thermal store temperature low when CH on & limited HW

[thermostorebore]

Hi Guys,

I am kinda needing a bit of direction on what might be wrong with the performance of my thermal store CH and HW system. I would appreciate any advice you can give me.

My current system -

Ideal Icos boiler feeding a Mcdonald Engineers Thermflow 210 thermal store vented mains pressure system

http://www.mcdonald-engineers.com/images/literature/Instruction Booklet - Boiler Cylinder.pdf

http://www.idealheating.com/products/icos_he.php#techspec

Boiler heats up coil inside thermal store. Water inside thermal store is pumped round radiators.

What the issue is -

I just can't seem to get enough hot water out of the system when the CH is on. I can get about a 3-4inch bath but then the water starts to go cold. When just the hot water is on, the thermal store gets up to ~75C (on store gauge). Boiler will switch off when temperature reached and will kick back in with a bit of hysteresis (couple of hours). When the CH is on, the temperature of the thermal store can't get above 60C and the boiler is on all the time. The CH and the radiators are all warm and the house gets warm but the HW is so limited

I have done various tests. Getting the thermal store right up to 75C and then switching on the CH. Within 1 minute, the temperature will drop by 20-25C on the thermal store gauge and will take about 20 minutes to return to around 60C. It won't recover to 75C

I live in scotland in a soft water area.

Is this the normal performance of a thermal store? Anyone got any ideas of what could boost performance.

Best Regards

C

 

[Norcon]

Anyone got any ideas of what could boost performance.

A new unvented cylinder as a replacement.

 

[swbjackson]

Sounds like the boiler is undersized.

 

[snb]

what size icos boiler has been fitted?

what is the pump speed of the pump from boiler to thermal store?
(should be on speed 3)

When the rads are on is the boiler on continually (blue light on continuous)?

is the temp on the front of the boiler at max?

The icos boiler can have it's performance altered by a simple jumper setting on pcb from outputs 15kw to 24kw.

To do this you should be gassafe registered

 

[thermostorebore]

Thanks for your replies so far.

SNB, Appreciate your helpful questions. Here are replies to your questions

The ICOS boiler is an HE24 which has min of 9.1KW to 24KW. The thermostat dial on the front is set to maximum.

I don't know what the pump speed is from the boiler to the thermal store. Is that set from inside the boiler? I know that the pump that supplies the radiators from the thermal store is set a maximum 3.

I will check on the blue light tomorrow when the heating is on

There isn't a temp gauge on the boiler unless its underneath the cover.

I'll try and have a look at the jumper settings on the black PCB box to find out what it is set at. I won't adjust anything as per your warning.

C

 

[Dan Robinson]

We really need to know what the boiler is doing whilst trying to recharge the store.

It should run at full tilt for nearly its whole burn time.

You might need to get some thermostats put onto the pipes to measure.

My 24kW boiler runs my ten rads and heavy hot water demands without a problem.

Admittedly I have a 350 litre store... but the bottom third isn't really being used.

 

[snb]

don't bother looking at jumper settings as it will almost certainly be set correctly for 24kw.

What we need to find out is wheter the boiler is connected correctly to the thermal store.

flow from boiler on to flow connection of thermal store etc.

At the very worst the system can be altered so that the boiler heats the rads directly and the thermal store is used purely for hot water but that is the last option.

But as you have said the rad pump is on speed 3 could be part of the problem.

How many rads do you have?
and what size is the pipe work connecting on to the rads?

Check that the lockshield ( side without trv) is only open 1 to 1.5 turns.

And then try to run the rads with pump on speed 1

 

[thermostorebore]

Dan, (and to follow up on snb's question)

Thanks for your post.

When the CH is on, the blue light on the boiler was on solid. After about half an hour and with the thermal store approaching 60C on the store gauge, the blue light on the boiler started to flash. Which I assume means its throttling back. The CH was still on and the room thermostat was still on.

Snb,

I have 14 radiators. 3 are small ones. Most are set at 3 or 2 as we don't use the rooms too much. Only 3 rads are set at 4 + the hall one which is on all the time for the room thermostat.

