thermal store temperature low when CH on & limited HW

[thermostorebore]

thanks Dan, I inherited this system so I didn't really have a choice.

From what you said in a previous post.

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

The boiler must have feedback to tell it to go in to cycling off mode. If I reduce the flow temperature thermostat on the front of the boiler, it will go in to this cycling mode. I don't think its the thermostat on the thermal store thats telling it. From reading around it looks like the boiler monitors the flow return coming back from the thermal store. If the temp is not that much lower than it was sent out at then the boiler thinks the store is coming up to temp and so starts to throttle back.

If this is what is happening then I guess it could be a number of things related to this.

1) poor transfer of heat from boiler coil inside thermal store. This is what Dan said. Scale??? but I live in a soft water area.

2) Broken thermistor on flow return side?

3) Broken thermistor on flow source side? I can check with a mulitmeter.

Could the thermistor on the flow return side be disconncted and just run off of the thermostat on the thermal store which can switch off the boiler.

I think we are getting there

C

 

[mikely]

How are the boiler and pump in the primary circuit controlled? In my thermal store system the boiler and pump are simply controlled by the tank thermostat. The boiler fires to bring the tank back up to the temperature set on the tank, so it always recovers to the set temperature; although this can take some time if there is a high CH demand. It sounds like your system achieves this when in HW mode but not in HW + CH mode.

 

[Dan Robinson]

Scale would have been the first thing I mentioned, but you pre-0emptied it.


I strongly suggest you get some pipe stats attached to the boiler flow and returns.

You need the boiler up to maximum - the boiler thermostat goes from 55 - 82 IIRC.

Mine for example runs with a top flow temp of 84 degrees. the temp controller on the store is set to turn off the boiler only when the store reaches a temperature I set at about a third of the way down. Currently it is a little higher than normal due to me using a controller not really designed for the purpose.

However, my store heats the water in it directly through the boiler - not a coil so it is far far more efficient. You are governed by the heat transfer capacity of the coil and that will reduce as the store temp increases.

I am afraid I might have to agree with earlier posts, but if you are staying there for any length of time, consider upgrading to an Unvented cylinder. Although a lot of these stores were put in because of restrictions imposed by G3.

The boiler will cycle as a result of the flow and return temperatures and its own programming.

If you flow temperature is rising too quickly that will shut the boiler down and it will enter a pump over-run cycle which is probably 5 minutes. This will cripple your thermal store.

Hence the need for pipe stats. I don't know how the boiler would behave; but the installation of another pump might help if this rapid flow temp rise (without a corresponding return temp rise) is the issue.

 

[snb]

can you confirm that the boiler stat is set to max.

Just to make sure the boiler flow and returns are set correctly on the top of the boiler the right hand connection is the flow.

So if you turn the system off the boiler and allow the rads or hot water to cool the store down first then you can be sure when you turn back on that the flow around the circuit is correct.

It won't be the first time a system has been connected the wrong way round and this boiler will still function where most would come up with fail codes.

Scale should not really be a problem.

As dan as said it would be helpfull if you could get some temp readings from the boiler flow and return.

I would expect 80 plus flow temp before the burner cuts out ( flashing blue)

If it is piped correctly and you are getting 80 degree flow temp then the problem would indicate a pump problem. ( you have checked that both of the pump valves are fully open!!!)

 

[simond]

I don't know Ideal boilers very well because we have never specified them. My scrap man has seen three from me this December, all condensers, all ruined beyond repair. Good for business, because my business is only fitting new boilers!

Thinking about the problem here and in contrast to the other suggestions, I do wonder whether the problem is flow rate. Not lack of it, but excessive flow rate.

A thermal store coil will provide very limited resistance, so the boiler electronics may be modulating down.

The boiler would normally be expecting to see a 20C temp difference; if the water is being pushed around the load too quickly the temp differential across the boiler will be low and it will modulate down accordingly. Eg: there might only be 8C difference.

We have already seen that the boiler is well up to heating a 210 litre TS and the house load connected is less than 12kW heating.

So I would find the pump, and put it on speed 1.

If there is no pump speed switch, see if there is a flow balancing valve on/near the cylinder and close it to 3/4 turn only.

 

[Dan Robinson]

Annoyingly the Icos had two version - one with and one without an internal pump.

Lord knows how its brain works though - I know what you're saying about the temperature differential and thermals stores. We had similar issues wit InterSystem boilers and very large systems. The boiler's logic would panic because the sensors weren't registering what it thought and went into an overrun mode until the return temp rose and it was happy again.

result. Cold house.

solution? Big ass pump

 

[thermostorebore]

Thanks Guys. I appreciate all your time trying to guide me in the right direction.

I think I will need to get the cover off of the boiler now and start working through your suggestions.

snb, Yes I do have the boiler set at maximum.

Regarding the flow and return temp measurement. Can I just measure the temp of the copper pipe? Is that good enough?


I'll keep you guys informed on what I find.

C

 

[Dan Robinson]

Yes mate - measure the pipes and make sure they are plumbed the right way.

Is yours the boiler with the internal pump or an external one?

 

[thermostorebore]

Dan, its the internal pump version. I only have an external pump on the central heating.

Please bear with me for a few days while I collect the data.

Simon, I'll have a look to see if there is a speed control switch inside the boiler.

C

 

[snb]

didn't realise yours was the system version boiler


The pumps on these boilers are notorious for failing. From all that you have said so far it appears that boiler is ramping down and going in to cycling mode due to the temp within the boiler being too hot due to poor flow.

You will need the serial number and wether the boiler has a wilo or ideal pump.

Just replace this and it you should see a vast improvement.

The only part that could have failed is the AAV but you need to be gassafe reg to work on this area of your boiler and you would normally have had some L9 error codes with this fault

 

[Norcon]

Poor flow or poor heat transfer at the coil.
I'd be more inclined to go with the latter.

 

[thermostorebore]

Guys,

Here is the data.

I recorded a 10 hour overnight trace and a short 30 minute one. the images have text and a legend on them which explains what was going on.

//www.diynot.com/network/thermostorebore/albums/

All these measurements were taken at the pipes entering the thermal store

The temperature readings of the flow and return at the boiler were

when boiler kicked in to full switch on 84C (flow) 69C (return).
when boiler went into cycling mode 84C (flow) 77C (return).

Presumable the change to cycling mode is to protect the coil as the differential is too small

So pump running too fast or poor heat transfer?

C[/b]

 

[Dan Robinson]

A little tricky to read on my mobile, but first guess is rubbish heat transfer. As I mentioned earlier, heating a thermal store through a coil is s daft idea anyway.

 

[mysteryman]

Just a bit of general guidance for anyone considering a thermal store:

Don't do it if there is only one heat source, or one heat source plus solar. The reasons are throughout this thread - you have to get the whole store very hot just in case the rads need high temperatures.

Put your investment into rads fed straight from the boiler with weather compensation.

 

[Norcon]

2 hours to raise the store by 10c?
If I'm reading your data correctly.

During this two hours do you know exactly how long the boiler was/is running for?
And did you notice if the circulator turned off when the boiler turned off? Or did it stay running?

120/l and 140/l stores have a 26kw coil rating.
180/l and 210/l = 40kw coil
250/l and 300/l = 60kw coil.
Thats based on a 50c delta.

These are Mc Donalds ratings and should be ample to enable your hot water store to re heat much faster.