Was your stat installed too high? My boiler cuts in about 20 seconds after opening a tap. Besides it has enough capacity to fill two baths, so I'm not worried about the boiler's additional 28kW cutting in. Also, I have a TMV on the output, so the exact temperature of the store is irrelevent to me. Temperature overshoot simply means greater energy storage, rather than a trip to the burns department with the children.
Christian.
This isn't the simplest, and I am not sure that it has value in terms of usability either.
This would need at least a latching relay arrangement to work. The top stat, as I read the way you describe its use, would delay the commencement of reheating until most of the contents had been used. If you happen to be at the point when the cylinder is down to 25% of capacity and then run a bath or shower, you are back into the same old game of the water running out quickly and relying on the instantaneous delivery rate of the boiler. Having the flow switch idea would help the timing a bit at least but doesn't replace having a full cylinder of water.
Boiler cycling would occur only if the heat production rate exceeds the capacity of the cylinder to absorb it. That can be avoided if the cylinder is a fast recovery type or if the boiler modulates down.
I think you mean water stored at a higher temperature at the top, which when mixed gives more at the wanted temperature. Even this is not going to compensate for letting the stored amount of heat fall to 25% of the full capacity.
This could be worth doing to deal with the poor response of bimetal thermostats.
Actually it does the opposite as far as having the top thermostat used. The flow detection is worth having and would "increase" the effective capacity simply because heat is being added back earlier than it would otherwise be.
The boiler efficiency argument here is marginal and is at the expense of performance. You are sacrificing the stored capacity for a presumed improvement in efficiency.
Whether there is an efficiency gain to be had really depends on the type of boiler and cylinder.
If it's an older type cylinder, incapable of taking all the heat from the boiler, then the cycling will happen anyway if the mismatch is great even if the coil is effectively immersed in cold water because the cylinder was allowed to drop to 25% of capacity.
If we assume that the cylinder is fast recovery, so will absorb all the heat and therefore no cycling during recovery, then the question becomes one of how long did the inefficient parts of the boiler cycle vs. the efficient parts last as a proportion. The inefficient parts relate mainly to energy used to heat up the heat exchanger before useful heat is delivered to the load, .and at the end of running, heat that is not usefully delivered to the load. If the heat exchanger is not a high thermal inertia type such as cast iron, then unless the recovery time is very small, the inefficient periods will be a small proportion of the total.
With a condensing boiler, having a cooler load means that the run time at lower temperatures is longer and hence the boiler is operating at a more efficient point - however when it does run, it runs for longer.
I did some experiments on my system where the cylinder was recovered totally from cold. I recorded the temperatures of flow and return water and rate of rise of the cylinder temperature and also measured the amount of gas used to do the reheat. The temperatures can all be logged to a PC from the boiler. Modulation down of the output only comes in the last 30 seconds or so of the run and is there to prevent overshoot of temperature.
I then repeated the experiment in two different ways. The first was to run off the same volume of hot water (basically a cylinder full) continuously and letting the boiler do its thing of starting to replenish when determined by the sensor. The second was to run off small amounts of water more slowly so that the boiler would come on as the cylinder temperature at the sensor had reached 55 degrees. This is indicative of smaller use during the day. I stopped running water as soon as the boiler came on and waited for recovery. In total I drew the same amount of water in each of the three cases.
In terms of total gas used for each, the amount was the same to within about 2%. So in this context at least, I don't buy the long burn argument as making a huge practical difference with recent equipment.
I don't think so. I experimented with it a quarter, third and half way up. When lower, it came on sooner, but still not brilliant. Using an electronic thermostat with sensor was rather better.
I increased the cylinder size to a 200 litre one as well and had double normal thickness of foam fitted. So there is pleny of storage, which I prefer anyway.
I suppose that with a heatbank, having additional temperature control makes good sense anyway, since presumably if it's fully charged and you open the tap to a trickle you would otherwise get very hot water.
A trip to the A&E at RBH is never a pleasurable experience at any time :-)
However, it could well produce good results, provided that the thermostats are both near the bottom. It allows a much more precisely defined hysteresis to be used. I'd probably use that at the >95% full (to turn the boiler off) and
Yes, I last went when my GLW decided to squirt half a bottle of superglue in my eye from 2m away. She was quite a good shot and the nozzle does produce a very tightly defined stream.
