CH OK, HW only luke warm

[benwhite]

We have recently moved into a new house with a Halstead Quattro combi boiler. Everything has been working OK but recently we have noticed that the time for the water from the hot tap to actually get hot has increased.

This morning the HW came out hot for about 30s, and then went luke warm and stayed luke warm. Also, the flow rate seems to have increased, because it's coming out much faster than it was a couple of days ago. This is the same on all taps in the house, the hot comes out hot for a bit and then goes luke warm and stays.

Haven't fiddled or messed about with anything on the boiler, short of bleeding the radiators and refilling the system a couple of weeks ago.

Thanks in advance for any pointers.

 

[chrishutt]

it's coming out much faster than it was a couple of days ago.

This probably explains why the hot water is cooler. The combi boiler puts in so much heat per minute. The temperature of hot water will then depend on the flow rate (typically around 10 litres/minute) through the combi heat exchanger.

If you turn down the outlet taps, does the hot water reach a reasonable temp?

When you bled and refilled the system, might you have altered the setting on the cold inlet isolating valve below the boiler?

 

[benwhite]

If you turn down the outlet taps, does the hot water reach a reasonable temp?

No, it didn't seem to. I did try this because as you say, the higher the flow rate, the lower the temperature for a given input power. This is why I'm really confused

When you bled and refilled the system, might you have altered the setting on the cold inlet isolating valve below the boiler?

No, infact I probably should have been clearer when I said "bled" and "refilled"... There was some air in two of the radiators so I just bled out the air and topped up the system to the same pressure. Certainly haven't changed anything else under the boiler.

 

[chrishutt]

Well, it might be something as simple as a failing diaphragm. This is a relatively cheap spare part but can be fiddly to fit.

However I think the fault needs a little more diagnosis before any assumptions are made. I would still want to know exactly what happens when you turn down the flow through the hot taps, which you haven't described.

 

[benwhite]

Thanks for your replies so far, Chris. Sorry I can't give any more information at the meoment... it only happened this morning and I had to leave the house for work or face being stuck for hours in the London traffic !

I'll investigate the effect of slowing the flow rate this evening.

 

[chrishutt]

If you're in Surrey, you may be in the patch of ChrisR who posts here and is a very good (for when you accept the inevitability of calling in an engineer). You should find his email in his profile.

 

[benwhite]

Thanks for the contacts Chris. I had already accepted the inevitability of having to call an engineer, but would still like to understand this problem.

Slowing down the flow has no effect - the temperature is still the same... in some cases it gets worse - when the burner starts to modulate you get cold patches in the flow.

I thought maybe it was a problem with the diverter valve... i.e. most of the hot was going into the CH circuit, but I checked the CH pipes when running hot water only and they're definitely cold.

I had a look through the boiler manual and it says that it's fitted with an automatic flow rate controller... the flow rate has definitely increased since 2 days ago, so maybe this is at fault?

What I don't understand is why the water doesn't get hotter when the flow is decreased... maybe the HW thermistor or driver PCB is at fault?

 

[chrishutt]

maybe the HW thermistor or driver PCB is at fault?

You obviously know a lot more about combis than you let on. I'm not familiar with Halsteads but I'd be thinking along the same lines, although the diaphragm (or flow switch / flow rate controller) is still suspect.

 

[Agile]

The temperature output depends on a lot of variables.

These include the amount of gas being burnt, control system within the boiler, thermal transfer rate of the secondary heat exchanger, pump flow rate etc.

Only a competent boiler engineer can properly check these and a DIYer will usually be changing parts on a hit and miss basis.

Sometimes the problem is only caused by a miss adjustment.

Tony

 

[ChrisR]

Sometimes the problem is only caused by a miss adjustment.

Blokes fiddle too sometimes, not just the women..

 

[benwhite]

Thanks everybody for your replies. I'll get an engineer in the new year... too busy at the moment to do anything about it.

Miss or Mr Adjustment certainly haven't been tampering. One day it was fine, the next not.