Firstly there is two types of modulating control. Modulation to maintain a flow set point, as on most combi's. If set to 70C when approaching 70C the burner lowers to keep the heat moving out. The second is load compensation, which senses the flow and return temps. When the temps close up it sees the building is getting up to temp and lowers the flow temp and hence the return temp. They are very different.
The Siemens BLC1.A controller I gave details can be set from load compensation to weather compensation. It does it externally to the boiler by switching with integral boiler anti-cycle control.
Velocity? Pump speed. Some do, but these are on very expensive boilers. In some cases this may be a disadvantage as the pump when modulated down may not reach the far points of a largish system.
Remember this USA/Canda, where things are done a little different. There is a diagram of a UFH system taken of the fresh water of a cylinder. Potable water running through the heat emitters. Not allowed here.
I had a look. The document is specific to UFH and the USA/Canada variantions and heavily leans towards the makers solutions. Look at figs. 2.1, regular boiler & 2.2, condensing boiler. Condensing biolers should not mix return water that woudl raise its temperature. That is why a thermal store/heat bank is the best alternative when using a condesning boiler, which is what I have been trying to get you to understand.
The North Americans are way ahead of us in dosmestic forced air and vent, to the point we are hardly in the race.
Sorry if I've missed the point about a heat store.
I've spoken to technical engineers at Keston, Viessman, MAN Heiztechnik and Geminox, and they have all recommended that a low loss header should be used for my application. It's difficult to argue against that weight of opinion.;
These people mainly deal with commercial applications, where headers are the norm. You intend to use UFH, contact the UFH people and just about everyone of them will say use a thermal store. So their weight of opinion would be greater, as there are more of them. Using a header will lower a condensing boilers efficiency, that is certain. Ask the boiler people about connecting the boiler to an integrated thermal store with the UFH and rads taken off the store. Not one will say no.
You also have to take into account that the tech depts have people who read from crib sheets. Did you ask them about a header specifically? Or did you ask them about running a UFH and rad system off the boiler?
Look at this:
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scroll down to Twin Zone Thermal Store Injection System
A dedicated UFH thermal store shown with the old fashioned type of waether compensation of stats. Cheap and effective. Of course an outsise weather compensator can be fitted to maintain the UFH section to what the outside temp dictates.
Rads can be taken off the top of the store.
Also go to:
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down to underfloor heating and see the solutions to underfloor heating.
There are quite a number of muddled points on their underfloor heating page.
For example, they say:
"Reducing cycling in itself will improve efficiency, however gains are also to be achieved by keeping return temperatures to the boiler low at all times. Without a thermal store this is very difficult to achieve, unless the boiler has built in electronics. "
This is erroneous. If you have a modulating, condensing boiler, which most are nowadays, they have temperature sensing and electroniics to optimise behaviour.
Obviously a simple boiler with on/off burner control will operate more effciently with a buffer. They don't present a convincing argument for the case of most new boilers which modulate. Their whole "buffer store" argment is based on the idea that the boiler will otherwise cycle when driving the load, and this argument is flawed with a modulating boiler.
I get the distinct impression, that as a vendor of thermal stores, they are bending the situation to justify buying their product.
It is. You have never dealt with boiler manufacturers. At times their knowledge of systems is minimal. They after all only make a water heater, which is the new name for a boiler. Say boiler and you fail the corgi test. Headers with a condensing boiler, UFH and rads in a domestic system is ludicrous. I can them suggesting that if two boilers are used, or a regular boiler, but a header which raises the return temp is ludicrous.
I contacted Ferroli about combining the output of two combi's. They said no. I rang their top techie, who wasn't very good at all, and he was going on about back pressure to the heat exchanger. I said a non-return valve would be fitted on each and one also on the main in, so it highly unlikely this would ever occur. I told him I had seen two Ferroli's connect this way in France. He murmured and said no. They want people to fit them as per the instructions as they can deal with that. Any deviation and they are screwed.
I rang Worcester Bosch about doing the same. They said yes sir, and sent a coloured diagram on how to do it. I mentioned what Ferroli said and they confirmed my view that there is no reason at all why any two combi's of any make cannot have their draw-offs combined as long as non-return valves are used and there is enough flow.
Even the makers can't agree.
That is so.
That is so.
What sort of modulation? Load compensation as yours is. Few have that. Most modulation is maintaining the boiler temp setpoint. The burner lowers when reaching setpoint. Very different.
Only the up market expensive boilers.
And one with flow setpoint burner modulation.
They do.
Not so. They say that a low temperature will be maintained from a heat bank/thermal store. This because of stratification and re-heating in one pass of the water through the boiler. A tall heat bank that has dropped to say 25C at the bottom and 55C at the top, and most of the centre 30C will be reheated in "one pass" of the stores water. This is important. The boiler will dump heat into the top of the heat bank, without mixing the stores water, heating it up from top down. It may be 75C at the top and 25C at the bottom. For most of the reheat the stores return temp to the boiler will be very low and high efficiency follows. They also say once the heat bank water starts a second pass the return temp is substantially higher.
