Why is my flow fluctuating so much?

[Ambaz79]

Hello All

 

I have an Ecodan 8.5kW Heat Pump with a 200L hot water Cylinder and a 100L Buffer Cylinder.  I am using the HP in Weather compensation mode and when I check the hourly tempreture in Melcloud it shows the Flow is fluctiting allot.  

 

Settings for the Wether compensation are as follows.

 

Low is 40 Degree Flow at -1 Degree outside

High is 25 Degree Flow at 15 Degrees outside

 

This morning the outside temp was around 8 Degrees so I would expect the flow to be around 30 to 32 Degrees however it looks like it runs at around this level and then keeps spiking to over 50 degrees.  I am sure this means we are using allot of electricity.  

 

I only have the UFH downstairs and have Rads in the 3 bathrooms upstairs that are oversized.  bedrooms dont generally need heating but we have AC in every bedroom that we use for heating if needed.  

 

Downstairs I have Thermostats in every room and they control only the manifold.  Thermostats are set to a few degrees above the target tempreture.

 

House is fairly air tight with triple glazed windows and 100mm PIR Insuration in walls all the way around.  loft roof is also very well insulated.  

 

Is there something wrong with the settings or is there the weather compensation curve now correct?

[Gary68]

The increases in temp to above 50C is your hot water reheating you can see the tank temperature increase in line with this.

 

If you don't want it to do this you can adjust the time hot water is heated in the schedule, its probably set to be on permanently rather than on a schedule.  Or you can increase the temperature drop before it reheats

 

When it is heating it is running at 34ish  in line with your Weather compensation curve, although at 11am it is cycling on and off every 7 mins which implies only one UFH zone is calling for heat and there isn't enough water for it to heat continuously so it has to turn off till the water cools down enough then restart.

Edited November 20, 2023 by Gary68

[Ambaz79]

Hi Gary

 

Totally makes sense now.

 

I am surprised how fast it shows the tank tempreture drops.  In the graphs it looks like every now and then there is a 5-7 degree drop in the water cylinder within the space of 20 minutes.  Does this look normal.  I would expect for a tank to drop but very slowly as its very well insulated. 

 

I will set a better schedual on the control panel and see how this makes a difference. 

[markc]

Regardless of the heating source a sudden drop in measure tank temp would be correct. Temp sensor should be near the bottom in order to give a realistic reading, but if water is drawn off, cold enters at the bottom which is then picked up by the temp sensor, the upper part of tank will still be hot but the sensor sees a sudden drop

[Ambaz79]

We have a 200L tank and there are 5 of us that live there.  Would it be ok to set it to a 1 hour window at night.  Im guessing I can answer this really by setting it like this for a few days to see if we run out of hot water? 

[Pip895]

20 minutes ago, Ambaz79 said:

I am surprised how fast it shows the tank tempreture drops.  In the graphs it looks like every now and then there is a 5-7 degree drop in the water cylinder within the space of 20 minutes.  Does this look normal.  I would expect for a tank to drop but very slowly as its very well insulated. 

 

Isn't it just people using the hot water rather than the tank loosing heat?

[Ambaz79]

on the screen shots attached we leave home at 8am so after this point no one should be using any hot water.  

[Beelbeebub]

Your tank temp is taken at a single point which is also the control point.

 

As the tank heats up from top to bottom there is a fairly small vertical zone where the temp goes from target temp to cold.  This zone moves down as the tank heats up until it crosses the control point which goes from being in cold water to hot water quickly. 

 

Then the heat source shuts off. At this point the temp sensor is only just in the hot zone and just below it is the cold water.  

 

So the sensor cools quickly even though the tank as a whole hasn't lost much heat.

 

 

 

[Dave C]

All true, but if nobody's home after 8am, and no hot water being used, you wouldn't expect the tank temperature to drop any significant amount between 12:00 and 12:30. Particularly if everybody left at 8am and the tank was heated back up to full temperature just before 9am.

 

Are you sure you don't have a leaking tap or similar anywhere? Only time my ASHP came on unexpectedly in the middle of the day last year it was because my daughter had left a hot water tap not quite fully turned off and there was a tiny trickle running out constantly - as explained above, it doesn't have to be much water drawn off before the sensor starts to see a drop.

