Boiler problems in big house - Change required?

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Goku

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Let me first list the set-up we have:
  • Vaillant ecoTec Plus 637 system boiler
  • Zilmet unvented hot water cylinder with external expansion vessel
  • 1 zone valve for Central Heating
  • 1 zone valve for hot water cylinder
  • 1 Grundfos 15-50/60 circulation pump (on flow pipe from boiler)
  • 16 radiators (including towel radiators and hot water cylinder)
  • 3 floor house: 4 bedrooms + 3 bathrooms (with towel radiators in each) + outbuilding (2 radiators)
  • Powerflushed the system out extensively over the weekend in both directions and flushed the boiler as well (don't think the system was ever flushed before)

The 2 main problems we have are:
1. Struggle to get good heating to all radiators despite balancing
2. Can't have hot water and heating on at same time as hot water cylinder ends up getting priority (next to boiler) and the heating system suffers

I have tried to detail both of the above in the two headings that follow.

Heating problem:
The boiler stopped working a few days after the power-flush (keeps overheating and switching off then restarting). The plumber sent by the home emergency insurance said (off the record) that the boiler's internal pump and the external pump were not enough for our house and between them, they were causing circulation problems (i assume that they run a different speeds so this could be a reason). He advised us to add an auto air vent (which we have done now), and he recommended that I change the boiler to a heat only boiler (ecoTec plus 438) and add a bigger pump (Grundfos 25-80).

Questions that I have:

1. Is this a good recommendation?
2. Can we get away with changing just the pump for a bigger one?
3. I understand the wiring diagrams for boilers (we have an S plan system). Is it straightforward to fit the new boiler (i.e. connect flow, gas, return, power from relay, switched live back to relay) and then to configure it?

Simultaneous heating and hot water problem:
The plumber mentioned above said that the cylinder thermostat should switch off the zone valve when the temperature of the water in the cylinder reached the target temperature. However, I have never known the zone valve to switch off (not checked the zone valve's manual level, but I am assuming) so the boiler is constantly heating the water; the water temperature does get hotter if we turn up the cylinder thermostat. Does this indicate:

1. The plumber is mistaken?
2. The zone-valve switch-off function of the thermostat is faulty?
3. The thermostat is wired incorrectly?

Thanks in advance
 
Their is a lot of info to process here, and as im sure you appreciate its over the internet. Has it ever worked perfectly or has it gone worse over time? The basic set up sounds fine. Turn the cylinder stat down and check the motorized valve shuts down then open cylinder stat up and recheck that should confirm all is correct. Is there a gate valve fitted on the hot water circuit it may need balancing down. If it nevet overheated before " flushing" it would suggest its something that has been done during it that has caused it. finally balancing is a trial and error job keep persevering untill you hit the magic formula.
 
You need hydraulic separation and a bigger pump at a guess.

Where in the country are you?

Not bother with your home emergency insurance or whatever.
You need an independent engineer who know how to work in a system of this size.

Hot water priority would be worth looking at.
 
Keep the 637 and put in a low loss header, is it possible to zone the rest of the heating ?
 
Lol. I used that exact term just last night... “a nice brace of San Miguel’s please barman” :)
 
rochdalegas said:
Their is a lot of info to process here, and as im sure you appreciate its over the internet. Has it ever worked perfectly or has it gone worse over time? The basic set up sounds fine. Turn the cylinder stat down and check the motorized valve shuts down then open cylinder stat up and recheck that should confirm all is correct. Is there a gate valve fitted on the hot water circuit it may need balancing down. If it nevet overheated before " flushing" it would suggest its something that has been done during it that has caused it. finally balancing is a trial and error job keep persevering untill you hit the magic formula.
Click to expand...


I agree that it's a lot to process. Hopefully my breakdown has helped make it easier to digest.

