Low boiler return temperature

[ForestHillRob]

I have a 4 year old Vaillant EcoTec 428 system boiler which has been showing an F23 error code which is apparently down to a high differential between flow and return temperatures. It is a large 4 bed house with 16 rads.


I had a plumber in who changed the pump on the system, but he said that the return pipe at the boiler was still cool and he thought that maybe there is some restriction in the piping. There are no valves that could be closed causing a restriction and all of the radiators are getting hot. It is just that that HW isn’t flowing properly back to the boiler.


After the pump was changed the boiler ran ok for 36 hours and then I got the F23 fault code again. As the boiler is still under warranty I got a Vaillant engineer in. He checked the boiler and said that it was all ok, including the heat exchanger. He also drained some water from the system and checked the Magnaclean and said that it was all clean. I had a powerflush 2 years ago. The Vaillant engineer checked the flow and return temparatures and although the flow was 65-70 degs, the return was only coming back at about 30 or so. The fault comes up if the differential is greater than 35 degs. He also thought that maybe there was some “restriction” in the return somewhere. The F23 fault has reappeared a couple of times since his visit, but I have been able to restart the boiler each time.


My question is, how can I find out if there really is a restriction somewhere and how can I identify where it is? I had the powerflush 2 years ago and Vaillant say the system seems clean, so I’m not sure that another £750 powerflush is going to do anything to fix the problem.

 

[bernardgreen]

The fault comes up if the differential is greater than 35 degs.

I may be missing something bu how can a return temperature be too low. ?

The cooler the return temperature then :-

1/ more heat has been distributed around the house by the radiators

2/ the water can take more heat energy from the combustion heat exchanger.

Or is back end rot still considered to be a problem.....

 

[ForestHillRob]

Logically I would agree with you that if the return temp is low then the radiators are doing their job and are transferring the heat from the hot water to the house.
However, as I understand it (and I'm not a CH engineer), the flow and return temps shouldn't have a differential of more than 35 degs due to potential damage to the boiler (thermal shock?). Vaillant's website says this differential is set at 35 degs and the F23 fault code results if that differential is exceeded.

 

[Madrab]

An engineer needs to go through a methodical process.

1. Understand the system - pipe runs - flow directions - bends, effective lengths and diameters, work out resistances and stuff.
2. Shut down all the rads, does the return get hotter, open each rad in sequence and monitor temps working with the pump speeds etc.
3. Bypass parts of the system depending on results to narrow down possible problems sections etc

Might be a long process.

 

[ForestHillRob]

Thanks Madrab.
Yes, I think that what I need is someone who is good on diagnostics and can methodically work through the system to find the problem. The first plumber suggested a powerflush, but I'm not convinced that spending £750 on that is going to fix the problem. I had one 2 years ago and the Vaillant engineer who came in this week reckoned that the system is clean. I just need to find someone who can do more than just say "There's some kind of restriction, Mate"

 

[shambolic]

Fit a gate valve between the flow and return and crack it open to allow circulation into the return quicker heating it up!!
I’ll get my spurs and horse

 

[Gasguru]

There's no such thing as a Vaillant EcoTec 428 system boiler, do you mean an Ecotec 428 heat only boiler?

If so there's dozens of postings on these models...do a search.

Essentially they must be plumbed in such a way as to ensure a sufficient flowrate of water circulates through the boiler to keep the temperature drop within limits (the heat exchanger has significant resistance).

An auto bypass valve (as specified in the manual) and high head pump is required for starters...a standard domestic 6m pump may not suffice.

I would check D40 and D41 (flow and return temperatures) with the burner on as a simple check to ensure the sensors are clipped to the correct pipes.

 

[nwgs]

I may be missing something bu how can a return temperature be too low. ?

The cooler the return temperature then :-

1/ more heat has been distributed around the house by the radiators

2/ the water can take more heat energy from the combustion heat exchanger.

Or is back end rot still considered to be a problem.....

If you understood how heating works you wouldn’t need to ask.

The boiler is faulting becuse there’s too big a difference between flow and return. Which indicates a restriction. The boiler won’t keep pumping out heat if there’s no where for it to go. So it faults.

A blocked return filter gives the same issue

 

[bernardgreen]

The boiler is faulting becuse there’s too big a difference between flow and return. Which indicates a restriction.

But it could also indicate that most of the heat in the crculating water is being given out to the ( very ) cold rooms before it returns to the boiler.

The boiler won’t keep pumping out heat if there’s no where for it to go.

So if the house is very cold and the amount of heat from the radiators into very cold rooms is too high then the boiler shuts down ?

 

[nwgs]

As I’ve said, you’ve no idea. Why would the boiler manufacturer take the time, effort and expense to design their boilers to fault with too much temperature differential?

A properly specd and fitted heating system wouldn’t behave like that.

Get your self back on to google and read a bit more

 

[bernardgreen]

Get your self back on to google and read a bit more

What should I read ? A book of boilers or another book of basic thermodynamics ?

 

[nwgs]

What should I read ? A book of boilers or another book of basic thermodynamics ?

Anything. It’s clear you can’t absorb the material matter.

 

[Dan Robinson]

Why would the boiler manufacturer take the time, effort and expense to design their boilers to fault with too much temperature differential?

Do you think it might have something to do with the heat exchanger not self destructing?

Good old Blowhardgreen, arguing black is white and only demonstrating his own ignorance..

 

[fezster]

Interesting that the boiler does not go into S53 (which is when the flow and return temp differential is greater than 30 degrees). F23 seems to be the same fault - maybe a vaillant engineer can explain why it would hit the latter?

I would monitor d40 and d41 as the boiler fires up to see how quickly the flow temp rises in relation to the return temp. Some things to ask your engineer and/or check yourself:

1. Has the system ever worked?
2. Is the pump working properly? Do a google for some checks which can be done.
3. Is there an ABV installed? What is it set to?
4. What are the pipe sizes in the CH system? Was the index circuit calculated when it was installed?
5. Has the pump been sized correctly? Which pump is it?
6. What is the heat loss of the house and what is d0 set to?

 

[nwgs]

Is there a return/internal filter for the heating ?