True, or if the bypass opens before the alpha "effect" kicks in what is the point of an alpha? Basically they both do similar things so its debatable which you need.
How does a bypass remove residual heat? For that the pump has to run so you need pump overrun not bypass. However, you may need a bypass and pump overrun to cover the situation of all rads with TRVs (which you shouldn't have) and needing to get rid of some heat (to the pipe work presumably).
How would the bypass operate without a pump?
.andy
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OK. Nothing is perfect but the TRVs will close enough to reduce the flow otherwise what are they doing, nothing. The flow has to reduce to lower the average radiator temperature and so lower its output. So at the bit of the alpha curve where the head rises to keep the flow up then all that will happen is the TRVs close more. They must reduce the flow.
The heat flow from the boiler is the flow, kg/s, and the difference across the boiler. The heat output from the rads is the temperature difference. Since the input water temperature is fixed then TRVs must slow the flow down so the output temperature is lower and thus the average water temperature lower. Ergo TRVs must reduce the flow AT any time they are having an effect. NOT just when they are about to shut the rad off completely.
So what is the point of an alpha? If the bypass is kicking in before the alpha "effect" why not use a normal cheaper pump! If the alpha "effect" kicks in the bypass won't open. Both effects are driven by pressure, ones got to happen first then the other won't.
An alpha would suit something like a coal fired room heater where the best way to run it is slow continuous release of heat, not super-burn/off/super-burn/off as you get with normal pump + stat (no bypass). Also a condensing modulating gas boiler (as long as it could cope with low flow (coal fires can).
The flow reduces naturally as a result of the increased restriction. The point is to reduce the power to the pump to stop it from screaming its nuts off.
Only while the boiler is firing. At the point where the heat used is less than that produced it will begin to cycle.
It is proportional to the temperature difference and flow.
It isn't. It will move up and down as the boiler cycles.
That wasn't my point.
That would depend on how the bypass is set.
Have you read through the Alpha datasheet?
You're missing the point. It is far better to have the pump reduce output as the heat flow reduces rather than shortcircuiting it back to the boiler with the pump on full power.
This misses the point completely.
.andy
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By providing a flow path for the water when the pump is over-running and all the TRVs and/or zone valves are shut. The residual heat is actually dissipated by the pipework as the water flows round the by-pass circuit. If the residual heat can't be carried away from the boiler, the boiler temperature continues to rise to the point where its overheat stat trips - which requires manual resetting.
Well, the door is painted, a nice grey. Gunmetal, actually.
You guys seemed to be enjoying yourselves while I was away.
I have upstairs on a separate zone, with TRV's on all but one Rad. I've just installed a wireless stat in that room.
Don't know about the pump yet. I'll probably go for the alpha and i can always close the abv fully if necessary.
Francis
In this scenario the boiler is required by the regs to turn off so the bypass is only in operation for a very short while
No. I have a conventional boiler, Alpha pump and an automatic bypass. Plus
4 zones valves run from room thermostats and only a couple of TRVs. This gives lots of degrees of freedom in setting up the system to operate efficiently - or in getting it wrong if you don't set it up well.
This is true - but the by-pass *must* operate under these conditions, albeit for a short period - otherwise the boiler trips. But the by-pass *shouldn't* operate at any other time.
I don't have any direct experience of this, but I was speculating that - with an Alpha pump - it may be difficult (or perhaps even impossible?) to adjust everything so that the by-pass *only* operates during pump over-run conditions.
Oh you can adjust an Alpha to do almost anything. You can even make it run like a normal pump if you want though it gets a lot louder then. But I found that by setting it a 'bit of a turn' more clockwise than is really required then I can make the by-pass operate.
At last an interesting comment of practical experience. However, I think the alpha is now operating in almost standard pump mode. The alpha data sheet curves show for a normal pump change of flow from Q1 DOWN to Q2 gives INCREASE of head from A1 to A2. As expected. But for alpha Q1 down to Q2 gives DECREASE of head from A2 DOWN to A3. Which is what you want if don't have an auto bypass.
With an alpha and an auto bypass you have to adjust the alpha to behave like a normal pump for the bypass to operate, that is flow down, head up. OK, an alpha has more adjustment than a normal pump but is the extra cost worth it?
For standard boiler I'd say no as you don't want flow reduction and low return temperatures with a standard due to the risk of condensation. For a condensing boiler I would be tempted by an alpha as with the better adjustment you are more likely to be able to delay the operation of the bypass and so work sometimes with a lower flow, lower return temperature, which makes a condenser more efficient. However I wouldn't like to say for sure that you would recover the extra GBP35 and alpha costs in the lifetime of the pump.
That's not my point, which was pump overrun is what you need to get rid of boiler excess heat.
Bypass does not figure, except that you need to allow flow. You could have an always on rad for that. You always need to allow some flow, during normal firing primarily so if you have that you have it during overrun to.
You can use an automatic bypass purely to take care of the over-run case.
.andy
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This ignores the issue of the effect of short circuiting the flow back to the return if a bypass is used for the low flow rate case.
Obviously. The bypass is simply set to the appropriate opening point, if one is used at all.
That's a red herring, because if the flow has reduced (which it will have done), then relatively little low temperature water will be being introduced into the heat exchanger. In a conventional boiler, the heat exchanger is not designed to allow condensation anyway and so the low flow will simply cause the boiler to cycle off as the heat load falls.
