get about 10C delta between flow/return.

My question is; What is the most efficient way of heating the DHW from say 25C to 55C?

• a) A high burner output and flow temperature (d.77/78 )
  
  b) A low(er) burner and flow temperature to get better condensing efficency (but taking longer to heat)?
  
  c) Something else?

d.77 is currently set to 7kW (max output for heating DHW)
d.78 is currently set to 75C (max flow temp for heating DHW)
d.17 is currently set to Return Regulation (if that makes a difference for heating DHW!?)
I've currently set the VRT392 to heat the DHW twice per day (morning/afternoon) for 30 minutes each. I tried setting it on all day but that used more fuel for similar DHW usage.

I'm currently using a "trial 'n' monitor" method by trying different options and logging usage at the meter, which is slow and hit & miss

What should I set d.77/78 to give the most efficient operation? Is there a mathmatical way of working it out?

Any optimisation suggestions appreciated!

 

[bengasman]

My question is; What is the most efficient way of heating the DHW from say 25C to 55C?

For starters, you need to get the cylinder to 65C or more, not 55. To achieve that, you need a flow temperature at the inlet of the cylinder of more than 65C, or the boiler will be on as long as the timer says it is on, because the demand will never be satisfied.

If you want to optimise that, you should start with insulating the flow pipe from the boiler to the cylinder in order to keep the flow temp at the boiler as low as possible to improve condensation.

 

[UpgradeME]

My question is; What is the most efficient way of heating the DHW from say 25C to 55C?

For starters, you need to get the cylinder to 65C or more, not 55.

Can I ask why you suggest 65C? Doesn't legionella die instantly at 60C and take +/- 5 hrs at 55C? - The VRT392 does have an option for "anti-legionella", (not currently enabled) whereby once per week it will heat the water to 70C (which I understand is considered the "disinfection" temp for legionella.)

To achieve that, you need a flow temperature at the inlet of the cylinder of more than 65C, or the boiler will be on as long as the timer says it is on, because the demand will never be satisfied.

The flow temperature will currently reach upto my d.78 setting (75C) but I have a feeling my demand is not currently satisified in the 30mins the boiler is on. The VRT392 limits the time period for heating DHW to 45mins (I assume for safety reasons) so I can increase it by 15mins if needed.

If you want to optimise that, you should start with insulating the flow pipe from the boiler to the cylinder in order to keep the flow temp at the boiler as low as possible to improve condensation.

Not sure I understand how insulating the flow pipe will keep the temperature lower? My understanding was I need a return temperature below 55C to improve condensing!?

 

[dumbrill]

The heat transfer via the cylinder coil will improve as the velocity of the water increases. Therefore to get the minimum heating time you should have your balancing valve wide open. Since I assume the system operates with priority DHW this shouldn't be a problem.

If the boiler can be set to have a higher pump flowrate for DHW then make sure it is setup to do this. Also set the boiler output to higher than 7kW. The boiler will then modulate to maximise the heat input within the confines of maintaining the correct temperature differential across the heat exchanger.

To answer your question then getting some heat back by maximising any condensate recovery might be beneficial, but the savings must be minute. All you are talking about is the heat recovered by condensing for the few times a day when the boiler is in DHW. I would say that life is too short to worry about it.

Also is the money saved by heating the cylinder only twice a day really that much? A modern cylinder will only lose around 2- 2.5kwh/24 hours by cooling, ie. probably around 10-15p per day. If you heat twice a day then you only make a saving on the heat loss if you use all the water and leave the cylinder cool after each cycle. If you've got a family then permanent hot water outweighs the savings and hassle of running out.

 

[dumbrill]

The flow temperature will currently reach upto my d.78 setting (75C) but I have a feeling my demand is not currently satisified in the 30mins the boiler is on. The VRT392 limits the time period for heating DHW to 45mins (I assume for safety reasons) so I can increase it by 15mins if needed.

To heat all the 117 litres by 30 degC will take 8kw for half an hour. Therefore if you're limiting your boiler to 7kw then I'm not surprised that you're not getting the water fully hot. Also the 8kw figure is a simple calculation that doen't account for losses, etc so the real figure required from the boiler will be slightly higher.

 

[UpgradeME]

To heat all the 117 litres by 30 degC will take 8kw for half an hour. Therefore if you're limiting your boiler to 7kw then I'm not surprised that you're not getting the water fully hot. Also the 8kw figure is a simple calculation that doen't account for losses, etc so the real figure required from the boiler will be slightly higher.

First of all thank you for the very useful information in the above few posts!

