Vaillant ecotec plus 837 combi

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Hi
I have recently had an ecotec plus 837 installed and am becoming a bit concerned by the gas consumption. The boiler was fitted to an existing system of 13 radiators having a nominal heat output of 15.5 kW. Although the boiler output of 28kW far exceeds my requirements, I opted for the boiler because of the higher hot water flow rate.

I was recently away from home for 10 days during which time I had the heat set to come on between 7am and 9 am and 9 pm and 11pm with the room thermostat set at 15 C. At all other times the boiler would switch on if the thermostat fell to 12 C. I was surprised to discover that maintaining these temperatures consumed 81 cubic metres of gas.

1) Should the output of the boiler be reduced to (say) 18 kW being the radiator output plus an allowance for heat loss in the pipework or will the boiler automatically modulate itself to run at optimal efficiency?

2) Although the thermostat (Vaillant VRT 392) shows the room temperature has reached the target temperature, the boiler will shut down, the pump will run on for a period of 5 mins and then the boiler will fire up again. Shouldn't the anti-cycling feature prevent the boiler re-firing for 20 minutes?

3) When the individual thermostatic radiator valves kick in and stop the flow to the majority of the radiators, I get vibration in the pipework which disappears when a TRV is opened. I never had this problem with my old boiler. I have noticed that the flow of water round the system is opposite to what it used to be and that the TRV's have a arrow on them which is now opposite to the flow direction. Will the reversal of the flow be the cause of the hammering in the pipework and will the TRV's be damaged by the reversed flow?

4) Although I get 18 l/min from the cold water tap, the most I can get from the hot tap is 13 l/min. The spec for the boiler is 15.2 l/min at 35C temperature rise. With the boiler output temperature for the hot water set at 38C shouldn't the boiler be able to produce the 15.2 l/m specified? Even when set at this temperature it still only produces 13l/min.

Your comments will be much appreciated
 
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first of all the reversed water flow is the reason for the hammering noise you are getting as your TRV's are not bi-directional, who ever installed the new boiler has obviously changed the flow direction, I'd get him back if i were you to correct the problem or put new TRV's on, The flow rates listed for boilers will pretty much never be achieved, these were listed as a kind of bench mark for people to compare boilers, the 837 will adjust itself to the size of your system although a good engineer would have gone into the boiler program and adjusted it for you ( I would have ), have a look and see if you have the Comfort setting turned on on your boiler as this does use a small amount of gas to keep the domestic hot water hot, so when you turn on the tap you won't have to wait so long for the hot water to come,
 
Hi


2) Although the thermostat (Vaillant VRT 392) shows the room temperature has reached the target temperature, the boiler will shut down, the pump will run on for a period of 5 mins and then the boiler will fire up again. Shouldn't the anti-cycling feature prevent the boiler re-firing for 20 minutes?

If it uses the same control curves as the EcoTec 415, which I suspect it does, then the anti-cycle time depends on the setting of parameter d2, and the target water temperature (heat), i.e. the bottom dial.

The default for d2 is 20 minutes at 20C, which equates to 4.5 mins at 65c and 6 mins at 60c.
I would guess that your flow temperature setting in within this range.

You can increase d2 to prevent short-cycling, as it does not make sense to keep firing the boiler every 5 minutes.

Try setting d2 to 40, this should give you an anti-cycling time of about 11 minutes at 60c.

Watch the return temperature during this time and see how far it drops. If it drops more than about 15c from its original figure, then decrease d2.

If it still short-cycles, then increase d2.

These observations are based on my own system, and with reference to the Vaillant lockout table for my 415.

Keith
 
Morris40 - Will the TRV's have been damaged by the reverse flow? I had adjusted the boiler output to 20kW but it did not seem to have any effect on gas consumption, although it did seem that the radiators took longer to get hot. I have now turned the comfort (warmstart) feature off but, as you say, that should only account for a small amount of gas consumption.

Keith - I had seen the default anti-cycling time was 20 minutes but hadn't realised the significance of the temperature. Any thoughts as to why the boiler would fire up again when the target room temperature had been reached?
 
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Keith - I had seen the default anti-cycling time was 20 minutes but hadn't realised the significance of the temperature. Any thoughts as to why the boiler would fire up again when the target room temperature had been reached?

The VRT 392 may not actually act as an interlock, and shut the boiler totally down. Reading the online manual, it is not very clear how it works. To maintain a constant temperature, it probably modulates the power output.
If there is still too much output, then it will cycle, which it what you are seeing.

It seems to depend on the setting of parameter C9 when it was commissioned. There are two options, thermostat or modulating. I would imagine that yours is set to modulating, which is the preferred option, hence its why your boiler never switches off, unless your heating is turned off.
Its all in the commissioning section of the Operation and Installation manual, which you should have.
If not, it can be found here http://www.vaillant.co.uk/stepone2/...operation-and-installation-manual-vrt-392.pdf

I would be inclined to increase the value of d2, and try to reduce the short cycling. You may have to increase the value of d1 (pump overrun) so you can monitor the return temperature, otherwise the pump will stop, before the anti-cycle times out, and you won`t see the result!

I will see if I can upload the boiler lockout table, so you can see the relationships.

View media item 30654
Assuming that you have not got the weather compensation unit, what temperature do you have the heating flow set to? Setting it lower will improve efficiency. If its cycling, then you could probably work with it lower. Try 60C to start with, that will increase the anti-cycle time.

