BG or local rgi for install?

[bluey831]

D_

The heating requirement is directly proportional to the difference between internal and external temperatures. Assuming an internal temp of 20°C. you would get results similar to this:

[code:1]
Outside temp kw required
-1°C 14
2°C 12
5°C 10
8°C 8
11°C 6
14°C 4
17°C 2[/code:1]


If an 837 or 937 is installed it would only be able to use modulation of the outside temperature is below about 2°C. Any higher, and the only way it can produce a lower output is by cycling the boiler on and off.

In addition, I'm told the Vaillant will knock off for 5 minutes rather than continue to recycle.

But that's still cycling.

So from what I can ascertain, the amount of boiler cycling would be minimum if I got the 837. My priority is a good flow rate to the shower so on balance I am left with is obtaining the best flow rate versus the overhead of some unnecessary boiler cycling below 12kw.

The 837 and 937 would both be the worst choice for cycling.

You seem to be considering only Vaillant and WB products. I think you need to widen your net and include the Broag Remeha range. They are well made and the firm has a very good reputation. All boilers will modulate down to 6kW (equivalent to an outside temperature of 11°C ), so the only thing to consider is the HW requirement. The 35C will give a 35°C rise at 14l/min and the 39C at 16l/min.

Ok - I have looked at the average temps for Nov to Feb. This reads as
Average:
Nov 7C
Dec 4.5C
Jan 3.5C
Feb 4.5C

The minimum reads:
Nov 4C
Dec 2C
Jan 1C
Feb 1C

Now we would have our heating on in the mornings for possible two hours or so 5.30am to 7.30am and then to come on in the evenings at about 5pm. On that basis it is likely we would be running the heating when the temps are at minimum - except maybe weekends. Even at the average temps for Dec - Feb the Kw required will be 11Kw - just marginally below the minimum for the V837.

If the V837 knocks off for 5 minutes - though you say it is still cycling - it is much less than one going on and off continuosly? Certainly for Dec, Jan and Feb I'm not sure there would be any boiler cycling?

The other question is of course, for the amount of time it will be cycling, i.e. the time between dropping below 12kw and the thermostat shutting the boiler off, just how inefficient is this? My understanding is that it would be neglible?

I have indeed been looking more closely at the WB and Vaillant - only because they seem to come up as generally being good boilers as well as being common etc. That is not to say others might not be as good or better - but I am at it from layman perspective. For instance the difference between the V837 with the minimum 12kw and the Broag Remeha with the minimum 6kw - I ask myself how much of a difference this would make in those winter months with the temps as displayed above.

 

[ChrisR]

12kw and the Broag Remeha with the minimum 6kw - I ask myself how much of a difference this would make

Quite. As I wrote just before your answer - I suspect it's not much.
I'm pretty sure I've heard all the theories, but precious little in the way of numbers, and nothing whatsoever that's independent.

Regarding Remeha, at the moment I wouldn't go for them. Customer service, some poor design issues... Give them a couple more years producing stuff for our market.

 

[bluey831]

A 937 would give you 15 li/min which is half of your bath flow rate.

Your current stored water is likely to be around 55 (could be more).
This is complicated a bit when you use a storage combi (they work better than you might expect) but I'm sure you get the point.

Ours is a lot more than 55 as there is no thermostat on the hot cylinder. When we have used the immersion it comes out near boiling!!! I understand the 937 has a storage amount to put out to taps instantly and the WB837 has a pre-heat system which operates similarly. However, there is a degree of inefficiency here presumably as the storage or preheat has to be maintained throughout the day - unless programmed otherwise??

I don't see the problem with "oversized" boilers to be as critical as some others do. OK, so the boiler has to turn off when you're at the low end of demand. SO the outlet temp from the boiler varies by a few degrees.
There's lots of barrack-room-lawyers telling you how bad that is, but show me figures from independent sources.
I don't buy it.

This is what I am trying to narrow down - and there does seem to be varying views on this. One sales guy I had around said what I was gaining by going to a combi I would be losing by it being oversized and cycling. When quizzed further he was not able to provide a satisfactory explanation and vered off in different directions.

From what I can ascertain, there does seem to be a minimal affect of the boiler cycling when the heat requirement goes below the minimum modulation of the boiler.

 

[bluey831]

Did a bit of further looking up. We have a Glowworm Ultimate 60ff. It says the Heat output min is 14.65kw and max is 17.59kw.

Now if I am reading this right this means that it has to be below -1C for it to be working efficiently?