If you could let me know the calculation you used for the above, that would be great? I can then play with the maths and apply the relevant settings as I optimise!

The heat transfer via the cylinder coil will improve as the velocity of the water increases. Therefore to get the minimum heating time you should have your balancing valve wide open. Since I assume the system operates with priority DHW this shouldn't be a problem.

If the boiler can be set to have a higher pump flowrate for DHW then make sure it is setup to do this. Also set the boiler output to higher than 7kW.

For some reason I had 'assumed' the opposite so if that's true, I can optimise the other way now! - Is there a max / temp calculation to work this out (i.e. cylinder litres, flow rate, temp rise, kW required) ? (And yes, I have DHW priority and can increase the pump speed during the summer when no CH is on)

Does flow temperature come into the equation? i.e. If I set the max flow temp to 80C, does this transfer the heat quicker? or would I get better effeciency having the max temp closer to the DHW set temp (55C)? Is there a mathmatical formula for that bit?

A modern cylinder will only lose around 2- 2.5kwh/24 hours by cooling,

What would that be as a temperature drop? i.e. If it was heated to 55C, assuming a loss of 2.5kW what would the temp be 24hrs later?

If you heat twice a day then you only make a saving on the heat loss if you use all the water leave the cylinder cool after each cycle. If you've got a family then permanent hot water outweighs the savings and hassle of running out.

When I originally did some (limited) testing, the fuel usage was at least twice as much when on all day. The boiler was cycling for short periods to maintain the temperature. This may be due to the control design (i.e. The VR10 is not designed for my cylinder) causing the excess fuel usage!?

I do have a family but they are not at home in the daytime during the week, I figured I was unecessarily maintaining the water temperature so went back to twice per day.

 

[dumbrill]

If you could let me know the calculation you used for the above, that would be great? I can then play with the maths and apply the relevant settings as I optimise!

It's the based on the specific heat capacity of water which is 4.18 kg/kg °C. So for every kg of water heated by 1 deg C you will need 4.18kJ. 1 kw is 1kJ/s so a 18 kW boiler could deliver 18kJ/sec to the water. I say could because that is only possible if the coil in the cylinder will transfer the energy at that rate

For some reason I had 'assumed' the opposite so if that's true, I can optimise the other way now! - Is there a max / temp calculation to work this out (i.e. cylinder litres, flow rate, temp rise, kW required) ? (And yes, I have DHW priority and can increase the pump speed during the summer when no CH is on)

I had forgotten that the 418 is a heat only boiler. In that case I wouldn't bother changing the pump speed. I only suggested that thinking that it was a system boiler.

There is no simple calculation for what the coil in your cylinder is capable of. The two variables you have control of are the temp difference and the flow velocity. Increasing either of this improves the rate of heat transfer. Note the rate is not the same as the efficiency.

What would that be as a temperature drop? i.e. If it was heated to 55C, assuming a loss of 2.5kW what would the temp be 24hrs later?

The figure for the standing heat loss for a modern 120 l tank is a lot less than the figure I quoted. Based on a 1.25kwh/day loss then the temperature drop would be around 9°C (back to the SHC calculation above).

When I originally did some (limited) testing, the fuel usage was at least twice as much when on all day. The boiler was cycling for short periods to maintain the temperature. This may be due to the control design (i.e. The VR10 is not designed for my cylinder) causing the excess fuel usage!?

You need to make sure that the VR10 is setup with a dead band of perhaps 5°C around the setpoint so that it turns off DHW at say 57°C and turns it back on at 52°C.

 

[The831Bunny]

The boiler software will regulate the modulation and flow temperature from the boiler while looking at the rise in the stored temperature. This is also referenced to the return temperature.

Open any valves on the cylinder return fully (you do not want any restriction).

I'd raise D.77 to 10Kw at least on a standard cylinder, and check D.40/D.41 for a difference of 15C to 20C on a cold cylinder.

I'd also recommend raising D.78 to 80C

In a nutshell the aim of the software on the boiler pcb and VR65 is to heat the cylinder in 'one hit'. Even with D.78 at 80C the flow temperature is NOT 80C. Its always <15C than the set DHW temp, only raised to 80C if the return water is cool due to large volume HW usage at the time the cylinder is being heated. This ensures the return is kept in the condensing 'zone' for as long as possible.

Just leave the software to sort itself out! The more you meddle the more the optimising will be confused! (The software calculates the switch off temperature and any overshoot in the first five cylinder charges).

The biggest factor here is the efficiency of the cylinder coil to transfer the heat. Is it less than 3 years old? Part L compliant?

