Vaillant Ecotec Plus 428 with UPS2 Pump

[ChrisNason]

So it looks like I should open the Automatic Bypass AND set the pump to position 2.

But I still can't get my head around the 'shortcut' that the bypass provides straight back to the boiler! The boiler is heating the water to 70c and a portion is going straight back in the return side with almost no heat loss. And why would the flow bother to get to the furthest radiator when there is this handy shortcut.

 

[fezster]

As has already been said, ideally a low loss header or close coupled tees would be used to maintain the desired flow rate through the boiler, and separating the flow to the CH system which would be driven off of the variable pump. I have a 438 and struggled for a long time to maintain the required flow rate through the boiler until I had a LLH installed.

 

[robmort]

I found this explanation very clear from Chris Watkins on https://www.ukplumbersforums.co.uk/...s-headers-or-close-coupled-tees.102592/page-2:

"Hydraulic separation using a LLH (expensive) or close coupled tees (cheap) allows modern boilers to receive the minimum flow rate required by MI's, their control systems can then make adjustments to the output (burner modulation) & anti-cycling routines based on the heat being used against a typical 20degC difference between the F&R. In conventual designed UK systems the flow rate changes as zone valves, TRV's & automatic by-passes open & close, they can't make sense of what the system actually requires.

This is more apparent the larger boilers & systems as the burner can not go low enough & turns off as heat is not being carried way. Then they fire back up after a sort while (cycling) this is very wasteful so many have a program which stops it, limiting the stop/starts per hour, trouble is areas of the building then go cold or HW cylinders don't heat up."

I wonder if a close-coupled tee would solve the problem on my 428 system.

 

[ChrisNason]

Having looked at the minimum flow rates for my 428 I can see a number of settings to get 20.3 litres per minute. I've settled on pump speed 2 which is a head of 3.5 metres with the Honeywell bypass set to 3.5 from their chart. But I could have selected a pump speed of 1 to give 2.5 metres of head and then set the Bypass to 2.8. Equally, I could have found a setting for Pump Speed 3! So what is best?

When the only demand is DHW and that is getting near to the trip point for the thermostat the flow and return temperatures are nearly identical. Still don't understand why it's good for the boiler to have a lot of flow going through the bypass before it's lost any of its heat.

I can see why disconnecting the boiler circuit from the demand circuit could be good but an LLH or Close Coupled Tee don't do this. It would be much better if Vaillant were to tell us how they expected their 428s to work in full demand or light summer DHW?

 

[robmort]

.......... Still don't understand why it's good for the boiler to have a lot of flow going through the bypass before it's lost any of its heat.

It's necessary to maintain a minimum flow rate through the boiler e.g. to stop kettling which damages it.

.....I can see why disconnecting the boiler circuit from the demand circuit could be good but an LLH or Close Coupled Tee don't do this. ....

On the contrary that's exactly what an LLH or CCT do!!

 

[fezster]

I can see why disconnecting the boiler circuit from the demand circuit could be good but an LLH or Close Coupled Tee don't do this.

That's precisely what they do. The boiler side of the LLH/CCT has it's own pump responsible for overcoming the boiler's HEX resistance (and any minimal pipework), whilst the CH side has one or more pumps to overcome the resistance in the individual zones.

 

[ChrisNason]

But they are not disconnected. They appear to blend. But perhaps the LLH allows both sides to run at different flow rates although I don't see how the pump from one side won't interfere with the other?

So in an ideal setup the boiler pump would be a single speed constant and the demand could be variable?

 

[D_Hailsham]

It's necessary to maintain a minimum flow rate through the boiler e.g. to stop kettling which damages it.

Are you saying that the minimum flow has to be the same when the boiler is on minimum output as when it is on maximum output?

The flow rates in the manuals are always those required at max output for a 20°C differential. For example, a 20kW boiler requires a flow of 14.35 litres/min. Let's say the minimum output is 5kW. If the flow is the same, the differential reduces to 5°C. if the boiler is quite happy with a 20°C differential at 20kW output, why can't it be happy with the same differential at 5kW output? This would require a flow rate of 3.59 litres/min.

The 415 and the 418 both use the same heat exchanger; the 428 and 438 also use the same, but different from the 415/418. Yet the minimum flow rate is not the same for the 415 as for the 418, or for the 428 as for the 438.