Questions that I'll need to find out for you -

I'll need to follow the pipes to make sure the boiler flow in and out are not swapped over. They are definitely going in the correct bit of the thermal store, I checked this with the manual and also those pipes are burning hot to touch. I'll do this and get back to you.

Size of CH pipe work.

Try with pump speed reduced to 1. I have had this set at 2 for the last 3 months.

I appreciate all your help. Hopefully the flashing blue light might be a clue.

C

 

[simond]

Is this a new system, or it is one that has previously seen you through a few winters?

Has the thermal store temperature reading changed recently, or has it always failed to get above 60C with a heating load?

Check your boiler size using the online calc at www.sedbuk.com, its easy and will take about 5 mins. All you need is a tape measure, see 'Whole House boiler sizing'.

 

[thermostorebore]

Simon d,

The system has been commissioned for around 6 years. It has always had this issue. It has various parts changed in the boiler due to them dying. It has had the PCB changed 3 times, the pump changed 4 times and even the main burner (which actually didn't need changed at all). These issues where due to the system needing a flush. Once it was done, the parts dying issue went away. The problem still remains with the lack of HW. Every winter I say I must find out what is going on.

The temperature has always been 60C when the CH is on. It does rise to cut off at 75C when the CH is off but just HW on.

Went through the calculator. The boiler output was 14.9KW. I have max 24KW

Thanks for your questions

Best Regards

C

 

[Dan Robinson]

Went through the calculator. The boiler output was 14.9KW. I have max 24KW

Is that what the boiler is actually consuming? How did you calculate it?

If it's true then that is your problem.

You're lucky it works as well as it does

You need a decent RGI to work out whether the boiler is starved of gas or whether it is set to burn that amount.

 

[thermostorebore]

Dan,

sorry I maybe didn't make that clear.

The calculator said that I needed a 14.9KW output boiler based upon my house size.

My current boiler can output 24KW.

I doubt it is consuming this since when it is trying to recharge the thermal store when the CH is on, the blue light it flashing which I think is indicating throttling back

C

 

[Dan Robinson]

Blue light flashing means the boiler has cylced off.

The kW is what you need for heating. the flow rate through your taps will give the kW you need for hot water. If you are drawing 12l/min @ 35 delta with a full heat load of 14kW then your effective draw on the store is going to be around 46kW.

Simplistically, the size of the store will buffer the demand a little bringing the requirement of the boiler down a little.

But a properly set up boiler and thermal store should see the boiler running full tilt until shut down (store reaching target temperature).

If it is cycling before the store is up to temperature then there is poor heat transfer through the system between store and boiler.

In theory the extra 10kW or so from the boiler reduces the effect of the ch draw on the hot water. and allows for recharging once the HW demand is over.


Perhaps a picture of the setup will help further.

 

[thermostorebore]

Follow up to snb questions,

Boiler connections into the thermal store are the correct way round and in the correct place.

The pipework from boiler to thermal store and from thermal store to CH is 22mm.

I also put the pump down to 1 on the CH but the thermal store could still not get above 60C


Dan's reply got me thinking about the cycling. The boiler starts cycling off when the store reaches ~58C. If I switch on the heating when the store is at 75C, the store will drop down to ~55C within a couple of minutes. When the boiler is on, neon light is on all the time, as it tries to recharge, I can turn down the thermostat on the front of the boiler down to half way and the boiler will then start cycling with the light flashing. My question is where does the boiler get the feedback to know to start cycling?

Regarding pictures, here is the location of a schematic of the system I have. If you look at page 3, my system is the one in the middle, the THERMflow Combination Type Sealed System Boiler Mode.

http://www.mcdonald-engineers.com/images/literature/Therm Flow Lft.pdf

 

[Dan Robinson]

hhhmmmm not what I would call a good design personally. you won't catch me fitting one (and I am one of the forum's few thermal store advocates).

For a TS to work properly you need the boiler running full tilt. Especially with a dual raw on its charge.