VRC 470 set up

[FinnFrog]

Thanks for your comment. This makes sense, I understand why you would go for the third option "thermost." if it can't modulate down that low.

I have a small ecotec plus 824 combi, which apparently has a modulation range of 1:6. = 16%.. Another site states an output of 6.7 to 19kW. (I can't really work out how that corresponds to 16%, but all the same). So, it looks like the "modulation" should be a good setting for me?

In hindsight, I should have gone for a larger boiler. Its medium sized house, which I'll make bigger soon, and its old with limited opportunity for thermal upgrades to the fabric, and single glazing I'm not likely to be able to get changed. Sorry to say my heating engineer was not forthcoming with guidance on boiler size, and I had to push him to fit the weather compensator unit at all. He had not fitted one before and was jittery about it, thought it was rubbish. The boiler deal he originally purchased came with the on boiler standard control unit, and when I reminded him I'd asked for the weather compensator he just added the cost of the unit (delivered 3 days later) and would not omit the extra over cost of the standard unit (which he kept for another job). On the plus side, I can't fault his pipe work and he took pride in his joints and pipe alignment. He just likes to stick to what he knows I guess. Anyway enough moaning already, its boring.

Also I've learnt a lot about boilers, modulation, efficiency and heat curves. Its all pretty interesting I think.

 

[David937]

Your boiler modulates down to 3kW which is good. See the data table in the installation manual. The old model was 6.7kW. I would definitely use the Modulating setting. Re. The output of the boiler, if you measure all your radiators and add the outputs up, you'll be able to see how much of the 19kW you are actually using, it may surprise you!

 

[ALEC1]

Thanks for your comment. This makes sense, I understand why you would go for the third option "thermost." if it can't modulate down that low.

I have a small ecotec plus 824 combi, which apparently has a modulation range of 1:6. = 16%.. Another site states an output of 6.7 to 19kW. (I can't really work out how that corresponds to 16%, but all the same). So, it looks like the "modulation" should be a good setting for me?

In hindsight, I should have gone for a larger boiler. Its medium sized house, which I'll make bigger soon, and its old with limited opportunity for thermal upgrades to the fabric, and single glazing I'm not likely to be able to get changed. Sorry to say my heating engineer was not forthcoming with guidance on boiler size, and I had to push him to fit the weather compensator unit at all. He had not fitted one before and was jittery about it, thought it was rubbish. The boiler deal he originally purchased came with the on boiler standard control unit, and when I reminded him I'd asked for the weather compensator he just added the cost of the unit (delivered 3 days later) and would not omit the extra over cost of the standard unit (which he kept for another job). On the plus side, I can't fault his pipe work and he took pride in his joints and pipe alignment. He just likes to stick to what he knows I guess. Anyway enough moaning already, its boring.

Also I've learnt a lot about boilers, modulation, efficiency and heat curves. Its all pretty interesting I think.

Its almost certain you don't need a bigger boiler, a 24 kw heating load is pretty massive, I would multiply your radiators by 1.5kws, and that will give you a figure you can rate the boiler down to for heating..

 

[ALEC1]

its a bit of a myth that low modulation increases efficiency, it also increases proportion of air in the heat exchanger and this may decrease efficiency.

The people advising the government got all excited by low modulation and it became part of standards, and the marketing lead boiler manufacturers just produced what is wanted..

Thats why low modulation on tried and tested boilers is new.... nothing to do with efficiency..

 

[FinnFrog]

very interesting. thanks again for your time.

 

[David937]

its a bit of a myth that low modulation increases efficiency, it also increases proportion of air in the heat exchanger and this may decrease efficiency.

The people advising the government got all excited by low modulation and it became part of standards, and the marketing lead boiler manufacturers just produced what is wanted..

Thats why low modulation on tried and tested boilers is new.... nothing to do with efficiency..

How much does the efficiency drop? A good turn-down ratio reduces cycling and enables lower flow temperatures to be maintained for longer so do these benefits outweigh this decrease in efficiency you mention? I'm just interested.

 

[ALEC1]

I don't know is the answer..

low flow temps can be acheived with cycling too...thats not to say cycling is good, its pretty well inevitable in smaller systems as you find in uk housing stock.

If a building is loosing only 4-5 kws perhaps the boiler should not be burning gas at all...

 

[ringi]

its a bit of a myth that low modulation increases efficiency, it also increases proportion of air in the heat exchanger and this may decrease efficiency.

The people advising the government got all excited by low modulation and it became part of standards, and the marketing lead boiler manufacturers just produced what is wanted..

Thats why low modulation on tried and tested boilers is new.... nothing to do with efficiency..

How much does the efficiency drop? A good turn-down ratio reduces cycling and enables lower flow temperatures to be maintained for longer so do these benefits outweigh this decrease in efficiency you mention? I'm just interested.

I fail to understand way we don’t use a small tall buffer tank (say 50l) setup as a low loss header for the CH (but not dhw tank) to control cycling more often.

Low modulation is good as very short run time of a boiler is very bad and combi tend to be over rated on the heating side to give the hot water flow rate. But a complex control system is need to allow the boiler to understand when it should modulate down.

 

[ALEC1]

actually the control algorithms probably aren't that complex...they time the on time as well as the off time around the outside temperature...my weather comped boiler has an average burner run time of 29 minutes, and that includes hot water from a storage combi (vitodens 242)


if any one has noticed that bigger out put boilers have bigger heat exchangers...a neat way of understanding the failings of low modulation is understanding that the gas input goes down but the heat exchanger stays the same size...now if the heat exchanger changed size it would be fine but it does not, it stays the same size, hence the excess air..

one way to test though is to look at the flue gas temperature at high modulation and then at low...I would bet the two are more or less the same!

