Discuss Time to change Boiler? in the Central Heating Forum area at PlumbersForums.net

I currently have a Vaillant ecoTEC Plus 438 system boiler which is 13 years old. I have been having problems with the boiler which potentially boils down to the heat exchanger so I am now looking to bite the bullet and have a new boiler installed.

My set up at home is that I have a 4-bed detached with 21 radiators (includes 4 towel rails), a 6m x 4m orangery/conservatory that was built last year with under-floor heating (UFH), and a Megaflo for storing hot water. Last year, I had all my radiators changed (unfortunately I did not have my system thoroughly flushed) and the main pump upgraded to a Grundfos UPS2 25-80 pump. I control the hot water, central heating, and UFH through the Heatmiser app. The boiler is located in the garage whilst the pump, 3 individual zone valves and megaflo are all located in an airing cupboard on the landing.

Quite a few years ago, I was getting an S.53 code on my boiler "the flow-return temperature difference is too great OR Boiler is in waiting period of modulation of block function (or operation block function) due to water shortage". I turned down d.0 of the boiler to 24kW which eliminated the S.53 status.

Last winter, I had 2 issues:
  1. I found the radiators furthest away from the boiler were hardly heating up. I had an engineer come round who eventaully found the issue - debris in the main pump. After removing this, these radiators heated up perfectly well.
  2. when I only had the UFH turned ON, I found the boiler would fire for a minute and then stop. This would be continuous (short firing). The only way I would get round this would be to turn on the central heating so that the UFH would remain on. The same engineer looked into this and believed there was a problem but as he had not installed the UFH, he was not 100% sure where the issue lay. He said there could be a blockage somewhere and/or the heat exchanger could be partially blocked. He suggested a low loss header to resolve the issue since it would elimate the problem of return & flow tempaerates being too great, and therefore the boiler turning off.

Since my boiler is 13 years old, I am happy to replace by boiler but I obviously want to avoid spending lots of money but still find I am in the same position where I cannot run my UFH independently. I have had a few heating engineers come over to quote for a new boiler. The majority have recommended the Vaillant EcoTEC Plus 430 (30kW) heat-only boiler, and believe the installation of a heat-loss header is an overkill. They pretty much believe the heat exchanger on my existing boiler is partially blocked. A power flush is highly recommended.

Can anyone recommend if the Vaillant EcoTEC Plus 430 will be suitable? I do have a heating engineer in mind who will be returning again next week to carry out a thorough examination of my system. Is there anything I should make sure he looks into?
Problem with the ufh is min heat load eg if the boiler only goes down to let’s say 6kw and one ufh zone is 800w you will get increased cycling

Heatloss will tell you the size of boiler but don’t include the hot water cylinder in this calculation
Thanks @ShaunCorbs. Does this mean I need to find a boiler that can go down lower than 6kW?

I did have an engineer go round and look at the sizes of each of the radiators fitted and allowing for 3kW for the UFH, he strongly recommended a 30kW boiler.

Could the issue I am having is that my current boiler cannot go below the minimum required for the UFH to work on its own?
Yes and ideally yes but you can work around it to maximise burn time but slightly less efficient
You could install a llh or a buffer
@ShaunCorbs - the heating engineer who will be carrying out a thorough investigation did mention about a LLH but said that the cost would be around £1,000 for this.

What do you mean by a buffer?

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