On the other side it can only get up to 17.59kw max - is this why it is not up to heating 11 radiators with our house heat loss at about 13kw?

The other point about this is that there are only two of us and we currently have the hot water on for about 3 hours per day - approx 1 hour morning and 2 hours evening. In reality, we only use the equivalent of two ten minute showers per day and two washes. If I am reading this right, we will be going from 3 hours of heating water in a hot cylinder that has no thermostat to about 20 -25 minutes in the Combi. In other words, this should cut our hot water costs by more than two thirds??

 

[D_Hailsham]

Did a bit of further looking up. We have a Glowworm Ultimate 60ff. It says the Heat output min is 14.65kw and max is 17.59kw.

On the other side it can only get up to 17.59kw max - is this why it is not up to heating 11 radiators with our house heat loss at about 13kw?

But the Ultimate 60FF is not a modulating boiler, the only way it can control the output is by turning on and off. The max and min heat output figures you quote show the range over which the installer can set the output. Once it has been set, it does not vary.

As for not heating 11 rads, what is the total kW of your rads? If it is more than the boiler output, the rads will feel cooler as each rad will give out slightly less - the rad's can't give out more than the boiler produces.

If I am reading this right, we will be going from 3 hours of heating water in a hot cylinder that has no thermostat to about 20 -25 minutes in the Combi. In other words, this should cut our hot water costs by more than two thirds??

But the cylinder is not being heated continuously during the three hours.

I don't see the problem with "oversized" boilers to be as critical as some others do. OK, so the boiler has to turn off when you're at the low end of demand.

Why do you think modulating boilers were invented? It's because they are more efficient than on/off boilers.

 

[bluey831]

As for not heating 11 rads, what is the total kW of your rads? If it is more than the boiler output, the rads will feel cooler as each rad will give out slightly less - the rad's can't give out more than the boiler produces.

How do I work out the Kw of the rads?

If I am reading this right, we will be going from 3 hours of heating water in a hot cylinder that has no thermostat to about 20 -25 minutes in the Combi. In other words, this should cut our hot water costs by more than two thirds??

But the cylinder is not being heated continuously during the three hours.

True - though presumably it will be heating for the majority of it - particularly as there is no thermostat on the cylinder?? Not sure how I would work out the difference between the hot water being on for 3 hours versus 20 minutes usage on a combi.

 

[Andygasman2010]

With access to manifolds for possible repair & changing the radiator valves, there's no reason for replacement of whole of your existing system. 10mm is easy to clean & great for most smallish systems.

says it all

Sorry kevplumb, I have to agree with DeltaT here. 10mm is fine for your average 3 bed semi.

Why isn't it?

 

[D_Hailsham]

How do I work out the Kw of the rads?

If they are the standard flat steel rads (with or without fins on the back), you can use the Stelrad Elite Catalogue. There is very little difference between manufacturers for this type of rad. Just use the nearest size if necessary.

But the cylinder is not being heated continuously during the three hours.

True - though presumably it will be heating for the majority of it - particularly as there is no thermostat on the cylinder?? Not sure how I would work out the difference between the hot water being on for 3 hours versus 20 minutes usage on a combi.

The heating time will be controlled by either the room thermostat (winter) or the boiler thermostat (summer).

I'm not sure what ChrisR has against Remeha boilers; I suspect he has never installed one. The company has an excellent reputation for their commercial boilers in the UK, but have only been selling their domestic range for the last five years. Remeha own the Baxi Group and servicing can be carried out by Heateam (part of Baxi). The warranty is two years labour and five years parts; but the heat exchanger has a five years parts and labour warranty.

 

[DeltaT]

With access to manifolds for possible repair & changing the radiator valves, there's no reason for replacement of whole of your existing system. 10mm is easy to clean & great for most smallish systems.

says it all

Sorry kevplumb, I have to agree with DeltaT here. 10mm is fine for your average 3 bed semi.

Why isn't it?

Ignore him Andy, he's my official stalker!!

 

[bluey831]

As for not heating 11 rads, what is the total kW of your rads? If it is more than the boiler output, the rads will feel cooler as each rad will give out slightly less - the rad's can't give out more than the boiler produces.

Well I think I have worked out the Kw of our 11 rads which comes to 15.3kw. Now if our existing boiler has a range of 14.65 to 17.59 then depending on what the installer set it at, it should be able to heat all rads eventually. Have I got this right? On the other hand, the new V831 will get 15.3kw to the rads super fast in comparison?

