Showers draining Water Storage Tank

I remember the other thread well.

Re present problem....You say that the Grant heats the cylinder each morning to 70C, so that means there should be ~ 200 litres of water available at 70C, it drops to 40C after taking the shower so simple calc shows that the tank gave up, 200*(70-40)/860, 6.98 kwh while the shower (assuming 45C showering temperature) required 100*(45-10)/860, 4.07 kwh so theoretically the cylinder lost 2.91 kwh or 18% of its stored capacity. In the other post we calculated that the cylinder was loosing ~ 2.5C/hr so to it would take ~ 5 hrs to loose that 2.91 kwh, would it be reasonable to assume a 5 hr delay between the grant off (cylinder heated to 70C) and the shower usage?. If so, then you are back to your original problem I think which we thought may have been thermo syphoning through the stove.

Where is the cylinder temperature that you are quoting measured?.

How long is the boiler programmed for in the morning.

Am I correct in saying that the cylinder thermostat for the Grant is located near the bottom of the cylinder?.

From the above info, the grant is heating ~ 150 litres to "70"C, if one assumes that the tank temp was 40C initially then it would require 150*(70-40)/860, 5.23 kwh and assuming the grant output at 25 kw and firing continuously then it would only fire for 13 mins before cutting out, if the tank temp was 25C it would fire for 19 mins.
But it is impossible in either case to have the the temperature at the probe and the temp at the thermostat the same (70C) due to the temperature rise through the grant which will depend on the flow rate through it, I wouldn't expect the flow rate to be > 30 LPM which results in a deltaT of 12 C which means that IF the cylinder stat SP is 70C then the cylinder temp will end up at 82C which I think you mentioned in your other thread? If the grant is cutting out at a indicated cylinder temp of 70C then it means that the cylinder stat is operating at ~ 58/60C. or else the grant is cutting out on its own boiler stat and you actually havn,t got 150 litres at 70C after 30 mins.

To try and pin this down, I would suggest just run running off some hot water until you see a cylinder temp of 40C then stop, fire up the grant for exactly 30 mins, note how long it is actually firing for during this 30 mins, it may just fire continuously for one period or a number of times which you can add together and you will then get a very good feel for what's going on.
Also, try and see the output of your grant or at the very least its output range.

The blending valve could be incorrectly set but should'nt affect the amount of heat taken from the cylinder, if too high then more cold water has to added somewhere else, ie at the shower, if too low, then you will certainly know that when under the shower but even if defective it won't cause any energy imbalance.

I think a more accurate test would be to keep draining off hot water after your shower(s) with boiler off until the cylinder is filled with cold water, then run the boiler until the cylinder is 70C while monitoring the burner running time. It should require ~ 14 kwh and a burner (total) run time of ~ 32 minutes. (assuming a 26 kw boiler). You may find that the burner fires continuously which would make the monitoring far easier, if you havn't the time/inclination, just write down the cylinder temperature before & after say a continuous burner run time of 10 minutes.

Have you seen the cylinder temperature higher than 70C without the stove in service??

What is the Grant boiler Setpoint temperature now?

funkey_monkey wrote: »

Would I be best heating back up to 70degC and then turning off the boiler and the mains water into the house. That way if there was a leak when water turned on again I should hear/feel the tank filling again?

Or what is my next step?

Yes, that would be the way to go initially I think. You could do a quick, say 1 hour test first and that is to isolate the cold water (mains) supply to the coil in the cylinder, there should be a isol valve at the cylinder, if there is a leak in the domestic hot water somewhere in the house then that will very quickly identify it.

What is the Grant model, age and output?, normally given in something like 20/26 kw or 70/90.

I think that if that stove flow was moved to the low connection that it may cure that thermosyphon as the water will be cooler there.
Before doing anything I would suggest leaving the stove return closed for a day or two (attach a large notice to the stove that it must not be lit and the reason why not) and monitor the cylinder temperature overnight/for a few days using your normal routine, then re open it and monitor it again.

I would lag every where there is access.

I have everything lagged as its cheap and you can split it to go around the pipe(s) without disturbing anything.

You might be able to get a restrictor to fit in the shower head, you said in your first post that you are getting ~ 10 LPM from some of the showers, if this is a different shower then just measure the flow rate like before, = or > 10 LPM is considered a power shower and a lot now come with restrictors fitted to give 6 LPM, a 9 kw electric shower by comparison will flow 3 to 4 LPM to give a comfortable showering temperature.

funkey_monkey wrote: »

Well, the water heating issue seems to be resolved by turning off the valve to the stove. I might want to do something to reduce the flow to the showers, but I can do that myself - I think.

My plan is to get the plumber back to reconnect the stove to the low flow connection and lag all the pipes.
Would it be best to put in an anti syphon loop when he is at it?
If so, what form do these take?

The Loop is as shown in the attachment.

I think that's the way its supposed to work, my only fear is that because the stove flow pipe goes up then down that it might interfere with the natural convection forces when the stove is on, because hot water likes to rise, you could consider just connecting the flow straight into the low connection, the worst that can happen then is that you get reverse thermosyphon but with a much reduced volume of hot water due to the reduced height between the stove low flow connection and the oil boiler return as there will be little or no hot water below this with the oil boiler on only.

The cylinder (store) vent is venting the store, the oil boiler and the stove as they are all the same body of water.

My own view but I don't think it's a great idea having both the boiler and stove connected directly to the store, when the boiler is running on its own then I feel that the circ pump is also going to cause circulation in the stove leading to further losses apart from the above syphon losses.

I would have a separate coil for/from each heat input.