Yes, the system has worked over the years, albeit not in an ideal fashion (see https://www.diynot.com/diy/threads/overheating-and-cut-off-on-vaillant-ecotec-plus-637.497900/). Two radiators just never worked and I don't ever remember the heat output from the radiators in the house being that great; except for the ones with the easiest flow from the flow. I have balanced the system a few times to find something that we could live with; but as you can see in the other thread, the boiler would keep shutting off every 10 minutes because I suspect the hot water wasn't being pumped away quickly enough. This means that although we could live with it, it wasn't ideal.

I just turned down the cylinder stat and observed that the zone valve didn't switch off.

There isn't a gate valve for the hot water circuit.

Yes, I agree that flushing the system has definitely introduced issues; which I want to discuss in the dedicated thread above.
 
Phill Peck said:
You need hydraulic separation and a bigger pump at a guess.

Where in the country are you?

Not bother with your home emergency insurance or whatever.
You need an independent engineer who know how to work in a system of this size.

Hot water priority would be worth looking at.
Click to expand...

I agree that I need a bigger pump. Reading on hydraulic separation, are you suggesting that to help our problems with the hot water circuit being given priority over the heating circuit due to its proximity? Sounds like a good idea if we have multiple zones in the house, but as we only have the two, I think I need to look into replacing the cylinder thermostat to begin with. But I will keep this suggestion in mind.

We're in London.

Counting on the emergency plumber to get the system working for now. Hoping that I can get some advice on here first before looking to an independent engineer.

We have a separate, unvented (pressurised) hot water cylinder so I don't think hot water priority is going to be suitable for our system.
 
Fit a gate valve to the hot water return to the boiler to allow you to balance it. Alternatively, just ensure the HW is heated before the heating comes on. If the zone valve is not working, it is almost certainly stealing all the heat from the CH side, and probably causing the boiler to cycle unnecessarily, so get the zone valve replaced if it is faulty.

Fitting a Low Loss Header would allow you to use the internal pump in the boiler to drive the boiler, and then fit a suitably sized pump to your heating system. As the boiler will now be hydraulically separated, you may even find your existing 15/60 pump is sufficient. I did something similar recently and have a big system, which required a 25-80 pump to work adequately. After fitting the LLH, I'm able to use an Alpha 2L 15-60 in PP2 mode to drive my entire heating system.
 
picasso said:
Keep the 637 and put in a low loss header
Click to expand...

Just had a look at a video on low loss headers:
Is this the same as a hydraulic separator? Would this be useful in my set up where I only have 2 zone valves?

Would a bigger pump still be beneficial with a LLH?

picasso said:
is it possible to zone the rest of the heating ?
Click to expand...

You mean add more zone valves to separate out parts of the house? Why would this be beneficial to do in my circumstance/set up? I would like to do this with the outbuilding, but as the pipes are all teed off from a single main flow, it will not be easy to zone parts of the house.
 
fezster said:
Fit a gate valve to the hot water return to the boiler to allow you to balance it. Alternatively, just ensure the HW is heated before the heating comes on. If the zone valve is not working, it is almost certainly stealing all the heat from the CH side, and probably causing the boiler to cycle unnecessarily, so get the zone valve replaced if it is faulty.

Fitting a Low Loss Header would allow you to use the internal pump in the boiler to drive the boiler, and then fit a suitably sized pump to your heating system. As the boiler will now be hydraulically separated, you may even find your existing 15/60 pump is sufficient. I did something similar recently and have a big system, which required a 25-80 pump to work adequately. After fitting the LLH, I'm able to use an Alpha 2L 15-60 in PP2 mode to drive my entire heating system.
Click to expand...

I'm worried about fitting a gate valve because I want the hot water cylinder to get a good amount of heat when it needs it. The zone valve is working because it switches on and off with the programmer. I think the cylinder thermostat needs replacing because it's doing nothing to cut off the zone valve (no sound of a 'click' when I turn the dial down).

I'm intrigued by this low loss header concept. You're the third person to suggest hydraulic separation on this thread; so I will definitely have to look further into this. How does your set up look?
 
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