New unsized boiler

Nickbo222

Hi. I’m hoping for some advice. We recently had a gas engineer come round to look at our boiler as it broke down. He gave us a quote to service and fix it. A day or two later it completely died so we asked for a quote to replace it. It was very cold and he was very busy but fitted us in on a weekend. We got two other quotes, but preferred this first one. 

The boiler was fitted but we are now finding that it does not heat the house sufficiently. We also have water flow issues, when using the shower and a tap goes on.

I had very little knowledge of boilers at the time and didn’t do a lot of research as we just needed it replaced ASAP and I trusted him.

I have since learned that the old boiler was a 35kw combi and he put in a 30kw combi. A similar house two doors down has a 37kw system boiler with tank. He is only now going to do a heat loss calculation to demonstrate that the boiler is adequate (which I’m sure he make sure it does). He did a flow measurement after the boiler went in and said we don’t have sufficient flow for the boiler.

he has given us quotes to change pipe work, radiators etc to overcome these issues. My worry is we can spend this money and it will make little difference, when in fact, had the right boiler been installed from the outset we would not be in this situation now. 

He is not accepting any blame and is suggesting that because we asked for a combi that’s what he gave us and he considers it powerful enough for the house based on his experience. But, given that he is the expert, we feel that he should have told us if a combi was not suitable and recommended a system boiler with tank if that was more appropriate. Also replacing a boiler with a less powerful one seems like a bad idea now, but I didn’t think to check this before.

any thoughts welcome.
 Thanks 

MWT

To be fair to him, if you have a combi and you ask for a replacement he is going to fit a combi, and the similar house two doors down with a 37kW system boiler is probably dramatically over sized.

How many radiators do you have in the house?

daveyjp

30kw should be adequate to heat a very large property, was the system powerflushed before the install and balanced correctly afterwards?

The boiler size also needs matching to your cold water flow rate, test the flow rate and see if it is.

A drop in water flow is to be expected when two taps are opened.

[Deleted User]

It is worth remembering that 30kW boiler only runs at 30kWs when it is heating hot water and at the start of the heating cycle. The boiler then modulates down to a much lower output level. Unless your home has no insulation, then a 30kW boiler should be more than big enough. Indeed, if it ran constantly at 30kWs you would be paying upwards of 70p an hour for heating. The lack of flow suggests that there has been a build up of sludge in your radiators etc which has been disturbed when the new boiler was fitted; sadly, this can settle in the heat exchanger. For this reason, it is usual when fitting a new boiler to an old system for the installer to recommend a system clean. Most boiler warranties exclude failure due to sludge in the system

btr30

Were all three quotes looking to instal the same sized boiler?
Is there an option to increase pump speed?
Is the boiler running constantly to try and achieve room temperature or is it cutting out on its own thermostat ( and is there scope to increase setting on this).
The downsize from 35 to 30 kW will affect hot water capacity provision.

matelodave

As everyone has said, the size of the boiler will only affect the flow rate of your hot water so it will take a bit longer to fill the bath.

There's more than enough power to provide heater for a very quite a substantial house so your heating should not be affected unless the boiler flow temp has been set to a lower flow temperature than the previous boiler. If the old one was set to 60 or even 70 and the new one is now set to 50 then it will take longer to heat the place. It will also use less energy as the boiler should be working closer to it's optimum settings.Try increasing the flow temperature a bit

Cardew

As the other have said, the reducion in power output from 35kW to 30kW will only affect your water heating capability; not the heating capability. In any case the increased efficiency of the latest combi boilers should go some way to offsetting the 5kW shortfall. 

If your radiators heated the house sufficiently with the old boiler, why do they need replacing? The CH system should have been flushed when the new boiler was fitted; failure to do this can sometimes affect the boiler warranty.

What is the make/model of the boiler?

Nickbo222

Thanks all! 

It’s a large house with 21 radiators over 3 floors and 5 bedrooms. Some rooms have 2 radiators and some of the radiators are very large. But they are old and need to be replaced. 

The system was flushed prior to installation.

We now know the flow rate to be 11 l/min but I believe the boiler can accommodate 20 l/min. It’s much higher from the external taps so he wants to try replacing the stop tap as it may be a restriction. 

The boiler is set to 70 degrees and it’s constantly on. It can’t reach 21 degrees in the rooms downstairs where the thermostat is located.

In the top floor radiators barely get luke warm. The boiler is on the lower ground floor so a long way from the top floor and I did wonder if a separate pump would help. 

It’s a vailant eco tec 938. 

The radiators must be 30-40 years old. 

Whist a lot of the gunge was removed I suspect there is more. 

Any further thoughts welcome
Thanks 

Nickbo222

A few people have suggested that the 30kw is for hot water. The boiler is 38kw for hot water and 30kw for CH. My main issue is with the CH, as the rads are just not getting hot enough on the top floor. 

niktheguru

Why did you get a combi boiler for a 5 bedroom house!! Particularly if you cold water flow isn't great. Its a recipe for disaster. Surely this plumber should have advised you of the options. Did all the other quotes suggest a combi boiler?  Lets hope you don't want to use hot water in more than one room at any time!! Whilst its not forbidden to use a combi on a large house, you do lose water pressure if you use more than one tap with a combi, but you do have that advantage of instant hot water. For big houses system boilers are a better option.

Perhaps I would suggest getting another plumber to have a look at the situation too, just to make sure this original person isn't covering his tracks and overcharging you with unnecessary interventions.

[Deleted User]

Older boilers were designed to run at high flow temperatures of 80C plus. To be efficient, condensing boilers need to run at a lower flow temperature and will not condense unless the return temperature is below 56C. It follows that modern boilers need radiators with a larger surface area to give out the same amount of heat. You could adjust the pump if it isn’t at maximum already and rebalancing your existing radiators would be advisable.

https://www.onlyradiators.co.uk/blog/how-to-guides/balancing-radiators-guide