So, the secret is:
- Have a tall thin cylinder to aid stratification.
- Controls to ensure store water is heat in only one pass through the boiler
- Stats to ensure the store is re-heated when most of water has cooled. This also prevents boiler cycling.
They are telling it as it is. Also with thermal stores/heat banks, a cheaper simpler boiler can be purchased.
Their Twin Zone Thermal Store Injection System, is not as good as the sectioned temperature store I described to you some time back, where a weather compensator maintains the bottom section (UFH) of the store to the desired temp the FUN needs.
The majority of US houses have a direct fired water heater. There are companies in the US who are aggresively selling systems which use hot water from a DHWS heater to supply UFH (e.g., Radiantec). I think that the wording of the Warmrite manual was probably chosen to avoid any legal action from such companies. They do say that the preferred approach to such "dual use" systems is to separate the space heating from those containing domestic water using a small stainless steel heat exchanger.
This 'dual-use' system is banned in some states and there are many US heating professionals who are vigourously opposing it's use.
They also have a large section on external drive heating. Some nice UFH layouts, but primary loops everywhere raising the return temperature, and we are going over to full condensing boilers in April. Overall the manual is of limited use to the UK market.
I have actually - several - and thoroughly tested their technical departments.
I'm not surprised. If I find an issue like this and there are signs of disagreement or I am not convinced of the soundness of the argument, I would probe deeper.
However, to be fair to them, what proportion of Ferroli's target customer base is likely to combine two combis in the way you describe? One in a thousand? It's not going to be one in ten.
Their description and justification is based on boilers running in on/off mode between 60 and 80 degrees, and the argument is that a heatstore is needed to smooth out that effect and keep the boiler running for longer periods. I don't have any argument with that.
I also realise that there are different algorithms used in modulating, condensing boilers, but the effect is still that the output is reduced as the load requirement falls simply because the temperatures will be maintained if that happens. It may not be the same mechanism as load compensation, but the effect is crudely the same - the boiler does not cycle in the same way as one with full power on/off would.
Modulation is still based on sensing *a* temperature somewhere and modulating on the basis of it. My point was that the argument that a heatstore is needed to stop cycling is really not true for this type of boiler. DPS are intimating that cycling is going to be a big issue needing a thermal store *unless* there are some electronics and modulation. Most condensing boilers are modulating types so they are really trying to make a selling point out of what has become a corner case.
Are you saying that this form of modulation would cause the burner to cycle on and off rather than up and down? I'd find that hard to believe.
It doesn't convince me. Once somebody does this kind of thing, then I go through the rest of their claims with a degree of scepticism.
How can that happen? If you are taking water from the cylinder at
25 degrees and passing it through a heat exchanger that can manage a dT of typically 20 degrees or 25 if you're lucky, then the flow temperature can't be more than 45- 50 degrees. I assume that you are talking about a directly heated store where the bulk water goes through the boiler. This means that the water will need to go several times through the boiler to reach 75 at the top.
If the boiler is a modulating type, what is the mechanism for it cycling? This will only happen if the rate of heat production by the boiler exceeds the rate of use. If the burner has modulated down and stays lit, then it will be running in an efficient range.
They are selling a thermal store, partly on this argument. There are cases where a simpler non-modulating boiler may be what has to be used (e.g. oil). However, most modern condensing boilers are at least modulating types and this effectively knocks that argment on the head.
The sales argument is a reasonable one as a sales pitch, and results in the customer buying a heatstore to make a simple boiler more efficient - revenue for DPS. What I believe not to be reasonable is the impression given that condensing boilers *need* this unless they are in some way sophisticated when the reality is that it's nowadays teh exception that condensing boilers are not modulating. While not untrue it's at least misleading.
Not the point. They are supposed to know their product and systems in general. They don't.
The ouput of the boiler falls, not the temperature given to the building.
You have lost it. It reduces cycling.
An expensive load compensation boiler, not flow setpoint modulation.
Correct.
You don't understand the various types of modulation.
With a heat bank high efficiencies can gained from a very simple cheap condensing boiler. A great bonus.
The burner modulates down when approaching setpoint reducing, not eliminating, cycling. A heat bank can eliminate cycling and produce high efficiencies.
because you don't know much.
No direct.
Yes, and so are they.
No. One pass and it heats up top down and the heated water from the boiler does not mix with the water already in the store.
If it has load compensation control, it may drop the flow temp tom unacceptable levels.
Only one of the benefits.
Depends on what type of modulation.
You fail to grasp.
Heat banks are not big sellers, but getting better, as most plumbers fail to understand them.
Nothing misleading, you just fail to understand. That is sad.
Actually several times for myself and for others as well.
Why? They are there to support the requirements of the vast majority of their customer base. For a domestic boiler, this is going to mainly be installers of single boilers in houses. How often do you imagine they get questions on how to hook two of them together? Once a year? Twice?
.