It has been completely rebuilt in the last few years, so isn't the shabby shell it used to be.
Christian.
It is.
You are not sure. I am sure. It has.
yep.
The stats can be set to give a half full cylinder if you like.
Fast recovery is highly desirable, modulation down? No.
temperature
No. water stored at a higher temp all through the cylinder.
Wrong.
Wrong again.
Wrong again.
equals more efficient.
When using a DHW blending valve overshoot doesn't matter.
As boiler cycles heat from the boioer is lost through the flue and primary pipes when the boioer is in an off cycle period.
Over storage is a bad thing. No need for it when you have the boiler power to have the least amount of storage you need.
recovers
If the store is full up to temp and you turn on the bath tap with the flow switch activating the boiler, by kicking the latch will do nothing as the bottom cyl stat will hold off the boiler.
Irrespective, the boiler needs to come on. If say the store is up to temp. and the flow switch/boiler comes in, while the flow switch is bringing in the boiler the high limit stat prevents any problems. When the flowswitch de-activates the boiler the system goes over to the main bottom cyl stat. If the store is over the setpoint of this, the boiler stays off. This situation could occur if a shower set to a flow flowrate is used. In reality if a bath is drawn off it will be below the store setpoint. You could eliminate the high limit and rely on the max temp of the boiler.
Set the by-pass accordingly.
The simplest is to have one thermostat or sensor and done with it.
You've added an additional thermostat, a relay and a flow switch.
So how does reducing the available volume of hot water in a storage system improve usability?
Or just have one at the bottom, maybe plus a flow switch and done with it.
This would only happen if the cylinder is incapable of absorbing all the heat or as part of a control system to avoid overshoot.
Yes, but *much* higher at the top due to stratification.
Please explain how this scheme improves usability and makes more stored water available.
Please explain how this scheme improves usability and makes more stored water available.
I demonstrated, with figures how it makes little diference with recent equipment. It might make more difference with older boilers.
Obviously. The question is whether that is significant.
More complexity.
If the boiler is of a fanned flue type, especially with small gauge flue pipes, the flue loss is negligible. It might make more difference with an old cast iron or natural air flow model.
Heat loss through primary pipes is in the building envelope so is largely irrelevant.
It makes very little difference as long as the cylinder is well insulated.
I think it might be knackered TBH. I've got it set around 0.8 bar or somesuch, which shouldn't be achievable with an indirect coil right next to the pump with no balancing valve.
I just don't have time to investigate.
Christian.
Not it isn't "simplest" is a simple thermostat, not two and a latching relay.
Actually, I've changed my mind. Simplest is no thermostat at all. Just connected directly to an always on boiler.
Christian.
Yes. But I can wait until the store gets below 95% before lighting the boiler. The addition of boiler power before this is of marginal benefit, given the additional complexity.
Christian.
Except in summer, where it would be even worse with air con.
Christian.
To improve efficiency, reduce boiler wear and in effect extend the cylinder size, without installing a larger cylinder.
When you require that volume of hot water.
That creates inefficient boiler cycling and excess wear on boiler controls.
99% of boilers do not have sophisticated DHW controls.
Usability? Stop making things up.
Read again.
Usability? Stop making things up.
Read again.
Figures from one sophisticated boiler which few people have heard of.
yes.
They are to be mandatory soon, so get used to them. They are simple mechanical devices.
It is lost and adds up over the year, and over millions of houses too.
In the summer it is not. There is also the wear of the boiler controls with excessive unnecessary cycling.
Extra cost and space taken up there ARE standing losses.
The extra cost is negligible as is the space.
We had the discussion about standing losses several months ago, when I demonstrated that these are insignificant as well if the cylinder is well insulated.
Nonsense.
You set it up so that it will always give a bath, and have a quick recovery cylinder er and a powerful boiler taking all its heat.
< snip drivel >
Nonsense.
You set it up to suit. Duh!
I have just fitted a W-B Greenstar heating only boiler and it acts in no such way.
Total nonsense.
There will be.
Now you know and your life is more complete.
You never.
< snip much tripe by a man with an incomplete life >
Look at the DPS web site, and two stats is a common option.
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