Showers draining Water Storage Tank

funkey_monkey

Hi,

We have a Maxipod 210 Direct which is heated up every morning via a Grant boiler. If a shower (of 7-10min duration) is taken then the tank drops from 70degC to just over 40degC. So, there is either an issue with the tank heating or that water usage is too high.

I've timed the flow at the shower heads and it is putting out 2L per 12s, so 10L/min which from reading about does not appear to be excessive.

What should I be looking at here?

Could this be related to a previous (and still unresolved issue) with heat being lost overnight: https://www.boards.ie/vbulletin/showthread.php?p=109388798


Thanks.

John.G

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?.

funkey_monkey

1. Delay between Grant off to shower approx 2hrs. Although if i have a slightly longer shower it will drop by end of shower to 40degC.
2. Cylinder gauge located 2/3 way up tank.
3. Boiler programmed for 30 minutes.
4. Yes, thermostat located in bottom 1/5 of tank.

We forewent an electric shower in ensuite under the impression that the power required to drive the electric shower could be taken from the hot water tank which would only require topping up in the mornings by the Grant.

I believe setup is something like the following:




A - Expansion tank
B - Cold feed
C - Blending valve
D - Hot out
E - Cold in

F - Oil/Gas boiler stat set at 70°+
G - To radiator or under floor heating
H - 28mm expansion
I - Flow
J - Oil/Gas boiler

K - Pipe stat set at 50°C
L - Pump
M - 28mm flow/expansion (copper pipe)
N - 28mm return (copper pipe)t

O - Low flow connection
P - Metal expansion tank

John.G

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.

funkey_monkey

Just a thought, could the blending valve be incorrectly set pulling too much heat from the tank?
I suppose the only way to check would be to run heat the tank and run a tap onto a thermometer...?

John.G

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

Initial temp: 37degC

30min temp: 68degC (continuous firing)

34min temp: 70degC
(boiler then cuts out)

I thought that was that. Left it on boost for the remainder of the time (26 min) - not sure if it fired up again or not.

Two hours onwards from hitting 70deg, tank gauge now showing 52degC.
Took a five min shower. Gauge now hovering around 40degC.

The start on boiler does not show figures just a progressively thicker mark as the knob is screwed. Sitting at almost max. Return pipe not lagged for approx 0.7m.

Just to add, system was pressure tested during install and was said to be okay.

funkey_monkey

John.G wrote: »

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

What is the Grant boiler Setpoint temperature now?

No. Boiler turns off at bang on 70degC. Stove would put it over a bit and then fire excess around zone 2 rads. Don't know exact figure it goes over 70degC by as we've not been using stove recently in this weather.

Boiler temp is an unknown as it is a thickening line scale and not graduated numerically. How've, knob is turned round to almost max setting.

funkey_monkey

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?

John.G

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.

funkey_monkey

John.G wrote: »

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.

Is a 50/90 (15/26).

Testing now. Was sitting at 60degC after morning heating and some usage. I typed it up and when it clicked off the temp was 74degC.

funkey_monkey

John.G wrote: »

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.

Couldn't get the isolation valve to move so didn't force it. Ended up turning off the mains.

The grant boiler is only 1 year old. This is a new build.

John.G

That's fine, suggest leaving for a hour or so then check temp, re open mains and listen for header tank filling through ball cock or not.

funkey_monkey

No difference. I've turned on mains again and left to see if that makes any change.
Definitely was a 14degC drop last night as I've photos to show it and both others in house swear they didn't use the hot water.

John.G

If there is no cylinder temperature difference? then that is "good" in one sense, if the temperature does fall again now with mains open then that will mean that you can start narrowing down the suspects, the first thing I would do is isolate the mains and change that seized DHW coil inlet isol valve, then carry out the test with the mains restored and that valve closed.

Its a bit surprising that a one year old valve is seized, a gate valve if fitted may need a little more force.

funkey_monkey

So turned mains back on.

11:02am - 70degC
11:30am - 66degC
12:05pm - 61degC

Is this leak territory or could it be a thermosyphon? I've went to the stove and both pipes flow/return at back of stove are hot - one moreso than the other.

funkey_monkey

12:30pm - 60degC

funkey_monkey

01:03pm - 56degC

John.G

funkey_monkey wrote: »

So turned mains back on.

11:02am - 70degC
11:30am - 66degC
12:05pm - 61degC

Is this leak territory or could it be a thermosyphon? I've went to the stove and both pipes flow/return at back of stove are hot - one moreso than the other.

Logically, thermosyphon is not the culprit as the thermosyphon is still active when the mains is off as long as the header tank is full and you don't seem to have any make up going to that after re opening the mains?, you mentioned in your other thread that you have a isolation valve on the stove return, shut that with mains restored and repeat the test but make very very certain that you re open it immediately after the test.

A leak in the DHW (maybe under ground) seems the most likely but I wouldn't be entirely happy that it is unless I can isolate it at the cylinder and then re run the test again.

funkey_monkey

Okay - will try that after lunch.

funkey_monkey

Stove IV CLOSED
13:57 - 71degC
14:19 - 71degC
14:25 - 71degC
14:57 - 71degC
15:50 - 71degC
STOVE IV OPEN
15:56 - 71degC (Pipes at back of stove luke warm)

funkey_monkey

16:21 - 70degC (Pipes at back of stove hot)

John.G

"Average" was 9C/hour with mains open and stove return open.

funkey_monkey

16:48 - 70degC

John.G

Something strange going on as no huge drop through the stove just now, presume mains is open?, so perhaps monitor for another hour or so. In the other thread you said the stove flow was connected to the lower of the two connections which maxipod recommend but only if it loops upwards first and then downwards to prevent a thermosyphon, see pic "maxipond stand" in post #8, of that thread and attached below.

funkey_monkey

17:30 - 70degC

No, I think stove should be connected to lower, but it is connected to upper. Connects to upper stove flow rises and would seem to go along attic before dropping into the stove. I'd need to confirm this route with the plumber though. However, it definitely rises directly upwards through the ceiling.

Upper connection point should only be for maxipod in floor above or attic location. When both are on same level, I believe that the lower connection should be used - which is not the case here.

I'm wondering if the drop is not happening as the temp in the room with stove is sufficiently warm now with people/cooking that there is insufficient heatsink for the syphon to occur?
I'll open windows now to see if that drops the temperature sufficiently to draw the heat out out of the maxipod.

funkey_monkey

17:30 - Windows open
18:00 - 68degC

funkey_monkey

20:05 - 62degC

funkey_monkey

20:35: 60degC

John.G

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.

funkey_monkey

22:04 - 54degC

Valve closed and note added. I'll keep an eye on it over the next few days and see what happens.