 

It would be consistent with the graphs - from 11:00 there's a slow but steady drop in tank temperature as the water trickles out, then the 'hot zone' in the tank is approaching the sensor so the temperature loss appears to speed up as the water continues to be used; then the heat pump kicks in to reheat the tank.

Edited November 21, 2023 by Dave C

[SteamyTea]

Why is the mixing tank temperature stable, at 25°C, in all the charts?

[eniacs]

16 hours ago, Ambaz79 said:

I am surprised how fast it shows the tank tempreture drops.  In the graphs it looks like every now and then there is a 5-7 degree drop in the water cylinder within the space of 20 minutes.  Does this look normal.  I would expect for a tank to drop but very slowly as its very well insulated. 

 

I have the same heat pump. I see this sort of loss when it starts to do a tank heating cycle. The 28mm pipework holds a lot of water that has after some time dropped to a low temperature. When a tank heating cycle is called for, the first thing that happens is the tank temp drops a lot as this cold water circulates through the tank, stealing its heat. Is it this? It looks like a few minutes later the flow temps jump and the tank is reheating. 

[Gary68]

16 hours ago, Ambaz79 said:

We have a 200L tank and there are 5 of us that live there.  Would it be ok to set it to a 1 hour window at night.  Im guessing I can answer this really by setting it like this for a few days to see if we run out of hot water? 

 

We have a 250L tank its on a schedule for heating up overnight to 48C for 90 minutes, its typically finished in an hour.  Then there is no hot water schedule till the following night, there is enough hot water for 2 showers, kids full bath and washing up with plenty to spare.

 

You could also try lowering the temp a degree at a time see if anyone complains, I have found 48C is hot enough for a adequately warm shower in the morning and the water is still more than hot enough for the kids bath 12 hours later.

 

Ignore what the tank temp says on the controller when the kids have their bath its says its 30C and after I have filled the bath it says it 18C,  its just the position of the sensor in the tank.

 

 

[JohnMo]

18 minutes ago, eniacs said:

 

I have the same heat pump. I see this sort of loss when it starts to do a tank heating cycle. The 28mm pipework holds a lot of water that has after some time dropped to a low temperature. When a tank heating cycle is called for, the first thing that happens is the tank temp drops a lot as this cold water circulates through the tank, stealing its heat. Is it this? It looks like a few minutes later the flow temps jump and the tank is reheating. 

You also need to look at the route the pipes take, especially the hot one from the top of cylinder, this pipe can takeaway heat quite quickly, especially if not really well insulated or goes up from the cylinder.

 

Ideally this pipe drops around 150mm minimum from the top of the cylinder as soon as possible, before going horizontal. This stops any thermosyphon effect.

[JohnMo]

2 minutes ago, Gary68 said:

Ignore what the tank temp says on the controller when the kids have their bath its says its 30C and after I have filled the bath it says it 18C,  its just the position of the sensor in the tank.

It daft that cylinders don't have a series of temperature probes up the cylinder (say 4 equally spaced), so you can see how much hot water you have.

[Ambaz79]

 

Sorry made a mistake on my Tank Size.  It’s actually a 320L Unvented Cylinder with a 25 Meter Coil pipe inside. It’s also got a 3kW Emersion heater that’s on a 2 week cycle for Legionnaire. 

 

What I did last night is put the water on Cycle to heat at 2.30am to 3.30am and the setting was on Eco.  Apparently it means it heaters the DHW with a lower temperature making it more efficient but taking longer to heat.  6am my wife went for a shower and after this the temperature just goes down and stays down.  I went for a shower at 8 and it was luke warm water.  Maybe 20-30 degrees.  So at 8.45am I turned off the timer function for the DHW and went back to it being heated as the tank required with the setting to heat after a 20 Degree drop in tank temperature. DHW is now set 49 degrees.  

 

Only other bit of info is I have a hot water circulating pump.  Its set to monitor the temp in the How water supply pipe and circulate it if the water get cooler.  I don’t know if this is making any difference. Grundfos UP 15-14BXA PM GB Brass Comfort Hot Water Circulating Pump.

 

I still don’t understand even though the water heated to around 40 degrees and the shower was only used once for around 5-7 minutes why the tank had no hot water at 8am?  Does not make sense to me.  

