Can someone explain this 'problem' to a newbie please?

[nickbb]

Hi All

I'm new to this forum and to solar technology. We have just had a pellet biomass boiler system installed feeding both our water and rads which is 'complemented' by 20 Joule evacuated tubes on the roof. The issue is this. We have been monitoring the controller which reports the temperatures on the roof (presumably at the manifold) , the top of the thermal store, and the bottom of the thermal store. We live in the south of England on the coast and yesterday was a mostly cloudless day with bright sunshine and a temperature of about 14 degs. When the solar cut in at about 10:00am the temps were as follows:

Roof 59.3
Top of thermal store 56.7
Bottom of thermal store 49.5

The temp curve then proceeded as follows over a period of 4 hrs between 10:50am and 2:50pm


Roof 59.0 55.8 56.4 55.3 61.8 56.4 46.9 50.7
Top of thermal store 56.2 54.8 53.4 52.5 51.3 50.8 49.5 49.1
Bottom of thermal store 50 48.8 47.6 46.8 46.1 46.1 44.4 43.5

Now during this time the solar was the only thing running, i.e. no pellet boiler, correct me if i'm wrong but I was kind of hoping that the temp in the tank would have gone up not down, or am I expecting too much in that period of time at that outside temp. By the way, the pump is set to cut in as and when it's needed, which it does. Any help would be appreciated before I sign this job off, which i'm not inclined to do as you can imagine.

[Naptown]

A few things here
Is this an internal static heat exchanger or external pumped unit. With a pumped unit there is some mixing of the water meaning the tank will become stirred up mixing the cold at the bottom with the hot at the top cooling the top.
Secondly it appears that some hot water was used during the day as evidenced by the dropping bottom temperature which will reduce the manifold temperatures.

[nickbb]

A few things here
Is this an internal static heat exchanger or external pumped unit. With a pumped unit there is some mixing of the water meaning the tank will become stirred up mixing the cold at the bottom with the hot at the top cooling the top.
Secondly it appears that some hot water was used during the day as evidenced by the dropping bottom temperature which will reduce the manifold temperatures.

Thanks for the quick reply Naptown, ours is an external pumped unit. I'm just trying to get me head round what I should expect from a system like this. So I guess you're saying that until spring then this is pretty normal?

[Naptown]

With the water at almost 60C during this time of year is actually not bad at all.

[nickbb]

With the water at almost 60C during this time of year is actually not bad at all.

To be fair though, the water temp in the thermal store that I have shown was the residual temperature from the pellet stove being on earlier in the morning. What concerned me was the fact that the solar seemed to have 'dragged' that temperature down to 43.5 from a starting point of 49.5 in four hours. But like you say, maybe that was due to a bit of washing up etc ?

[nickbb]

With the water at almost 60C during this time of year is actually not bad at all.

Ok, today was a really cloudless sunny and mild day. The roof (manifold I presume) temp got to 110, but the water temp in both the bottom and the top of the tank did nothing but drop over a period of 5hrs without any water (baths, washing up etc) being used. Can anyone throw any light on what the cause of this might be?
The bloke who installed it left it on the roof unconnected for a week and a half before connecting and filling, which I have read is not great. I have also read that some installers do that and that the tubes can stand 200 Deg before they go bad.
Any help is appreciated.

[Naptown]

carefully feel the pipes that go to and come from the collector Are they hot ? is the one coming from the collector hotter than the return to the collector.
do the same thing on the pipes from the storage. The return to the tank should be hotter than the pipe feeding the heat exchanger.
Verify that the pump(s) are actually operating and pumping fluid and water.

[nickbb]

carefully feel the pipes that go to and come from the collector Are they hot ? is the one coming from the collector hotter than the return to the collector.
do the same thing on the pipes from the storage. The return to the tank should be hotter than the pipe feeding the heat exchanger.
Verify that the pump(s) are actually operating and pumping fluid and water.

Thanks for that Naptown. Some progress this end i'm pleased to report, namely that I have convinced the guy that the solar is not working properly, i.e. 108 on the roof and 40 at the bottom of the tank and constantly droppping over 5 hrs ! Anyway, it turns out that the flow and return from the thermal store to the solar tubes are connected the wrong way round. In other words the pumped line is connected to the top of the coil in the tank instead of the bottom. I'm just hoping that will solve the issue.....what do you think?