Please don't ask 1000 questions till you do this and post your results. I've see 100's of ebus VR65/61/430 systems that have been 'tweaked' by button pressers with absolutely no heating engineering or Vaillant knowledge. Granted a few parameters can be changed to over come problems usually in system design, but generally do you really think manufacturers invest in a load of R&D for people to change parameters willy nilly?

Gets off soapbox

 

[Bunnyman]

That goes for me too! 831 is the ****** on these!

Untitled

• Bunnyman
• 26 Mar 2010
• 499

 

[UpgradeME]

The biggest factor here is the efficiency of the cylinder coil to transfer the heat. Is it less than 3 years old? Part L compliant?

The cylinder is copper with foam insulation, approx 15 years old.

Please don't ask 1000 questions till you do this and post your results. I've see 100's of ebus VR65/61/430 systems that have been 'tweaked' by button pressers with absolutely no heating engineering or Vaillant knowledge. Granted a few parameters can be changed to over come problems usually in system design, but generally do you really think manufacturers invest in a load of R&D for people to change parameters willy nilly?

Understood, I come from a R&D background in technology so can guess what they will likely go through during NPD! Products are usually designed for a range of scenarios/installations. During R&D a 'happy medium' setup is usually preloaded on products which may not be optimum for all installations. On my CH I found I could increase effeciency by >15% after 'tweaking' from defaults. My goal here is to tweak the settings from 'happy medium' to 'optimum' for my DHW setup!

Using a flow rate I had for CH (maybe different when heating DHW?) and using the following formula;

&#916;T (K) = power (kW) ÷ flow (l/min) ÷ 4.1 (kJ/l/K) × 60 (s/min)

...setting d.78 to 10 kW will give me a d.40/41 &#916;T of 14.5°C

I do have some questions but as requested I'll wait for these results! I've set d.78 to 80°C, d.77 to 10kW and fully opened the cylinder valve (pump speed currently on II (Grundfos UPS 15/50)). Target DHW temperature is set to 57°C (on VRT392). I will check fuel usage tomorrow morning and compare / post back results.

Should I change d.17 back from return regulation? (I changed it to 1 to overcome a short-cycling issue on CH... If it also affects HW I can switch it back for the summer if it will increase effeciency?)

 

[UpgradeME]

Okay, results from this morning compared to yesterday and a previous 15yr old 18kW open vent, non-condensing boiler from last year with same cylinder but using a tank stat rather than VR10:

Yesterday vs Today = +7% more fuel
Old boiler vs Today = +21% more fuel

Todays settings were d.78=80°C/d.77=10kW/Speed II/Set=57°C/Valve open
Yesterdays settings were d.78=70°C/d.77=7kW/Speed II/Set=57°C/Valve half closed

Old boiler settings largely unknown i.e. they were dials rather than temp settings. Usage comparrison from actual meter readings taken from gas bill over the summer last year, then averaged out across the number of days it covered and halved (as it was on twice per day). Hot water at the tap 'feels' the same as it was then...

If I can get my Vaillant's usage down 20% from here (I should be able to match the old one surely?), I will be happy as I know it's a lot more effecient for CH over the winter.

Does anyone have any suggestions on where to go from here with the settings?

Even with D.78 at 80C the flow temperature is NOT 80C. Its always <15C than the set DHW temp, only raised to 80C if the return water is cool due to large volume HW usage at the time the cylinder is being heated. This ensures the return is kept in the condensing 'zone' for as long as possible.

I assume thats a typo and you mean it's always 15C more than than the set DHW temp? So if I set 57C, the flow temp will be 57+15=72C ?

Just leave the software to sort itself out! The more you meddle the more the optimising will be confused! (The software calculates the switch off temperature and any overshoot in the first five cylinder charges).

That's interesting to know. Can you elababorate any more e.g. Does it work on a 'rolling' five cylinder charges? If not, can it be reset? Is the info visible anywhere to the user, perhaps via d.code? (The reason I ask is I assume after changing flow rates via the valve or pump speed, this setting will be revevant!?)

 

[Agile]

Its interesting but I disagree with many of the statements which have been made above.

Firstly, as I understand it the cylinder should be set to 55-60C and NOT higher.

Secondly, most of the replies have clearly been designed to ensure rapid reheating. But the OP very specifically asked how to make it run most efficiently.

To specifically answer the OPs request me reply would be to set the water temp to 55C ( or even lower, one of my customers uses hers at 45C ).