 

For my heating I have now reduced the flow in the manifold to the rooms and increased the weather compensation curve slightly to increase water temperature for heating.  I read somewhere that if the flow is too fast and water in and water our is similar temperature then it would stop the HP.  Yesterday I was seeing the HP cycling around 4 times per hour.  Today I see its Cycling around twice an hour.  The ideal scenario would be that we stop this cycling. 

 

Heat Pump is around 7-8M awy from where the Cylinders are and the pipes to and from the HP are insulated.  Other pipes connected to the Cylinder are not.  I will try and take a picture later on tonight. 

 

[JohnMo]

7 minutes ago, Ambaz79 said:

water heated to around 40 degrees

Reason it feels cool, is the shower mixer, always mixes in cold water, it's looking for the hot water to be about 10 degs hotter than the shower water comes out the head. So if you have 40 deg water you are having a 30 deg shower - cool shower.

 

48 is good for us, 46 is a not so good shower experience.

12 minutes ago, Ambaz79 said:

For my heating I have now reduced the flow in the manifold to the rooms and increased the weather compensation curve slightly to increase water temperature for heating

Not really the economical way to do things. Reducing flow decreases floor output, so you compensating by increasing flow temp, leads to a decrease in CoP.

 

14 minutes ago, Ambaz79 said:

Yesterday I was seeing the HP cycling around 4 times per hour.  Today I see its Cycling around twice an hour.  The ideal scenario would be that we stop this cycling

The heat pump cycles, when it's below its minimum modulation, it's normal. The amount of cycling is likely to be outside temperature related. If your heat pumps runs around 10 minutes don't worry.

 

17 minutes ago, Ambaz79 said:

Other pipes connected to the Cylinder are not

The DHW out pipe is always hot, by not being insulated you are constantly transferring heat to the room, away from the cylinder.

[sharpener]

5 hours ago, Ambaz79 said:

Only other bit of info is I have a hot water circulating pump.  Its set to monitor the temp in the How water supply pipe and circulate it if the water get cooler.  I don’t know if this is making any difference. Grundfos UP 15-14BXA PM GB Brass Comfort Hot Water Circulating Pump.

 

Depending on how it is wired/timed this pump will be mixing the contents of the tank so will spoil any stratification.

 

As @Gary68 says, your tank should be more than sufficient for two showers and a great deal else.

[Gary68]

20 hours ago, Ambaz79 said:

 

Sorry made a mistake on my Tank Size.  It’s actually a 320L Unvented Cylinder with a 25 Meter Coil pipe inside. It’s also got a 3kW Emersion heater that’s on a 2 week cycle for Legionnaire. 

 

What I did last night is put the water on Cycle to heat at 2.30am to 3.30am and the setting was on Eco.  Apparently it means it heaters the DHW with a lower temperature making it more efficient but taking longer to heat.  6am my wife went for a shower and after this the temperature just goes down and stays down.  I went for a shower at 8 and it was luke warm water.  Maybe 20-30 degrees.  So at 8.45am I turned off the timer function for the DHW and went back to it being heated as the tank required with the setting to heat after a 20 Degree drop in tank temperature. DHW is now set 49 degrees.  

 

Only other bit of info is I have a hot water circulating pump.  Its set to monitor the temp in the How water supply pipe and circulate it if the water get cooler.  I don’t know if this is making any difference. Grundfos UP 15-14BXA PM GB Brass Comfort Hot Water Circulating Pump.

 

I still don’t understand even though the water heated to around 40 degrees and the shower was only used once for around 5-7 minutes why the tank had no hot water at 8am?  Does not make sense to me.  

 

For my heating I have now reduced the flow in the manifold to the rooms and increased the weather compensation curve slightly to increase water temperature for heating.  I read somewhere that if the flow is too fast and water in and water our is similar temperature then it would stop the HP.  Yesterday I was seeing the HP cycling around 4 times per hour.  Today I see its Cycling around twice an hour.  The ideal scenario would be that we stop this cycling. 

 

Heat Pump is around 7-8M awy from where the Cylinders are and the pipes to and from the HP are insulated.  Other pipes connected to the Cylinder are not.  I will try and take a picture later on tonight. 

 

You haven't heated the water for long enough overnight, it was set to 50C but you only achieved 40C in the 1 hour schedule, increase the schedule to 2 hours so it can get to 50C then you may have enough to last all day, I assume you are heating overnight as you are on an off peak rate, if so, don't be stingy with the schedule, if its has to reheat during the day its going to cost a lot more.

