- Joined
- 15 Oct 2007
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- 41
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doitall -
Hmmmm, that's a good one. I'm not sure how accurate a number I can get for the system volume. I'll have a go. We used 22mm pipe out from the boiler for the initial part of the system. Then, as I remember, when the system divided up between upstairs and downstairs we stepped down to 15mm. I'm not sure where that point is in the house, so it'll be a bit of a guess. Then I'm not sure I can remember exactly where all the pipe runs go. Anyway, I'll have a go. Paradoxically, it might be easier to drain the system and measure it that way.
The hot water tank is actually a heat bank. There are 250 L in there, but this volume is only there as a heat store. Hot water is made by passing mains pressure CW over a heat exchanger on the side of the tank. It has worked surprisingly well. I'm not sure what the volume of water is in the heating coil in the tank, but I can probably get that from the manufacturer.
One thing to note is that this pressurisation problem only happens when the CH circuit is working. When the DHW circuit is working (and the CH circuit is off) there is no problem. I know, from reading some of the other forum posts, that this kind of problem happens a lot. Namely - after the summer when the CH system is turned on for the first time - there is some kind of problem. However the DHW system has been working fine all summer.
It's curious this because it's difficult to see exactly why there should be a difference. After all, the boiler is producing hot water. Does it know whether it is going for CH or for DHW? I wouldn't think so. So this over pressure problem must be due to some kind of dynamic of how heat is extracted from the hot water made by the boiler. Could it be a thermostat problem, for example?
bengasman -
The tank is actually a thermal store made by Dedicated Pressure Systems in Epsom. The concept of a thermal store is kind of new to me and I initially opted for it because I wanted to have mains pressure hot water without having a "megaflow" type of tank. This could have been wrong thinking, I don't know, but what I can say is that it has worked well in operation so far.
I actually went down to their fabrication shop to discuss it with them. It was based on their recommendation that I went with an "S" plan valve system. The bypass was part of their standard package. You could either have one (if the boiler did not have one) or not. The process of ordering the tank and ordering the boiler happened at the same time, so I opted to get the bypass on the tank rather than the boiler. So, as you say, the bypass and the motorised valves are all there in one spot on the side of the tank. The bypass looks like it's set at 0.3 bar by the way.
I've not had any experience with expansion vessels as such, although I understand their function in principle. The cap on this one was stuck on pretty tight and I needed plyers to get the thing off. It looks like is may have been sealed on at the Worcester factory, maybe. In any case, I got it off. I just gave the central part of the valve a quick tap and something - I assume it was air and not nitrogen - hissed out. There was no water. I didn't hold it down because the CH circuit was still kind of hot and pressurised. Also, I didn't know how important the exact pressure was and how much a short release would affect it.
Aside from this test, I haven't done anything else to the expansion vessel. However, what I can say is that I'm pretty sure from looking at it that there is no way I can change it.
Thanks guys for your help. I really appreciate it. My thoughts at this point are that solving this problem is probably beyond my ability to do. I have no test equipment or previous diagnostic training. Initially I was thinking that it was a matter of bleeding or balancing. In other words something simple I could do. But from all the advice I've has so far, it's beginning to look like it could be the expansion vessel, a thermostat, a pressure relief valve or something else beyond my ability to cope with. I'm gonna give Worcester a call today and see what they say, but I'm thinking it may be time to get an engineer involved.
Hmmmm, that's a good one. I'm not sure how accurate a number I can get for the system volume. I'll have a go. We used 22mm pipe out from the boiler for the initial part of the system. Then, as I remember, when the system divided up between upstairs and downstairs we stepped down to 15mm. I'm not sure where that point is in the house, so it'll be a bit of a guess. Then I'm not sure I can remember exactly where all the pipe runs go. Anyway, I'll have a go. Paradoxically, it might be easier to drain the system and measure it that way.
The hot water tank is actually a heat bank. There are 250 L in there, but this volume is only there as a heat store. Hot water is made by passing mains pressure CW over a heat exchanger on the side of the tank. It has worked surprisingly well. I'm not sure what the volume of water is in the heating coil in the tank, but I can probably get that from the manufacturer.
One thing to note is that this pressurisation problem only happens when the CH circuit is working. When the DHW circuit is working (and the CH circuit is off) there is no problem. I know, from reading some of the other forum posts, that this kind of problem happens a lot. Namely - after the summer when the CH system is turned on for the first time - there is some kind of problem. However the DHW system has been working fine all summer.
It's curious this because it's difficult to see exactly why there should be a difference. After all, the boiler is producing hot water. Does it know whether it is going for CH or for DHW? I wouldn't think so. So this over pressure problem must be due to some kind of dynamic of how heat is extracted from the hot water made by the boiler. Could it be a thermostat problem, for example?
bengasman -
The tank is actually a thermal store made by Dedicated Pressure Systems in Epsom. The concept of a thermal store is kind of new to me and I initially opted for it because I wanted to have mains pressure hot water without having a "megaflow" type of tank. This could have been wrong thinking, I don't know, but what I can say is that it has worked well in operation so far.
I actually went down to their fabrication shop to discuss it with them. It was based on their recommendation that I went with an "S" plan valve system. The bypass was part of their standard package. You could either have one (if the boiler did not have one) or not. The process of ordering the tank and ordering the boiler happened at the same time, so I opted to get the bypass on the tank rather than the boiler. So, as you say, the bypass and the motorised valves are all there in one spot on the side of the tank. The bypass looks like it's set at 0.3 bar by the way.
I've not had any experience with expansion vessels as such, although I understand their function in principle. The cap on this one was stuck on pretty tight and I needed plyers to get the thing off. It looks like is may have been sealed on at the Worcester factory, maybe. In any case, I got it off. I just gave the central part of the valve a quick tap and something - I assume it was air and not nitrogen - hissed out. There was no water. I didn't hold it down because the CH circuit was still kind of hot and pressurised. Also, I didn't know how important the exact pressure was and how much a short release would affect it.
Aside from this test, I haven't done anything else to the expansion vessel. However, what I can say is that I'm pretty sure from looking at it that there is no way I can change it.
Thanks guys for your help. I really appreciate it. My thoughts at this point are that solving this problem is probably beyond my ability to do. I have no test equipment or previous diagnostic training. Initially I was thinking that it was a matter of bleeding or balancing. In other words something simple I could do. But from all the advice I've has so far, it's beginning to look like it could be the expansion vessel, a thermostat, a pressure relief valve or something else beyond my ability to cope with. I'm gonna give Worcester a call today and see what they say, but I'm thinking it may be time to get an engineer involved.