[inetdog]

Thanks for that Naptown. Some progress this end i'm pleased to report, namely that I have convinced the guy that the solar is not working properly, i.e. 108 on the roof and 40 at the bottom of the tank and constantly droppping over 5 hrs ! Anyway, it turns out that the flow and return from the thermal store to the solar tubes are connected the wrong way round. In other words the pumped line is connected to the top of the coil in the tank instead of the bottom. I'm just hoping that will solve the issue.....what do you think?

That will certainly make a big difference! But you may still have problem with the capacity of the system. Best thing is to make more measurements once the flow direction is fixed.

One possible remaining problem is that if you have a separate boiler/heater to that tank, the solar should draw from the bottom of the tank and return to the middle of the tank so that it is not cooling the higher temperature water heated by the separate heater. Only if the solar heater output temperature is higher than the temperature produced by the heater should you connect the return to the top of the tank.
In your case, that would correspond to the stove coil being in the top half of the tank and the solar coil being in the bottom half.

[nickbb]

That will certainly make a big difference! But you may still have problem with the capacity of the system. Best thing is to make more measurements once the flow direction is fixed.

One possible remaining problem is that if you have a separate boiler/heater to that tank, the solar should draw from the bottom of the tank and return to the middle of the tank so that it is not cooling the higher temperature water heated by the separate heater. Only if the solar heater output temperature is higher than the temperature produced by the heater should you connect the return to the top of the tank.
In your case, that would correspond to the stove coil being in the top half of the tank and the solar coil being in the bottom half.

Thanks, The solar is indeed connected to the coil in the bottom of the tank. The upper coil is fed by a pellet boiler, so hopefully things should improve when the solar is re-connected. I will post the results, but thanks for your help Naptown.

[nickbb]

Thanks, The solar is indeed connected to the coil in the bottom of the tank. The upper coil is fed by a pellet boiler, so hopefully things should improve when the solar is re-connected. I will post the results, but thanks for your help Naptown.

Ok, pipes changed round and results as follows:

9:00am 10:55am 11:35am 12:05pm 1:20pm 2:35pm
Roof temp 21 48.5 48.3 47.3 46 45.8

Tank bottom temp 43.7 38.3 39.3 39.9 39.1 37.9

Tank Top temp 53.7 48.5 47.5 46.8 45.1 43.8

Now I know there are a lot of variables, but the day has been cloud free, the sun low in the sky with some tree obstruction. The first temp was just after the pellet boiler had switched off, and the last temp when the 'sufficient solar' symbol had gone off on the controller. My question is, given all of the above do you think the figures I have reported are reasonable for the time of year (in the south of England) or should I be getting more?

Thanx

[Naptown]

Was there any water use during this time?

[inetdog]

Ok, pipes changed round and results as follows:

                     9:00am       10:55am      11:35am      12:05pm     1:20pm      2:35pm
Roof temp            21            48.5         48.3        47.3        46          45.8

Tank bottom temp     43.7          38.3         39.3        39.9        39.1        37.9

Tank Top temp        53.7          48.5         47.5        46.8        45.1        43.8

Now I know there are a lot of variables, but the day has been cloud free, the sun low in the sky with some tree obstruction. The first temp was just after the pellet boiler had switched off, and the last temp when the 'sufficient solar' symbol had gone off on the controller. My question is, given all of the above do you think the figures I have reported are reasonable for the time of year (in the south of England) or should I be getting more?

Thanx

A couple of questions:

1. What would the tank temperatures be for comparison if the panel system is turned off completely (valved off so no flow even without pumping)?
2. The tank top temperatures are looking OK, but the steady reduction in tank bottom temperatures suggest, as Naptown asked, that there was some cold water being added to the tank as hot water was used. Without having an idea how much, it is difficult to comment about what your panel output was compared to expected.

[russ]

Right - a modern tank with good insulation should lose very, very little temp.

[nickbb]

A couple of questions:

1. What would the tank temperatures be for comparison if the panel system is turned off completely (valved off so no flow even without pumping)?
2. The tank top temperatures are looking OK, but the steady reduction in tank bottom temperatures suggest, as Naptown asked, that there was some cold water being added to the tank as hot water was used. Without having an idea how much, it is difficult to comment about what your panel output was compared to expected.

I have turned the system off and will monitor the temps for a couple of days and report. But in the meantime let me ask another (possibly) daft question. The 'sufficient solar' symbol is coming on and the pump is cycling. So assuming no water is drawn off, can a situation exist whereby the action of the solar pumping would actually cool the tank temp, or does it work that if it kicks in then it should always contribute a positive temp?