This misses the point. A condensing boiler typically modulates as well, so there is much less of an issue of mismatch between the rate of heat production and use.
That depends on the lifetime of the pump, but it certainly improves system behaviour as regards noise. If it saves the cost of an ABV, then it's a no-brainer, although the latter may be useful as a better alternative than a simple lockshield.
.andy
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Not accord Electronically controlled, the pump can, by means of a selector switch located on the terminal box, be set to: ? 2 constant pressure curves ? 2 proportional pressure curves ? 3 fixed speed curves.
So either constant head (I think I said that), proportional head (head goes down with lower flow), or 3 normal pump modes.
I'm glad we agree it seemed to me before that you said the flow did not reduce until the TRVs almost closed, which is of course not the case.
Protection of standard pump nuts can be achieved by a bypass.
No. If alpha is in constant or proportion head mode flow decreases. Otherwise it is in normal pump mode.
Of course but what is the relevance of that. It will always cycle when Watts in > Watts out. We are discussing TRVs and pumps.
No it isn't. Temperature output from a rad is proportional to the average rad temperature (or rather difference to the room). The flow does not come into the rad output.
What happens is that as you restrict the flow the output temperature drops so the average rad temperature making its output less.
We aren't talking about the boiler cycling but in any case at any point in time the temperature in to the rads is fixed. It is the output temperature that decreases, as the TRVS control it, in order to lower the rad output.
My point is that the flow varies since you seem to be saying that TRVs only reduce the flow as the close completely.
I'm glad we agree on that then as I read your post differently.
Yes, have you?
I'd agree that the fine adjustment on the alpha is nice. But you still have to operate the alpha in normal pump mode to use it with a bypass. If the fine adjustment necessary, or worth the money? I think we'll have to disagree on that.
Why? The alpha's modes of constant and proportional head are designed for system that work best with a variable flow. That has to be boilers that modulate their output. Also, those boilers that are happy with a low return temperature, which is another effect of low flow.
Set it any differently and it's just a fine adjustment standard pump.
You need to look at the technical manual for the Alpha.
They have some other products which have different selectable operating modes.
No I didn't say that.
Possibly, but there's no point.
Have a look at the technical manual.
I know. The point was that the behaviour is not simple because the burner is being fired on and off. This has a substantial effect on the temperatures.
From the perspective of heat output to the room, it is approximately proportional to the Mean Water to Air Temperature.
From the perspective of heat delivery to the radiator (which will be the same since none of it disappears) it will be proportional to the flow and temperature difference across the radiator.
That is because the rate of heat delivery to the radiator reduces.
You can't treat them in isolation though. If the boiler is cycling, the output temperature is far from being fixed,
I didn't say that at all.
Yep, and the technical manual.
Plus I have one of these and know how it behaves.
Up to you. You can effectively replace the need for a bypass and have quieter and better controlled operation.
Even a fixed boiler has varying output temperature as the burner cycles. There isn't going to be an issue with the return temperature being low at low flow rates anyway.
.andy
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The efficiency of a condenser is not determined by the return flow, only the temperature. You can get a lower return temp by having the flow faster, or in some instances slower.
They are quieter. Nevertheless, if a system is installed properly and the odd bit of vibration absorbing flexible pipe or plastic pipe is used, a pump should not be noisy, even if the TRVs are nearly all closed.
Are we taking about the same sort of automatic bypass, i.e. pressure driven or is this an electric bypass? As I fail to see how a pressure bypass can handle overrun without first handling the run into closed TRVs first.
If TRVs are open, stat goes off, boiler goes off, pump goes to over run then water will go to rads (or cylinder).
You may not want this in the summer! Consider an S-Plan system with the heating zone valve firmly shut, and the boiler running to heat the hot water. When this is satisfied, the HW zone valve closes and the boiler stops firing. BUT you still need some flow during the pump over-run period to stop the boiler from tripping. Where can this flow go if you don't have a by-pass?
I am looking at the pdf from the grundfos website alpha_data_booklet.pdf, 16 pages with graphs etc. from which the quote above is taken - page 6.
You need auto bypass for standard pump - I was being humorous referring to your comment it would scream its nuts off.
If the boiler is short cycling yes but if it is doing that you need to fix it since that is very wasteful. If it is on a long cycle then you can consider it as being in almost steady state. There will be a steady climb in temperatures but relative temperatures will be similar during this time.
Agreed (I think I said that).
Short cycling is a different problem and needs to be fixed. Normal long cycling is relatively stable temperature difference on an overall rising base line.
Why? Assuming you have a normal pump set to the right setting. In fact since there is a recommend minimum speed to stop sludging it could be argued that a bypass, which keeps the boiler flow constant is better as it stops the sludge settling in the boiler. Of course if you correctly treat the water then sludge is not an issue, hopefully.
I would tend to agree with you as it would seem reasonable that a low flow low temperature return is going to quickly warm up in the heat exchange. However, I'd want to check with the boiler manufacturer that he was happy with that. I suspect most might prefer the constant flow arrangement so as to avoid even the only perceived risk of condensation. So I'd reckon most would recommend normal + bypass (that's the result I had when talking to HRM). Of course, assuming this is a standard boiler, not a condensing.
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