Combi Boiler Question

[Craig-SM]

I plan on removing the radiators in the kitchen and replacing with K2 ones which will be more compact. I will obviously need to drain the system down to remove the old ones but as I'm going to making adjusting the pipe work, repairing the walls and repainting whilst they are off am I OK to carry on using the hot water as it will most likely be spread over a week?

It's a Worcester Bosch 28CDI sealed system if it makes a difference.

 

[123hotchef]

The boiler will shut down if you drain the system. So no there will be no hot water.

 

[gaz1]

no

once you drain system down the boiler is out of use

if you measure between the rads you can normally get the right size to refit the new ones back onto the wall again

and you then dont need to drain the system down as you can flip the rad down and drain that rad while connected just the valves turned off but to flip down you need to cause a leak on both sides to spin them so you will need towels and wood blocks to rest the rad on its side

 

[Craig-SM]

The boiler will shut down if you drain the system. So no there will be no hot water.

I thought this might be the case.

 

[Craig-SM]

no

once you drain system down the boiler is out of use

if you measure between the rads you can normally get the right size to refit the new ones back onto the wall again

and you then dont need to drain the system down as you can flip the rad down and drain that rad while connected just the valves turned off but to flip down you need to cause a leak on both sides to spin them so you will need towels and wood blocks to rest the rad on its side

There will be quite a bit of repair work to do so will take time, the pipe adjustment not so much of an issue. I will just cap the pipes off once removed so I can get the repairs done and refill the system until ready to refit the new radiators.

 

[123hotchef]

There will be quite a bit of repair work to do so will take time, the pipe adjustment not so much of an issue. I will just cap the pipes off once removed so I can get the repairs done and refill the system until ready to refit the new radiators.

Thats the best way cap them off. I wonder if you could just cut the feed and return near the boiler and join it and just pressurise the pipe work so you can have hot water.

 

[Craig-SM]

Thats the best way cap them off. I wonder if you could just cut the feed and return near the boiler and join it and just pressurise the pipe work so you can have hot water.

The feed and return pipe are compression joints at the boiler so could do so long as I have enough movement in the pipework as that part of the central heating plumbing has been done very well and don't want to disturb it too much.

 

[Jaeger_S2k]

Don't go near the boiler, you could cause overheating of the heat exchanger, as it needs radiators to soak the heat away when you stop demanding hot water.
There's only 1 heat exchanger, when you start the heating the heat exchanger does its thing and supplies heated water to the circulation pipes. When you open a tap and demand hot water a valve closes the heating circuit and opens the hot water circuit makes hot water until you don't demand it, turn off the tap. However, the boiler doesn't know how much hot water you want or for how long you want it so for safety there's a dump loop (usually a rad nearest the boiler that has no TVR and should be open circuit, there are systems that have close loops nearer the boiler. But there will be a heat dump somewhere). I prefer a dump rad as the heat I just made comes to the system and I benefit from it rather than just a dump loop somewhere.
The reason I'm constantly telling Mrs Jaeger why the radiator has just become hot, and the CH is off. Because she just ran the hot tap!

The easiest way, close the lock shield and TVR, do as @gaz1 said to take the rad out. But don't just leave the TVR dialled down and not blanked. If it gets cold, the TVR will open and dump water all over the floor. If you have the TVR cap it came with screw that on, alternatively, a washer 1/2 nut and a 5 pence piece will blank it off safely until you're ready to do the pipe changes. Then you're olny dumping the water out once.

 

[gaz1]

Thats the best way cap them off. I wonder if you could just cut the feed and return near the boiler and join it and just pressurise the pipe work so you can have hot water.

i wouldnt advise doing it as the piping in the system allows for expansion when needed pre firing of the boiler

depending on what your doing if you can leave one side alone whilst cut and adjust the other side to fit would be the main coarse of action

but if the rads in the way for your adjustment just remove everything and cap both the feed and return pipes but take note on which is feed and return before you do at least you can get them on the right side when reconnecting them back up

 

[gaz1]

The reason I'm constantly telling Mrs Jaeger why the radiator has just become hot, and the CH is off. Because she just ran the hot tap!

The easiest way, close the lock shield and TVR, do as @gaz1 said to take the rad out. But don't just leave the TVR dialled down and not blanked. If it gets cold, the TVR will open and dump water all over the floor. If you have the TVR cap it came with screw that on, alternatively, a washer 1/2 nut and a 5 pence piece will blank it off safely until you're ready to do the pipe changes. Then you're olny dumping the water out once.
View attachment 502744

if your getting a rad hot when central heating is off you have an issue with the diverter valve /selonoid not throwing over properly

its the diverter valve for hot water/central heating valve within the boiler

you shouldnt get any heat near a rad when its turned off

 

[Jaeger_S2k]

if your getting a rad hot when central heating is off you have an issue with the diverter valve /selonoid not throwing over properly

its the diverter valve for hot water/central heating valve within the boiler

you shouldnt get any heat near a rad when its turned off

So just how does the boiler dump the heat it's just made to make hot water?

