Vaillant Ecotec pro 24 high return temperature

[ajrobb]

You haven't answered my last question.

1200 litres/hour
So at 70°C flow, the total output drops from 11 kW to 8 kW and the difference across the radiator drops to from 11°C to 8°C.

How do you work that out?

Forgive linear approximations for simplicity:

900 litre/hour, flow 85°C, boiler power 11 kW:
11000/(900/3600*4187) = 10.5°C temperature drop
Combine this with a rated mean temperature difference of 60°C and we get flow and return temps around 85°C and 75°C

Drop the boiler output to 8 kW:
8000/(900/3600*4187) = 7.6°C temperature drop
naughty approximation: 60°C mean difference for 11 kW scale:
60*8/11 = 43.6°C mean difference => 63.6°C mean temperature
67.4°C flow and 59.8°C return

OK not quite 70°C - 62°C but not so far out.

 

[iany1954]

I've got it set at 70 degrees and seems to be ok, d.1 set to 8 minutes. All radiators (trv's open) set to about 10 degree drop. I have considered outside temperature monitor but I do'nt have a North facing wall (house is a semi joined on the North side).

It seems like when the house is up to temperature the heating will come on for 10 minutes every 30 minutes and just warm the radiators without cycling, and the outside temperature is -3 at present!

Obviously the trv's upset everything as they are all set differently, but since all boiler checks have been done with the trv's fully open this can only be a compromise.

Sometimes wish I could be the 'average' man in the street and not be too interested in how things work and getting the best from them (ignorance is bliss?).

 

[ajrobb]

OK, if you are still in the mood to play...

We have established that the temperature rise across the heat exchanger in °C is roughly equal to the heating load in kW with a flow of 900 litre/hour. As the TRVs kick in, the by-pass will open further and the temperature rise across the heat exchanger will drop with load until the boiler starts cycling.

With the house warm at -3°C outside, you state you are on roughly 33% load cycle or about 3 kW. You might be able to drop the boiler rating right down to minimum (7kW?) and still warm the house in a reasonable time (your radiators can only put out 8 kW at 70°C anyway). The good things about that theory is that the radiators will warm up slower so the boiler will spend a bit more time condensing. There should be fewer boiler cycles and the boiler will operate longer, making hot water instant more of the time. Some boilers are very slightly more efficient at lower loads but you're also paying for more electricity to operate it longer...

I thought about dropping the feed temperature but I think that would drop the maximum radiator load below the minimum boiler load and will get the boiler short-cycling, which is not a good thing. You might get the same problem with weather compensation, so I wouldn't bother.

 

[ajrobb]

It is plain that most boilers are far too big for average modern houses in Britain. We have the smallest houses in Europe and one of the mildest climates. A large sector of the market would be better served by a 5-10 kW system boiler that could modulate down to 1 kW and a well-insulated unvented cylinder. A single larder-size unit would be preferable, so there is only one installation charge.

 

[D_Hailsham]

It is plain that most boilers are far too big for average modern houses in Britain. We have the smallest houses in Europe and one of the mildest climates.

Not today!

A large sector of the market would be better served by a 5-10 kW system boiler that could modulate down to 1 kW and a well-insulated unvented cylinder.[/quote]
Geminox make them; look at the THi models. A system boiler which goes from 1-10kW!

 

[iany1954]

Well seems to be sorted to my satisfaction now (probably as good as it gets). Flow temperature set to 70 degrees, d1 at 8 minutes, d0 at 12 Kw, bypass fully screwed down and all radiators about 10 degrees drop (+-2). Despite the present freezing weather the system warms up well with no overshoot or cycling. Tends to operate for 10 minutes every 30. The average boiler flow/return temperature is 7 degrees. Room temperature seems to be more even than before (presumably with the less overshoot restricted by the d0 setting). Short of changing the microbore for 15 mm I think that will do. Thanks for the help, have a happy Christmas.
Ian

 

[iany1954]

Quite happy to play with this as I don't see why I should pay for an ineficient system if a little time may save a few pounds (anyway its fun to learn how things operate). I might try reducing the rating to 10 Kw (or less) when it gets a bit warmer (its -8 outside at the moment) and the complaints obout 'messing with the heating' reduce a little!!
Ian

 

[ajrobb]

Merry Christmas. I'd be happy with your system settings, so it's time to relax.

I suspect most of the restriction on operating efficiency are due to the boiler rather than the radiators; designed for bigger houses in colder climates (yes colder than our current weather). Your radiators are already oversized to some extent if you can heat the house in this weather on 33% duty cycle. I doubt you'll gain any efficiency from replacing them unless you spend far more money than you could possibly save on efficiency improvements by buying much bigger radiators able to dissipate more than the minimum boiler output at flow temperatures of 50°C or less.