[Ambaz79]

16 hours ago, sharpener said:

 

Depending on how it is wired/timed this pump will be mixing the contents of the tank so will spoil any stratification.

 

As @Gary68 says, your tank should be more than sufficient for two showers and a great deal else.

 

Yesterday night I switched off the Recirculating pump and the water cylinder has kept a stable tempreture overnight.  Almost flat lined at around 47-48 Degrees as I changed the water target temp to 48 degrees.  Only thing I noticed is in the morning when the 1st shower was taken the water did drop down to 20 degrees in the app.  My Water Cylinder has 2 places where we can inset the thermometer and its currently in the lower one.  Im guessing the reading is from the lower part where it takes in fresh water thats cold. 

 

I had it set to reheat the cylinder when it drops 10 degrees lower then the target temp and to heat for a maximum of 120 minutes so by the time we went for the second shower the reading was back at 48 degrees or slighly over.  What I have done this morning is put it back onto schedual to heat the water at 2.30am for 2 hours max.  It will be interesting if there is enough water to last a day or will I have to have it come on for a few hours in the day too.  I have 3 Teanagers so showers are a bit longer sometimes then 5 minutes.  

 

On the negative side it now takes allot longer to get hot water when we turn on the tap in the ensuits that are around 14M and 30M away from the tank.  Kitchen is even further in the run at around 40M ish.  This is why a re-circulating pump was chosen in the first place.  If this was the cause of the problem then is there any other solution to this? Pump does not have any other settings other then run at 100% all the time, Run when the water temp drops in the pipe line or adaptive auto where it uses algorythms to know when we will be calling for Hot water and circulates the pump at around these times. 

 

Pipe that makes the circle is not currently insulated so I can insulate them.  but not sure if this would really solve the problem overall. 

 

also on the point of stratification, does it not move around/mix the water when the fresh water is pumped into the cylinder anyway as hot water is called for? 

[JohnMo]

Secondary circulation really needs a thermostat and timer and pipes well insulated. So thermostat stops pump when returning water is comfortable temp, when the water cools, pump starts - repeat. Timer so pump only runs when useful. No use loosing cylinder temperature, when no one in house or everyone is asleep.

[JohnMo]

26 minutes ago, Ambaz79 said:

does it not move around/mix the water when the fresh water is pumped into the cylinder anyway as hot water is called for? 

It shouldn't, there is normally a diffuser to slow the cold water coming in and limit mixing. So the thermocline slowly moved up the cylinder as hot water is consumed. Once your thermocline gets close the the secondary returns port, plenty of mixing can occur and leave you just lots of cool water.

 

Our cylinder temperature pocket is really close to the bottom of the cylinder, cylinder at 0730 was 48, temp gauge now says 21 after wife had a shower. I am going to heat until tomorrow morning to see how my shower is tonight.

[Ambaz79]

Looks like there is some other problem. Circulating Pump been off all day. Left at 8am with water temp at 49 degrees. Target temp was set to 48 degrees. My son went for a 10 minutes shower at 4.30pm and then we have used it for washing up after dinner at 8pm 5 minutes into washing up the water went cold meaning no hot water in cylinder. I would expect a 320L tank to do allot more then this on a single charge.

 

I've had to change the ashp dhw to come on on a 10 degree temp drop and left it to be constantly on. 

 

Could it be the Cylinder is not fully filled up. I thought it could be loosing water somehow but then thought we didn't use the water all day and water has remained hot. 

 

But lost with this now. 

[Ambaz79]

Looks like there is some other problem. Circulating Pump been off all day. Left at 8am with water temp at 49 degrees. Target temp was set to 48 degrees. My son went for a 10 minutes shower at 4.30pm and then we have used it for washing up after dinner at 8pm 5 minutes into washing up the water went cold meaning no hot water in cylinder. I would expect a 320L tank to do allot more then this on a single charge.

 

I've had to change the ashp dhw to come on on a 10 degree temp drop and left it to be constantly on. 

 

Could it be the Cylinder is not fully filled up. I thought it could be loosing water somehow but then thought we didn't use the water all day and water has remained hot. 

 

But lost with this now. 

[Beelbeebub]

Are there any thermostat or sensor pockets on the tank?

[Ambaz79]

Yes there is that is lower down the tank. You might just be able to see a small wire going into the tank.