 

[Craig-SM]

Don't go near the boiler, you could cause overheating of the heat exchanger, as it needs radiators to soak the heat away when you stop demanding hot water.
There's only 1 heat exchanger, when you start the heating the heat exchanger does its thing and supplies heated water to the circulation pipes. When you open a tap and demand hot water a valve closes the heating circuit and opens the hot water circuit makes hot water until you don't demand it, turn off the tap. However, the boiler doesn't know how much hot water you want or for how long you want it so for safety there's a dump loop (usually a rad nearest the boiler that has no TVR and should be open circuit, there are systems that have close loops nearer the boiler. But there will be a heat dump somewhere). I prefer a dump rad as the heat I just made comes to the system and I benefit from it rather than just a dump loop somewhere.
The reason I'm constantly telling Mrs Jaeger why the radiator has just become hot, and the CH is off. Because she just ran the hot tap!

The easiest way, close the lock shield and TVR, do as @gaz1 said to take the rad out. But don't just leave the TVR dialled down and not blanked. If it gets cold, the TVR will open and dump water all over the floor. If you have the TVR cap it came with screw that on, alternatively, a washer 1/2 nut and a 5 pence piece will blank it off safely until you're ready to do the pipe changes. Then you're olny dumping the water out once.
View attachment 502743 View attachment 502744

TVR and a good 2m of pipe work is being removed and replaced so will cap at a suitable place but not the boiler

 

[gaz1]

depending on the boiler most use the same diverter valve for both hot water and central heating

when it goes faulty and they do

you get heat on both sides of the system hot tap on heats a rad up whilst the other side can give scalding water through the taps

look up rad heating when central heating is off

 

[Misterg]

I plan on removing the radiators in the kitchen and replacing with K2 ones which will be more compact. I will obviously need to drain the system down to remove the old ones but as I'm going to making adjusting the pipe work, repairing the walls and repainting whilst they are off am I OK to carry on using the hot water as it will most likely be spread over a week?

It's a Worcester Bosch 28CDI sealed system if it makes a difference.

You would need to connect a bypass between CH flow and return (min 22mm) and re-pressurise the system. (Per the installation manual S 8.7)

(I had a vague memory that they also specified a minimum length for this bypass, but is seems not.)

(As above: the boiler dumps heat into the CH system when hot water demand stops - regardless of whether CH is on or off.)

[Edit to add: The bypass must be copper pipe.]

 

[Jaeger_S2k]

Hot water is called for. The boiler fires up, makes hot water and passes it to the heat exchanger. The heat exchanger exchanges the heat it's carrying from the boiler through to the mains cold water supplying the hot taps and heats the cold water up (in reality, the cold water cools the hot water down. But that's splitting hairs).
That heated water is delivered to the hot taps and gradually gets hotter.
However, you decide you don't want any more hot water and close the tap, removing the demand. But, the boiler is now at full chatter and making the hottest of hot water to keep up with the heat exchanger, then the parameters are changed. It doesn't need to make any more, so how does it cope with the overexcited heat exchanger, hot flue and internals. How does it get rid/dump that heat that if kept in the boiler could cause internal damage.
It's not an elaborate question, it's simple, how does the boiler lose the heat soak that it's now suffering from @gaz1 ?

 

[Jaeger_S2k]

You would need to connect a bypass between CH flow and return (min 22mm) and re-pressurise the system. (Per the installation manual S 8.7)

(I had a vague memory that they also specified a minimum length for this bypass, but is seems not.)

(As above: the boiler dumps heat into the CH system when hot water demand stops - regardless of whether CH is on or off.)

I remember fitting a smallish boiler. They asked for a loop from flow to return, in 15 mm, but just the distance between the supply and return. I can't recall the make or distance from the casing that was called for, but it didn't lay well with me, but I was only the fitter. I guess they'd just over run the pump flat out till temps were stabilised.

 

[dholdi]

Its a combi.
It has two heat exchangers.
1 for heating and 1 for hot water.
If there is a heat demand the primary hx is used.
If there is a dhw demand the diverter valve operates and diverts water flow thru the dhw hx.
When there isnt a dhw demand it reverts to the heating hx dumping any excess heat there.

 

[Jaeger_S2k]

Its a combi.
It has two heat exchangers.
1 for heating and 1 for hot water.
If there is a heat demand the primary hx is used.
If there is a dhw demand the diverter valve operates and diverts water flow thru the dhw hx.
When there isnt a dhw demand it reverts to the heating hx dumping any excess heat there.

Not if it's a Viessmann, but it's irrelevant dissipating the heat soak remains the same, how is it done @gaz1 ?

 

[dholdi]

Not if it's a Viessmann, but it's irrelevant dissipating the heat soak remains the same, how is it done @gaz1 ?

Really, all of their current domestic combis do.

 

[Jaeger_S2k]

Really, all of their current domestic combis do.

I don't understand the point you're making?
The question wasn't how many hx are in the boiler, but how does the boiler deal with the heat soke of running a hot water tap.