Vaillant Ecotec Plus 637 or Worcester Bosch 8000 35kW

[windsortg]

I currently have a Vaillant ecoTEC Plus 438 system boiler which is 13 years old. I have been having problems with the boiler which 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, and a Megaflo for storing hot water. Last year, I had all my radiators changed 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.

I have been in discussion with a few plumbers and they have recommended either the Vaillant EcoTEC Plus 637 (37kW) system boiler or the Worcester Bosch 8000 boiler. I have found that those who are accredited as Worcester Bosch installers recommend the Worcester Boiler as it gives you a 12-year warranty whilst the Vaillant accredited installers receommend the Vaillant boiler (as it comes with a 10-year warranty). Some of the plumbers have no preference, they are happy to fit either. The advantage of the Vaillant they say is that you have more control on the Vaillant in terms of what you can set it to, and to make adjustments.

Can anyone recommend which boiler I should go for?

 

[ianmcd]

it has been a legal requirement in England and Wales for a few years now that a whole house heat loss calculation is carried out, it is unlikely you need 37KW, fitting an oversized boiler is inefficient and doesnt do the boiler any good, you would also be advised to change your new system to Priority domestic HW

 

[windsortg]

thanks @ianmcd. I have had quite a few registered heating engineers come over, and no one has mentioned they need to carry out a whole house heat loss calculation. You seem to imply that a 37kW boiler may be oversized.

Excuse my ignorance but what do you mean by 'Priority domestic HW'? From by current setup, the HW has priority over the central heating & UFH to be heated up first.

 

[ianmcd]

thanks @ianmcd. I have had quite a few registered heating engineers come over, and no one has mentioned they need to carry out a whole house heat loss calculation. You seem to imply that a 37kW boiler may be oversized.

Excuse my ignorance but what do you mean by 'Priority domestic HW'? From by current setup, the HW has priority over the central heating & UFH to be heated up first.

Then you already have PDHW, but yes a whole house keat loss calculation must be carried out, as already said unlikely you will need 37KW, your proposed installers are just looking at the old boiler and matching the size, that is not how you should do it

 

[windsortg]

From the calculations I have of the 21 radiators, it amounts to approx 24 kW (~82,250 BTU). I understand you generally allow for 3kW for HW but I'm not sure what the kW would be for the UFH. The UFH is heating a 6m x 4m orangery with plenty of glass, and the floor contains two loops of approx 100m on each loop run.

 

[muggles]

Certainly sounds more realistic but for the best result use www.heat-engineer.com

We don't add anything on for hot water

 

[windsortg]

as part of getting building reg permission for the orangery last year, I had to get a company to carry out a SAP2009 calculation to justify the heat loss as part of demonstrating compliance with Approved Document L1B (2010 Edition). I am not sure if this would cover the whole house heat loss calculation. I was just going through their report but I cannot for the life of me figure out how this would help me in establishing the right size boiler

 

[ianmcd]

I understand you generally allow for 3kW for HW

you dont add anything for HW as you have a PDHW set up so when the water is heating the CH is not used so all the energy is used to heat the HW Cylinder, that is a more realistic figure, it is very unlikely that all radiators will be on at the same time so oversizing a boiler by 30% is not a good idea

 

[windsortg]

the way my central heating is set up. I only have one zone valve that controls all 21 radiators (even though the majority of rads have TRVs fitted), so when CH is on, all radiators will initally be on.

Based on the info provided, are the Worcester Bosch 35kW and Vaillant 37kW both oversized for my needs?

 

[muggles]

Almost certainly. You probably need something around 20kW. The only way to be sure is to do a full heat loss calculation. You can do it yourself on Heat Engineer if you want. It'll cost you £12 and a few hours of your time

 

[windsortg]

ok, I've got a couple of more heating engineering companies coming over in the next few days. I'm just baffled that from the ones who have already been, apart from one (who believes a 30kW boiler will be sufficient), the others of all suggested a 35kw - 40kW boiler. From the reviews these companies have had, their past customers have been very happy. Also, none of these companies have suggested carrying out a heat loss calculation.

Coincidently, with my existing bolier which is a 38kW boiler, 4-5 years back I was getting the s.53 status on the boiler display. From the manual, this status means "flow-return temperature difference is too great OR Boiler is in waiting period of the operation of block function due to water shortage (VL-RL spread too big)". The way I have overcome this is by reducing the power rating of the boiler to d.0 = 24kW). So essentially, I believe, I am running the boiler at a maximum of 24kW output.

I may possibly have a partial blockage somewhere and/or a design flaw when the central heating was first installed many years ago, and it has then been extended over the years.

Is there any particular experience needed for carrying out a full heat loss calculation via www.heat-engineer.com ?

 

[ianmcd]

Well you have just proved yourself that you dont need 35KW

 

[ianmcd]

There are many free heat loss calculators available, nothing really difficult in using them

 

[windsortg]

There are many free heat loss calculators available, nothing really difficult in using them

if you can provide any that you are aware of, that will be greatly appreciated

 

[ScottishGasMan]

d.0 = 24kW)

If you changed this to 24kW a few years back you've just proved you're current boiler is massively oversized, which often is the root cause of the S.53/S.54 errors.

The reason most haven't picked that up is that they are not heating engineers, their boiler fitters.


As far as I knew the full heat loss calculation was only required when installing a full new system, not just a new appliance. I may be wrong, it's different up here. But either way its a good idea.