Then set the flow temp to about 15C above the selected water temp. The boiler power output can be set higher to perhaps 10-12kW. The heating coil should in my view be left to give a 10C delta. Then increase the heating times to perhaps 60 min as these settings will not always reheat within 30 min. In any case your older cylinder will probably have a lower absorbtion perhaps only 4-6 kW

I appreciate that these settings are not what most heating installers would use as our incentive is to provide quick reheat so that the customer never complains about the hot water running out.

Tony




PS Insulating the HW flow and return pipes minimises the heat loss to increase the efficiency. Ben meant that with an insulated flow pipe the water will arrive at the cylinder at virtually the same temperature as it leaves the boiler whereas if uninsulated might lose a degree or two. I am surprised with your quest for efficiency that you have not already insulated them. It is required under regulations that they are insulated in the vincinity of the cylinder!

 

[UpgradeME]

Then set the flow temp to about 15C above the selected water temp. The boiler power output can be set higher to perhaps 10-12kW. The heating coil should in my view be left to give a 10C delta. Then increase the heating times to perhaps 60 min as these settings will not always reheat within 30 min. In any case your older cylinder will probably have a lower absorbtion perhaps only 4-6 kW

I've now dropped the cylinder temp to 54C which if 831Bunny is correct, will mean the flow temp will be 54+15=69C. I have turned back the balancing valve to half way and will check later to see what my delta temp is on flow/return.

I've been trying to find out what my cylinder will absorb, there is little information on the label attached to the cylinder - I previously lowered my output kW to 7 on the basis that the cylinder would absorb upto 6kW and the boiler will modulate down to 5kW. However, with my output set at 10kW, arn't I 'wasting' 4kW of output?

PS Insulating the HW flow and return pipes minimises the heat loss to increase the efficiency. Ben meant that with an insulated flow pipe the water will arrive at the cylinder at virtually the same temperature as it leaves the boiler whereas if uninsulated might lose a degree or two. I am surprised with your quest for efficiency that you have not already insulated them. It is required under regulations that they are insulated in the vincinity of the cylinder!

There is a total flow/return pipework visible and capable of being insulated of around 2m max (the rest is between ceiling/floor). They are all short pieces between the floor / pump / valves in the vincinity of the cylinder. It would be very fiddly to fit neatly, I didn't think there would be that much heat loss on such a small amount of pipework so haven't bothered to date...

Would it be worth insulating 2m of pipe?

 

[dumbrill]

Okay, results from this morning compared to yesterday and a previous 15yr old 18kW open vent, non-condensing boiler from last year with same cylinder but using a tank stat rather than VR10:

Yesterday vs Today = +7% more fuel

Todays settings were d.78=80°C/d.77=10kW/Speed II/Set=57°C/Valve open
Yesterdays settings were d.78=70°C/d.77=7kW/Speed II/Set=57°C/Valve half closed

You suggested earlier that you thought the 30 mins was not sufficient time to heat water fully. My calc above indicated you would need 8kW for 30 mins and you limited the boiler to 7kW. How do you know that 'yesterdays' settings were for a full tank reheat?

How do you know that the starting temp for the water in the tank is always the same? There are a lot of variables that can affect the measurements you are taking.

If you are serious about doing this then you probably need to take readings over a period of days so that you can get an idea of variability. Each time before you start you should fully flush out all the hot water from the tank so that the starting temp is the same. Set the DHW time to 45mins so that it's always ths tank stat that turns off the boiler.

It's also worth bearing in mind that each tank reheat takes 4kWh that could cost you as little as 20p depending on your gas cost. If you save 10% then that's 4p per day for two reheats. At the end of the year you can all go out and have a MacDonalds to celebrate, but that's about all.

 

[Agile]

But he could have TWO of the McDonalds if he insulates his pipes!

With a 55C delta a 22m pipe one metre long loses 60 watts ! That often surprises people!

In your case the 4m will lose about 200w

Surely that must be worth insulating?

Tony


PS
I would also tentatively suggest you try the effect of increasing the delta on the flow valve. Thats assuming you have an auto bypass valve.

BUT the effect of the bypass opening would be to artificially elevate the return temp inside the boiler ( as measured on d40/d41 ) so restricting the flow may have the actual effect of increasing the return temperature. Its a try it and see situation with the objective of reducing the return temp inside the boiler.

Regarding the boiler power, I now think that about 6-9 kW may be more appropriate for your needs. The purpose is to have it low enough to prevent/reduce the boiler cycling off.

You can calculate the power adsorption by having the boiler stably heating the water and measuring the gas rate at the meter ( see FAQ )