 

[The831Bunny]

Well seems to be sorted to my satisfaction now (probably as good as it gets). Flow temperature set to 70 degrees, d1 at 8 minutes, d0 at 12 Kw, bypass fully screwed down and all radiators about 10 degrees drop (+-2). Despite the present freezing weather the system warms up well with no overshoot or cycling. Tends to operate for 10 minutes every 30. The average boiler flow/return temperature is 7 degrees. Room temperature seems to be more even than before (presumably with the less overshoot restricted by the d0 setting). Short of changing the microbore for 15 mm I think that will do. Thanks for the help, have a happy Christmas.
Ian

Sounds spot on to me, Well done

 

[Agile]

Well seems to be sorted to my satisfaction now

Despite the present freezing weather the system warms up well with no overshoot or cycling. Tends to operate for 10 minutes every 30.

Thanks for the help, have a happy Christmas.

Ian

If the boiler only needs to operate for 33% of the time and heats up quickly then I would be inclined to reduce the flow temperature.

That would lengthen heat up temperatures but increase efficiency somewhat.

Tony

 

[WeatherCompenation]

Yes the best thing now would be to reduce the boiler temperature by, say 5C) and see how that goes. It is having the boiler temp too high that causes the room overheating which causes the the TRVs to shutdown which reduces the flow which then causes the bypass to kick in!) the ideal would be to have no boiler cycling at all and for it to be running on constant load which provides constant heating which matches the heat losses from the key room (usually the sitting room). In an ideal situation weather compensation would be employed to do this and the TRVs in the sitting room would never be invoked to perform the "high-limit" role for which they are intended. My main sitting room radiator TRVs are set to shutdown at around 23C this only occurs if the room is full of people or if I light the open fire.

Remember the power output of the radiators is heavily dependent on the flow temp (an increase/decrease of 10C produces something like a 20% change in kW output). That's why I suggest trying just a 5C change to start with. The modulating capabilities of the boiler are crucial to achieving a satisfactory outcome. and reducing the temperature of the boiler out equally reduces POWER output of the boiler. In the best boilers the boiler logic not only controls the flame size but also the pump speed so that the boiler can run continuosly on a lowish power EXACTLY matching the heat output from the radiators with the heat input from the boiler. The ideal point that you want to reach is that the main living/sitting room radiators are running constantly without the TRVs limiting the temp and that you are comfortable if, and it's a big IF, the radiators are correctly sized in the rest of the house it should be a matter of, perhaps, reducing the temp in unused rooms using the TRVs.

One drawback of reducing boiler temp is that you lengthen the warm-up time for the house because the POWER output (POWER is the AMOUNT OF HEAT PER UNIT TIME). BUT it's better for you to set your heating to come on 20 or 30 mins earlier and have it running more efficiently for several hours. Again, in boilers with weather compensation which automatically control the boiler output temperature ( rather like you standing there and turning the control as the outside temperature changes) they have an adjustable "boost" period to warm up the house from overnight temperatures before settling back to the reduced flow temperature neede to maintain the daytime temp.

 

[ajrobb]

As I stated before, you might not have much scope to reduce flow temperature. Doing so will drop the maximum radiator load (currently 8 kW at 70°C) below the minimum boiler output (7 kW). This could cause short cycling, increasing the likelihood of breakdown.

To reiterate, the flow through your heat exchanger is a constant 900 litre/hour as the pump speed is fixed and the ABV is set to 350 mbar. The maximum flow through your radiators is about 700 litre/hour, so the ABV is active all the time.

 

[WeatherCompenation]

Good point ajrobb. However, his theoretical radiator load will be further reduced if the TRVs start shutting down which is why it might be worth experimenting with a lower boiler temp and then obeserve what actually happens at the boiler. If this allows the TRVs to stay open and leave the radiator "on load" then he may get a better result because all the radiators remain on load and the flow more evenly distributed.

 

[iany1954]

OK,
With the boiler set at 12 Kw it was taking about 3/4 hour to reach the 70 set boiler temperature. I increased to 14 Kw and it heats up quicker and still does not short cycle, the lounge remains at a stable temperature with no overshoot. I could reduce back to 12Kw and start the heating earlier as its obviously a balance of longer warm up time compared to lower efficiency. I will try reducing the boiler temperature by 5 degrees during the Christmas period and see how this effects things (warm up time, cycling, etc.). Perhaps a compensator would carry out this action automatically? It seems like these sytems can be played with forever so long as you have the patience!

On the subject of fitting a compensator (VRC 400) would I also need to then remove the lounge thermostat (which acts as the interlock to boiler terminals 3,4) and fit TRV's to the radiators in the lounge (ie TRV's on all radiators)? OR could the lounge thermastat stay connected without changing to TRV's (the Valliant manual is not too clear). The VRC 400 would be fitted to the boiler in the kitchen. Is there any advantage in this system as the present control seems reasonable?

Ian