 

[ringi]

if any one has noticed that bigger out put boilers have bigger heat exchangers...a neat way of understanding the failings of low modulation is understanding that the gas input goes down but the heat exchanger stays the same size...now if the heat exchanger changed size it would be fine but it does not, it stays the same size, hence the excess air..

one way to test though is to look at the flue gas temperature at high modulation and then at low...I would bet the two are more or less the same!

This is going off topic…

I have seen lots of boiler specs that give the best efficiency at low flow temperature and the output modulated down. E.g. there efficiency is better at 30->50 with 30% of rated power output then they are at 30->50 with 100% of rated power output.

Unless my understanding is wrong, on any modem builder the air intake is controlled by a fun and is reduce to modulate the boiler down, with a sensor system that keeps the gas/air mix correct. So no excess air, just a lower flow rate of the flue gases therefore giving longer for the heat exchange to extract the heat before the flue gases leave the boiler.

For heating anything apart from the most well insulated house provided you wish to heat the complete house most modem boilers will modulate down enough to give long burner run times. But what about when you wish to just heat the study in the daytime?


Also how does the boiler know the power output to give when the “call for heat” comes from the control system? (Getting that right takes “leaning” logic in the boiler, and/or a control system that gives the boiler for information then just on/off)

 

[ALEC1]

I took this up with a friend who works in the industry, who pointed out that What I have said is wrong, lower modulation rates do enhance efficiency.

As said above boiler manufacturers quote rated out puts at high temperature 80/60 and lower temps of 50/30. The lower temperature always gives the higher kW rating, put simply less heat goes out the flue...

With respect to the point above about heating one room only, complications arise as the heat loss of the room could be so low that no boiler designed to heat a whole house will cope.

Then there is the hardware to set up zoning, costs that won't be recouped in the saving...

the problem isn't really a heating one, it's a building one... Why live in buildings that are so poorly insulated...

 

[ringi]

With respect to the point above about heating one room only,

the problem isn't really a heating one, it's a building one... Why live in buildings that are so poorly insulated...

I am hitting this with two building.

We live in a 1930 bungalow, heat loss is high, and as room usage is not fixed by the building layout there is no sensible way to have 2 zones. Upgrading wall insulation is a 10 year project as we decorate rooms.

More to the point, I am in the process of setting up a property to let out as a HMO, with each tenant having their own room (with auto closing fire door) – so if one tenant want their room heated in the day time, (as they are working shifts,) the heat demand could be very low.

 

[ALEC1]

the problem with zoning is the minimum flow rate through the boiler... to keep a boiler in its operational parameters you need the smallest zone to be the same as the minimum output of the boiler.

One room being maintained at say 21c may only need 200w of heat from a boiler that in minimum output produces 20 times that if its minimum modulation is 4kws... not a happy scenario...

if I was doing HMO, I would spend the money I save by not zoning on insulation, and maintain one or two temperatures (night and day) 24/7.... much like they do in Germany...where the boilers and their controls are made...

zoning complicates installs, adds to capital outlay by a margin, and has not been proven to lower running costs...much better to buy a solution out of a box, such as the vaillant vrc470 and use it as they do in Germany...

 

[ringi]

the problem with zoning is the minimum flow rate through the boiler... to keep a boiler in its operational parameters you need the smallest zone to be the same as the minimum output of the boiler.

One room being maintained at say 21c may only need 200w of heat from a boiler that in minimum output produces 20 times that if its minimum modulation is 4kws... not a happy scenario...

if I was doing HMO, I would spend the money I save by not zoning on insulation, and maintain one or two temperatures (night and day) 24/7.... much like they do in Germany...where the boilers and their controls are made...

zoning complicates installs, adds to capital outlay by a margin, and has not been proven to lower running costs...much better to buy a solution out of a box, such as the vaillant vrc470 and use it as they do in Germany...

Thanks for a very thoughtful response.

Giving the tenants control over their heating, and having large radiators is a selling point that will put up the rent I can get a bit. I am not seeing zoning as a way to reduce heating bills.

Each letting room must have an auto close fire door, so it not like a well insulated Garman open plan flat!

The least control I will have to give tenants is a TRV so they can turn down the radiator in their room; otherwise they will just leave the window open if they are too hot.

As each letting room must have an auto close fire door, I can’t see how I would get the same temperature in each room without some temperature control in each room. If the temperature monitoring in each room does not feedback into the “call for heat”, then I will have to hope that the hallway heats up at the same speed as the letting rooms. (I think this is the root issue I keep coming back to.)

The Vaillant zoning system for more than two zones seem to need a pump and electronic mixer for each zone, so even if the Vaillant bits were free, it would still be out of the question. (Also I don’t think Valliant does TPI control, so small zones may be an issue.)

I could control thermal activators on a UFH manifold (or in line on each radiator) using simple per room controllers, provided that the start of the TPI periods on each controller could be synced, so all rooms tented to call for heat at the same time.

No normal boiler will be happy unless there is a lot of thermal mass in the control loop, this could be a buffer tank, or large oversized radiators with a TPI control system. (It is a shame that there is not an affordable boiler like the ACV HeatMaster that is covered on a standard service contract – the ACV boilers are not covered by BG)

Another option is to put timed TRV on each radiator, (most of them don’t provide boiler feedback, or allow a max temperature to be enforced.) and have a small buffer tank (low loss header) with a Grundfos ALPHA modulating pump running most of the time. The Grundfos pump will only use something like 5w when all the TRV are closed. It then just needs a simple control system so the boiler recharges the buffer thank when needed.