Vaillant target flow temp

[wallaceoc]

Hi,

I have a Vaillant EcoTEC Plus 637 in my house which has 4 beds and a living room with 6 rads total. I also have underfloor heating running off the boiler and hot water in an unvented system.

My issue is that when the room stat for the radiators calls for heat the boiler fires but never reaches the target flow temp of 69 degrees. It will burn to roughy 45 degrees and stay there for a few mins, cut out and fire back up again. But for the underfloor heating the boiler fires up and burns close to the target flow temp and stays on between 55 and 70 until the room thermostat turns it off.

To me it doesn’t make sense. The radiators take an age to heat the rooms and they never get overly hot to the touch. I have bled the rads, checked the pressure etc and all looks fine.

Any suggestions would be appreciated.

Many thanks!

 

[muggles]

The boiler is massively oversized for the system, so it's reduced itself to minimum output to prevent damage. The solution is to either run the UFH and radiators at the same time, or get a boiler that is more appropriately sized (and thus more gas efficient as well).

 

[Razor900]

Can't be 4 bedroom with 6 rads surely??

Sounds like it needs range rating first

 

[fezster]

OP - What S code is shown when the boiler stops firing - eg. S7?

You need to further diagnose by looking at the following:

1. Check d5 to see what the flow temp target value is when the boiler is firing.
2. Monitor d40 and d41 to see what the flow and return temps are whilst the boiler is firing.

It's also worth checking parameter d.17 which is flow/return regulation sensing. It should ordinarily be set to flow regulation sensing.

What controls do you have? Are they vaillant ebus controls?

 

[wallaceoc]

Thanks for your replies Let me add a bit more detail to maybe help.

It’s 4 bedrooms. One rad per bedroom and a living room with 2 rads.

The house is approx 1700 sq feet (160 sq m) in total and the uderfloor heating is 2 zones over approx 600 sq ft (55 sqm).

Previously a Vaillant engineer had set the heating partial load from auto to 25 (before I had an extra bedroom added and the underfloor heating) and I just checked and last service the engineer must have set it back to auto.

Should I set the partial load back to 25 as the symptoms of the boiler firing for a bit and shutting down are similar to the symptoms before the engineer set it to 25 from auto.

Thanks

 

[wallaceoc]

OP - What S code is shown when the boiler stops firing - eg. S7?

You need to further diagnose by looking at the following:

1. Check d5 to see what the flow temp target value is when the boiler is firing.
2. Monitor d40 and d41 to see what the flow and return temps are whilst the boiler is firing.

It's also worth checking parameter d.17 which is flow/return regulation sensing. It should ordinarily be set to flow regulation sensing.

What controls do you have? Are they vaillant ebus controls?

Hi thanks for this reply. It came in as I was typing my last reply so I will check these out. Do I have to go to a diagnostics screen to see the s codes?

Thanks

 

[fezster]

The S code will be shown on the front display - you may need to press "i" to see it.

The d codes are accessible in the menu. You can check the manual for them here:

https://www.vaillant.co.uk/downloads/ecotec-installation-and-servicing-manual-261417.pdf

 

[muggles]

Thanks for your replies Let me add a bit more detail to maybe help.

It’s 4 bedrooms. One rad per bedroom and a living room with 2 rads.

The house is approx 1700 sq feet (160 sq m) in total and the uderfloor heating is 2 zones over approx 600 sq ft (55 sqm).

I think that's confirmed my theory. Normally work on a maximum of 100w/sqm for UFH so a 37kW boiler will (as a rule of thumb) be good for at least 370 square metres. If the UFH is only running at 75w/sqm your boiler would be good for around 490sqm. Even at a very high (and frankly unlikely) 150w/sqm UFH output it can do 250sqm. The boiler is simply too big for the system, by a very very long way. Yes it can modulate down, but the problem is that it fires up at approx 70% of its maximum output, overheats before it can modulate down, shuts off, and repeats. This uses an enormous amount of gas and results in slow heat-up times. Turning the maximum output down to 25 (or maybe even less) will certainly help. I'd go for 20 then turn it up again if it turns out to be insufficient, which I suspect it won't. Ultimately though, your boiler is just too big. As a very rough calculation, 55sqm of UFH at 150w/sqm plus six big rads at 1.5kW each is 17.3kW. If it's 100w/sqm for the UFH then your total system output is 14.5kW. Trying to pump 37kW or even 25kW into that just isn't going to work.