 

[D_Hailsham]

As for not heating 11 rads, what is the total kW of your rads? If it is more than the boiler output, the rads will feel cooler as each rad will give out slightly less - the rad's can't give out more than the boiler produces.

Well I think I have worked out the Kw of our 11 rads which comes to 15.3kw. Now if our existing boiler has a range of 14.65 to 17.59 then depending on what the installer set it at, it should be able to heat all rads eventually. Have I got this right?

Yes.

On the other hand, the new V831 will get 15.3kw to the rads super fast in comparison?

There shouldn't be any noticeable difference in the heat up time. The V831 may produce more than the total rad output, but all this means is that the water will initially reach temperature very quickly, so the boiler will modulate down to provide the 15.3kW. This may be fractionally quicker than the existing boiler takes to reach working temperature.

 

[bluey831]

As for not heating 11 rads, what is the total kW of your rads? If it is more than the boiler output, the rads will feel cooler as each rad will give out slightly less - the rad's can't give out more than the boiler produces.

Well I think I have worked out the Kw of our 11 rads which comes to 15.3kw. Now if our existing boiler has a range of 14.65 to 17.59 then depending on what the installer set it at, it should be able to heat all rads eventually. Have I got this right?

Yes.

On the other hand, the new V831 will get 15.3kw to the rads super fast in comparison?

There shouldn't be any noticeable difference in the heat up time. The V831 may produce more than the total rad output, but all this means is that the water will initially reach temperature very quickly, so the boiler will modulate down to provide the 15.3kW. This may be fractionally quicker than the existing boiler takes to reach working temperature.

Jeeze I'm confused again.... what a shocker.

When we talk about modulating down - do we mean it will modulate down to what the KW of the rads are which in this case is 15.3kw OR do we mean it will modulate down to what the heat loss is in the house which is 13.5kw.

Put another way what is the difference between the heat loss of the house (13.5kw) and the heat that the rads give out (15.3kw).

It might be totally obvious but I'm missing it totally!!

Bit miffed about the fact that its gonna take nearly as long to heat the rads with my brand spanking shining new boiler as it does on the existing old goose .... oh well.

 

[bluey831]

Any offers on the question in my last post i.e. the difference between rads giving out 15.3kw and house heat loss being 13.5kw.

For instance am I right in thinking the rads will only ever need to give what the house heat loss is - or are these two things totally separate??

 

[fluff29]

hi bluey

Just reading your posts for a change.

Look at my thread. DH explains modulating to me a number of times. Still not sure if I get it.

Now I know where the guys are when they aren't answering my posts, they are over here helping you out - they do not have their troubles to seek.

 

[D_Hailsham]

When we talk about modulating down - do we mean it will modulate down to what the KW of the rads are which in this case is 15.3kw OR do we mean it will modulate down to what the heat loss is in the house which is 13.5kw.

Neither.

Modulation adjusts the boiler output so the water temperature leaving the boiler stays at the temperature set. Think of it as being like the accelerator on your car. If you want to maintain 70mph you are continually making small adjustments to allow for the terrain. Older boilers could not do this they were either full on or off. Turn the boiler on and it would run at full output until the water reached the set temperature and the boiler thermostat opened; it then turned off and stayed off until the water temperature had dropped (about 10-20degC) and the boiler thermostat closed. Whereupon the boiler lit and gave out max power again. And so the cycle continued.

On a modulating boiler it will start off at max output while the water heats up from cold. The boiler will monitor the rate at which the water temperature rises and as the temperature approaches the set temperature, e.g at 70C if the temperature has been set to 75C, will start reducing the boiler output so the water temperature does not exceed the set temperature. In the same way that, if you want to increase speed from 50mph to 70mph, you push the accelerator right down and then bring it up as you near 70, so you don't exceed the speed limit. (I am being theoretical. )

Put another way what is the difference between the heat loss of the house (13.5kw) and the heat that the rads give out (15.3kw).

Heat loss of the house is calculated as the amount of heat needed to raise the temperature from -1C to 20C and maintain 20C. If the outside temperature is lower, more heat is required; if higher then less. The heat required is approximately proportional to the difference between outside and room temperatures.

So if the heat loss is 13.5kW, that means that your radiators need to produce 13.5kW to heat the house from -1C to 20C and keep it at 20C. If the rads give out 15.3kW it will do two things: heat the house from a lower temperature (approx -4C) to 20C, or heat the house up faster if the outside temperature is higher.