thermal store temperature low when CH on & limited HW

[Dan Robinson]

There is something very wrong with the heat transfer

On a combi that 2 hour cycling would be typical of limescale.

Yours could be a dying pump.

Let me think about this a little.

I stand by my earlier comments on thermal stores and coils.

 

[thermostorebore]

Great help guys. I appreciate all your time getting to the root of the problem.

I think I have now collected all the data.

The pump is an Ideal badged Wilo one. Has many numbers on it but HB 15 sticks out.

The pump has no speed control on it for me to adjust the speed.

Norcon, It takes 2 hours to get a 10C temp rise. The boiler heats for 2 minutes then switches on for 4 minutes. The pump is definitely circulating when the boiler heat goes off. The pump only stops when the thermal store is up to temp. This is shown after the 2 hours has elapsed on the graph. The boiler stops heating after the 2minutes when the return temperature goes up to 77C. Presumably the overtemp thermistor has kicked due to small 7C differential between flow and return to protect the boiler. Could the boiler run with the flow and return at 84C?

Dan, as always your advice seems logical and sensible. It just seems the coil can't get rid of the heat. While doing the temperature measurements the flow temp rapidly came up to temperature. The pump seemed to be behaving as I would expect but then again I am no expert.

I have contacted McDonald engineers to see if they can shed any light on what might be going on with the coil.

This is confusing the hell out of me. Can I collect anymore data for anyone?

Thanks again

C

 

[Dan Robinson]

Are we sure you water is soft?

What is water in the boiler like? Could you have a thick layer of sludge in the coil?

 

[alyne]

Firstly I'm no heating expert, but I've been researching boilers, temps and flow rates over the last few days because of an issue with my new boiler on my thermal store. So i've accumulated alot of info on it in the last 48 hours!

Looking at your graphs (how did you measure and log this by the way?) It looks like you've got a low temp rise across the boiler feed and return and low heat transfer to the store (I take it, it's and indirect store?).
That says to me that the flow rate through the boiler is far to high? If you had a lower flow rate (slower pump) then the temp difference between the boiler return and flow would be greater, but the water would be circulating through the store coil alot slower and be able to transfer more heat to the store, and thus the water returning to the boiler would be alot cooler, the boiler would then add alot more heat and so on.

It looks like the flow rate is too high, the water is coming back to the boiler at almost the same temp it left, your boiler is seeing the high return temp and just shutting off and going into anti cycle, once it reaches its max temp.

Quite HOW you turn down the flow rate I don't know. You'll need an expert in your boiler for that!

sorry -edit to add that someone said all that a couple of days ago!

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]

Lowering the flow rate is easy...

But if you look at the charts you can see that as the store temp rises, the effectiveness of the coil drops off quickly.

What would be handy is for the OP to do a chart with the store starting from cold, with no demand on it whatsoever followed by a heavy draw on the store, then leave it to recharge.

Going back to my store as an example ( I might do a similar log on mine as I have an 8 channel logger); and working from memory... When the bottom of the store drops below about 45 it kicks the boiler in. The top of the store is around 70/73 degrees. The boiler immediately kicks in and runs at full tilt until the stat is satisfied. There is no, or almost no modulation.

Annoyingly at the moment I am using EvoHome and a wireless cylinder stat to control the store, and there is a blip in the communication so it sometimes goes over temperature - but then it isn't really suppose to be controlled this way .

Remember I have no daft coil to interfere with heat transfer and the dhw system is a 100+kW plate.

 

[simond]

sorry -edit to add that someone said all that a couple of days ago!

Well, you know what they say about imitation and flattery

 

[simond]

Following on from being the lone voice in the wilderness (well, OK, there is another now).......

If you have an Ideal System boiler it should have isolation valves underneath on the flow and return. Choose one and set it 3/4 shut.

Then take some temperature readings. I would bet some money the reduced flowrate sorts it.

Best of all, this costs you nothing. Damn, I could have billed two hours on this one....

 

[Dan Robinson]

I am not disagreeing with you per se, but I worry that that might impair the reheat time somewhat.

Surely if the coil was working "properly" then it would take all the heat the boiler threw at it.... epsecially if it was rated at 60kW

I am concerned that you will cure the symptom, not the problem. Or do we think that isn't practical on this design?

 

[alyne]

That maybe the case, but if you balance the flow to get a decent temp drop through the coil then at least it will be alot better than it is now!
Thats the reason i chose a direct store, because I cannot see how an indirect one will really work, as the store needs to be almost as hot as the boiler flow temp, which is never going to happen with an indirect system.

 

[Dan Robinson]

Thats the reason i chose a direct store, because I cannot see how an indirect one will really work, as the store needs to be almost as hot as the boiler flow temp, which is never going to happen with an indirect system

Like what I said on page 1 then .

 

[astclair]

Looking at your "hot water only 30 minute data" plot it seems to take around 700 seconds to get a 1 degree rise in TS temp with a delta of around 10C. According to my abacus, with a volume of 210 litres, that's about 1.2kW or so actually getting through to the water, assuming no significant DHW draw. This ties in well with a boiler that modulates down to 9kW firing for around one seventh of the time, as also shown on the graph.

The coil is rated at 40kW @ 50C delta - shouldn't we expect something approaching a linear relationship i.e. 8kW @10C delta?

So I'd agree that there is a serious heat transfer problem in the TS.

Back on the overnight plot, even when the CH comes on the return is never much more than 7C below the flow despite delta 25C+. I reckon the boiler is never even getting out of first gear due to the heat transfer problem.

 

[thermostorebore]

Thanks Guys. To answer as many of the questions as I can.

The tool I used for obtaining the temperature readings was a picolog 12 channel data logger. I used K type thermocouples and attached them to the pipes.

As far as I can tell the water is soft. Can't extract the data from scottish water but their advice is that it is soft to moderately soft.

As far as sludge in the boiler coil, I am getting that checked out tomorrow.

I have Ideal Boilers coming in tomorrow to check the boiler out. I will get them to check the sudge in the pipe and speak to them about the consequences of reducing the flow of the return pipe.

It will take me a while to find a convenient time to bring the thermal store down to low temp and them heat it up due to the weather. This best I have got at the moment is an expanded version of the overnight plot. I have magnified the data at the point the central heating comes on. It shows the thermal store going down to 40C. The flow and return come up pretty quickly but there is a bit of a differential for around 50 minutes.

It can be found in my album


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

I'll let you know what the boiler man says.

C

 

[Dan Robinson]

Those Pico logs rock... I look for excuses to play with mine although your is bigger

 

[Dan Robinson]

Just hooked up my Pico to my store for a coldish test. Please ignore the shut off temperature as my control system is a little heath Robinson at the moment.

It will starting with the bottom of the store at in the mid 30's I believe. It is designed to heat to 59 as quick as possible. It currently always overshoots, but we are trying to give you a comparison of heat ups - so that won't matter .

Later I will do the same from a hot start for hot water only.

try not to be too jealous

 

[snb]

having looked at the graphs it shows that the coil only has about 8 degree differential.

This would indicate that the flow arond the primaries is fine but there is a problem with heat transfer in the thermal store.

I often see the reverse in gledhills with poor hot water performance.


I would recommend that you take a sample of the stores water to see what the ph value is.

I am guessing that this will be much more alkaline than you would expect and because of the store generally being above 65 degrees has scaled on the outside of the coil preventing heat transfer.

If this is the case then you have 2 options if you want to keep the store.

1, try to descale the store
2, have the boiler arrangement changed so that the boiler heats the store directly. ( luckily this boiler can be open vent aswell as sealed)