Combi boiler query about hot water

[steventaylor]

Hello

Please can someone help me with my query:

We are thinking of having a combi boiler / central heating fitted to our 2 bed house, as we currently have an old immersion tank (economy 7) and storage heaters.

My concern is with the water, I have heard different things:

If I am running a bath, having a shower or washing up, will the hot water reach the boiler's hot water temperature and cut out and come on again in say 2 minutes, like a normal boiler thermostat does when the temperature is reached/drops again? If so, this may be a problem with the hot water side for us.

If I am completely wrong, how does it work, doe sit stay on or get colder after running for a period of time ? Is it ok for my 2 bed house?

Thanks for answering my query/concern.

 

[Rico99]

Hi there,

Very simple really.

You tell the boiler how hot you want the water on the controls.

Turn on your tap and the boilers keeps going heating the water to your desired temp until you turn the tap off.

Rico

 

[kevplumb]

depending on flowrate / temp rise

you can run a combi all day on hw

 

[steventaylor]

sorry what do you mean depending on flow rate/temp rise? Do you mean a slower flow will mean more temp rise and then the water will run cool/cold for a short period??

Thanks for your responses, I asked as my friend has a combi and after about 20-30 seconds or so, his hot water runs cold for 20 seconds or so, then gets hot again, I wondered if this was the norm??

 

[kevplumb]

slower flow hotter water

your friend has a problem with his/her boiler

 

[steventaylor]

I see, so slower flow, hotter water and water will run cold for a short period as first thought as temperature may be reached?? Not that I want it running slow anyway.

Yes he said it is either a gas valve burner adjustment,scaled heat exchanger causing reduced flow, faulty themistor?? or i think he said flow rate problem but am not sure now. He's got BG coming round on Thursday to sort it. Apparantly it works well with central heating on, but when hot water only on the problem happens. He's got a worcester combi.

 

[kevplumb]

I see, so slower flow, hotter water and water will run cold for a short period as first thought as temperature may be reached?? Not that I want it running slow anyway.

water will run cold till the heated water runs through the pipe

it will then carry on ad infinitum if the boiler is working ok

 

[steventaylor]

Water will run hot until tap closed, but I was just querying about the flow rate, if this was incorrect (say set to slow) would this mean the water would run cold for a while like my friends?

 

[kevplumb]

Water will run hot until tap closed, but I was just querying about the flow rate, if this was incorrect (say set to slow) would this mean the water would run cold for a while like my friends?

then why not just ask the question in the first place ?

if the flowrate is low the boiler will modulate to keep the temp where it should be ish

 

[steventaylor]

thanks for your help. sorry for sounding rude.
I'll let you know how my mate got on with BG. What do you think will be the cause..... take a guess!

 

[kevplumb]

partially blocked dhw heat exchanger is first port of call

 

[steventaylor]

I'll let you know!! Actually he said it was 1 of about 4 things (or was told anyway): scaled heat exchngaer/reduced flow, wrongly set flow rate, wrong gas burner pressure or faulty thermistor on hot water side... but we will find out!

He has a worcester and apparantly they have different heat exchnager to most combi's and no diverter valve?? a single heat exchanger or something.

 

[jackecole]

Interesting post. Kevplumb:

How do you test for a blocked heat exchanger? Some of the fins may have melted possibly causing overheating, causing the water to go hot and cold hot and cold.

 

[kevplumb]

its called a biflow

both h ex are in one lump

in which case scale might well be the prob

not really fair throwing questions out when i dont even know the boiler model

 

[jackecole]

I know the Worcester 24/28i junior has a crossflow heat ex. A few probably do though. As mentioned could be scaled up (the hot part) as fresh water going in continually, especially if a hard water area and no scale redcuer fitted inline (on cold pipe in to boiler)

I wonder why biflow heat ex's are prone to scale more than other type of combi's with 2 heat ex's?