If the output falls it's as a direct or indirect consequence of the building requiring less heat and does not imply that the burner is cycling to do it.
That makes no sense. If the output is reduced, it results in less heat transfer. If the heat requirement of the load is matched by the boiler running at lower output, why would the boiler cycle. Your argument only holds true if the boiler output exceeds the load requirement.
The issue is that the output is reduced - the boiler modulates down, not off.
If the monitored point is maintaining the flow temperature set point, then inevitably if the load reduces, the burn rate will be reduced to match it - otherwise why have modulation?
Actually I do.
However the point is that the power output is being reduced. Their argument and yours is based on an assumption of cycling. Where does this arise unless the heat required is less than the minimum that the boiler can do?
It becomes an equation between
a) simple boiler (and there seem to be few new ones coming onto the market like that) plus heatbank, or
b) boiler with modulation.
The only bonus in a) for the customer is if he has to use a simple boiler for other reasons or is the cost of a) is less than b)
If the burner is turned down and not off, then cycling hasn't happened.
If the boiler would otherwise cycle. Where is the mechanism for that if the boiler modulates down such that it is producing heat at the rate required by the load?
So you believe that this can help a modulating boiler that is not cycling off because its output matches the load?
How can that happen? You can't have water entering the heat exchanger at 25 degrees from the bottom of the store, going through a heat exchanger with dT of 25 degrees max and delivering water into the top of the store at 75 degrees. That can only happen when the return temperature reaches 50 and that won't be in one pass.
It was a simple question. What is the reason that a modulating boiler would cycle without the store if the production rate matches the load?
That's only an issue if you put the store there. If you are driving the heating load of radiators directly and the UFH via a blending valve, the issue doesn't arise. There is only a need to heat the store to 75 degrees if it's driving radiators and they need this temperature to give required output or if it's running a DHW exchanger as well. If it's only driving a DHW exchanger and that is handled by a priority arrangement and motorised valve, then the whole thing is simple.
It would be on boilers that exhibit cycling. Other than that it's a hollow argument.
Why do you believe that that affects whether or not it will cycle if driving a heating load directly and matching the requirement with lower burn rate?
Then please explain. I want the mechanism by which you believe that a boiler modulating on flow set point and running with output at or below load requirement will cycle the burner on and off.
With embellishments and claims that are stretching reality it's not surprising.
Then please explain the mechanisms as requested I am specifically asking why you believe that the method of modulation has an impact on whether a boiler will cycle when running at or below the heat load requirement when directly connected to the load -i.e. radiators and via a blender valve to UFH.
Can't be. This John can write coherent sentences, spell his own name (Wol) and almost certainly do joined up hand writing (as well as joined up logic).
Doesn't the design of the header and control of the water temperature inside it mean that the return temperature is low? I found a Viessman manual on their domestic Vitodens 200 boiler which says a bit more about this
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(page 35).
For my application, it seems like the options are: (a) Rely on the condensing boiler pump and zones valves if the load has sufficiently low resistance. As soon as any external circulation pumps are required, however, it appears that zones valves cannot be used and a means of hydraulically separating the boiler from the rest of the circuit is required. (Heating system suppliers give conflicting advice on this, however.) (b) Use a low loss header and circulation pumps in each heating zone. (c) Use some other means of separating the boiler flow from the secondary circulation (such as IMM's method).
All UFH suppliers provide a circuit manifold with a secondary pump on the load side of a thermostatic mixing valve. That would seem to rule option out (a). However, I bet that a simple zone valve system is (erroneously) used with these manifold/pump arrangements in most UFH installations. The UFH suppliers don't seem to specify headers or heat banks.
shows a mixing valve between the pump and the boiler. With a low load, the rads could be off and the flow from the boiler could be very low. Why would this arrangement be acceptable without a low loss header?
You can count on one hand and have fingers to spare. On some commercial boilers, the company I once worked for knew more about their boilers than they did. We were maintaining them on a daily basis, they were not.
They are supposed to know their product and systems in general, they do not.
Probably more than you think. Not the point, they are supposed to know their product and systems in general, they do not.
But will cycle and sooner than load compensation.
I recently fitted a W-B Greenstar heating boiler coupled to a heat bank. It modulates on flow setpoint. Fine for a heat bank. More expensive load compensation would lower the flow temp. Not what I want.
They all eventually cyclcle. With aheat bank, correctly wired, they do NOT.
You don't.
Cycling will occur.
I just mentioned one. I can name a few more too.
A heat bank plus simple boiler is cheaper than an expensive load compensation model plus a heat bank or unvented cylinder.
AND... the heat bank creates a brilliant
- buffer-store
- neutral-point
- anti-boiler-cycle
- instant mains pressure hot water
- all the benefits of using plate heat exchanger, etc,etc.
- no expensive high tech boiler to expensively go wrong.
Nothing wrong with high tech boilers when the need dictates, but why bother when you can get the same plus more and a simple cheaper more reliable boiler.
NO contest.
< snip babble about attempting to justify an expensive purchase >
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