As you have a combi, your hot water is controlled separately, so you have full control over the heating, without worrying about stored water temperature.

Keith
 
it is unlikely the TRV's got damaged but you'll need to change them if you are not going to put the flow and return back around the correct way.
 
Hi

4) Although I get 18 l/min from the cold water tap, the most I can get from the hot tap is 13 l/min. The spec for the boiler is 15.2 l/min at 35C temperature rise. With the boiler output temperature for the hot water set at 38C shouldn't the boiler be able to produce the 15.2 l/m specified? Even when set at this temperature it still only produces 13l/min.

Your comments will be much appreciated

Oh dear!

The boiler heats water! It does not generate water flow.

You supply can only give 13 li/min through the resistance of the boiler so thats all the flow available to be heated.

Did your installer not discuss this with you?

If you badly need more flow than upgrads your supply pipe from the street. Possibly replacing an internal stop cock with a full flow valve might also help a little.

Tony
 
Keith - I will do as you suggest. If I understand you correctly, if the thermostat is set to modulating mode, I can expect the boiler to run almost constantly, although it will be modulating its output to maintain the target temperature. The boiler output temperature was set at 70C. Have you suggested setting it 60C to ensure the return temperature is below 55C for it to work as a condensing boiler?
 
Agile";p="1885074 said:
Hi

Oh dear!

The boiler heats water! It does not generate water flow.

You supply can only give 13 li/min through the resistance of the boiler so thats all the flow available to be heated.

Did your installer not discuss this with you?

Tony

Tony - Although I realised the resistance of the boiler would affect the flow, I had not appreciated it would be so significant. The installer didn't bring this to my attention. As I don't need the heat output that my boiler is capable of, it seems I would have been better advised to have the ecotec plus 831.
 
The boiler output temperature was set at 70C. Have you suggested setting it 60C to ensure the return temperature is below 55C for it to work as a condensing boiler?

Ideally you should set the flow temperature as low as possible, while still maintaining the room temperature.

If you have a reasonably well insulated house, and your radiators are large enough, a lower temperature will be sufficient.

An external weather compensator, where fitted, adjusts the flow temperature to suit the outside temperature.
In practice, you can usually find your own compromise.

I have to operate EcoTec 415 at 62C all the time, to maintain my stored hot water at a reasonable temperature. This gives me a maximum return temperature of 55C, when at minimum (5kw) output, just before it enters anti-cycle mode for 15 minutes.
I have d2 set to maximum (60). This eliminates any short cycles.

Your 70C seems rather high, hence the very short cycle. A lower temperature will increase the anti-cycle time, and reduce the return temperature, increasing efficiency.

Short cycles are not a good idea, and will put extra stress on the boiler, as well as wasting gas.

You just need to experiment to find the best balance. If the flow temperature is too low, then the target room temperature may not be reached. It depends on many factors.

Keith
 
Keith, thanks for all your advice. I'll try it out over the next few days and see how I get on. Should I be able to estimate gas consumption or are there too many variables? Maybe having the gas meter checked for accuracy would be the solution?
 
The meter is unlikely to be at fault. You can set the boiler to max power and check the meter consumption.

Water flow into a property should be assessed dynamically. Few boiler installers realise this hence disappointments like yours.

Tony Glazier
 
Should I be able to estimate gas consumption or are there too many variables? Maybe having the gas meter checked for accuracy would be the solution?

There are too many variable, including the British weather !
Its unlikely the gas meter is faulty.

Reduce the flow temperature, you can look at the return temperature d41. If you can keep it below 55C, then there will be something extra to gain, however the boiler is quite efficient anyway.

If you still have issues with the boiler coming on when it not needed, then it may be better to operate the controller in the thermostat mode.

At least it will shut down when there is no heat demand, and not keep cycling, and wasting gas keeping the circulating water hot.

It may be more efficient that way, as it looks like the minimum output from the boiler is still more than you need, as you said in your original post.

Keith
 
although a good engineer would have gone into the boiler program and adjusted it for you ( I would have )

As far as I can tell the installer left all the settings at their default values. Setting D.36 controls the hot water flow sensor. At the moment the value is 0. Does this setting affect the l/min of hot water the boiler produces or is it the flow rate at which the boiler will start heating the water?

I have read previous posts regarding TRV's. When mine were originally fitted (19 years ago) I recall the installer telling me they worked best with the valve fitted to the return side of the radiator with the head horizontal. The advice now seems to be that they should be on the flow side. As mine always worked well with my old boiler, I am not inclined to replace them and think the flow and return connections to the boiler should be reversed to take account of them being one-way flow. Despite all the valves working well, given their age do you think they are living on borrowed time and it would be prudent to replace them?
 
That setting only changed the minimum flow to detect a demand for hot water.

TRVs have always been designed for fitting to the flow.

About 10 years ago most makers realised this was causing problems with installers and made them bi-directional.

Its technically better for them to be on the cooler return. Few installers wanted to do that as a preference.

Its also better for them to be horizontal but that renders them liable to accidental damage.

It would have been unusual for a bi-directional valve to have been fitted 19 years ago.

All of these questions should have been explained/answered by your installer. You seem to have had bad luck choosing him!

Tony
 

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