It was different in the days of fixed - not modulating - boilers where the flow rate had to be maintained when zone valves and TRVs closed down. But with modulating boilers, which reduce output as valves close down, the only time you need to maintain a minimum flow is when the boiler is running at minimum output and can't reduce it any further if a valve closes.

 

[fezster]

But they are not disconnected. They appear to blend. But perhaps the LLH allows both sides to run at different flow rates although I don't see how the pump from one side won't interfere with the other?

So in an ideal setup the boiler pump would be a single speed constant and the demand could be variable?

Yes, there's hydraulic separation. There will be mixture in the chamber of a LLH, but the important part is that the resistance of the boiler no longer needs to be taken into account when sizing the pump for the CH system. Since installing mine, the boiler is far happier and maintains its approx 20 degree differential at all times.

 

[ChrisNason]

Are you saying that the minimum flow has to be the same when the boiler is on minimum output as when it is on maximum output?

The flow rates in the manuals are always those required at max output for a 20°C differential. For example, a 20kW boiler requires a flow of 14.35 litres/min. Let's say the minimum output is 5kW. If the flow is the same, the differential reduces to 5°C. if the boiler is quite happy with a 20°C differential at 20kW output, why can't it be happy with the same differential at 5kW output? This would require a flow rate of 3.59 litres/min.

The 415 and the 418 both use the same heat exchanger; the 428 and 438 also use the same, but different from the 415/418. Yet the minimum flow rate is not the same for the 415 as for the 418, or for the 428 as for the 438.

It was different in the days of fixed - not modulating - boilers where the flow rate had to be maintained when zone valves and TRVs closed down. But with modulating boilers, which reduce output as valves close down, the only time you need to maintain a minimum flow is when the boiler is running at minimum output and can't reduce it any further if a valve closes.

That's more understandable! So perhaps a variable speed pump can work with a 428.

 

[robmort]

Are you saying that the minimum flow has to be the same when the boiler is on minimum output as when it is on maximum output?

No of course I'm not saying that, but the MI requires 20.3l/min minimum for the 428. When the heat ouput from radiators is less than the minimum heat ouput of the boiler (when TRVs close under warm conditions or only a few radiators are needed) the differential temp across the boiler approaches zero as the ABV opens as designed to maintain the required flow rate, and these boilers start misbehaving.

 

[D_Hailsham]

It's necessary to maintain a minimum flow rate through the boiler e.g. to stop kettling which damages it.

Are you saying that the minimum flow has to be the same when the boiler is on minimum output as when it is on maximum output?

No of course I'm not saying that, but the MI requires 20.3l/min minimum for the 428.

But that is exactly what you are saying: "the MI requires 20.3l/min minimum for the 428".

The 428 and the 438 use the same heat exchanger, but they have different "minimum" flow rates. If this is just to prevent kettling I would have thought the flow rate would be the same for both boilers. I just don't understand why a boiler needs the same flow rate at 5kW as it does at 25kW.

 

[fezster]

As I've learned (from painful experience) - the minimum flow rate may well be required because of the high output of Vaillant boilers in the first 30/40 seconds, regardless of if they modulate down after this time. If minimum flow rate is not maintained for those first 40s, you'll probably hit an S53 before the boiler has a chance to modulate.

 

[ChrisNason]

As I've learned (from painful experience) - the minimum flow rate may well be required because of the high output of Vaillant boilers in the first 30/40 seconds, regardless of if they modulate down after this time. If minimum flow rate is not maintained for those first 40s, you'll probably hit an S53 before the boiler has a chance to modulate.

I experience the sudden shutdown as the temperature rise is quick. Usually when the system is only serving DHW and that is nearly up to temperature.

So what pump setting and bypass will help this? If the bypass is mostly open and the pump is on the lower setting the water is returning with almost no heat removed. If the pump is on a high setting with a mostly closed bypass the water is going through a nearly up to temperature DHW cylinder and again almost no heat removed.

I suspect that setting a big difference between the DHW cylinder and the boiler temperature might be the best I can achieve. At the moment this is 50c on the cylinder and 70c on the boiler.

 

[robmort]

But that is exactly what you are saying: "the MI requires 20.3l/min minimum for the 428".

............ I just don't understand why a boiler needs the same flow rate at 5kW as it does at 25kW.

You think Vaillant's MI's are wrong then?
I'm saying that clearly at 5kW the min is as given, and at higher power a higher flow rate is needed as given by opening TRV's etc., and of course this is matched by the boiler modulating.