BG or local rgi for install?

[D_Hailsham]

The only concern was the modulation frequency which is minimum at 12kw. Presumably the 937 minimum is higher still.

I think you mean the lower limit of boiler output is 12kW. It happens to be the same for the 937. However, this could easily be more than your heating requirement. You need to consider this as well as your hot water requirement - 37kW will be ample for this.

To find your heating requirement use the Boiler Size Calculator. The final result includes 2kW for a hot water cylinder, so you need to deduct 2kW to find your central heating requirement. The minimum output of your boiler should be as far below this as possible.

 

[DeltaT]

Well I got me some quotes for an install - no surprise BG is out there in front.

Upshot is BG is about £700 to £800 more expensive that WB or Vaillant Approved installers for the same job. Install + powerflush + Magna filter + programmable room thermostat. The quotes are as like for like as possible. Going from conventional system to Combi - either WB37CDI or Val837.

However, one thought I am having is that the current rads and pipework is significantly old - we think about 30 years plus. Pipework is 10mm.

There is a reasonable chance therefore that the powerflush will highlight some weaknesses or other problems etc. With BG it will be a case of them sorting it till I have my system up and running within the stated price (Am also an existing BG homecare customer). With the RGI I could be in for additional expenses - either extra rads or possible repiping.

What say you all on this one?

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.

'Upshot is BG is about £700 to £800 more expensive that WB or Vaillant Approved installers for the same job'

But a so called 'Approved' installer is going to charge you a premium rate for a job like this anyway and BG is not the best 'yardstick' to judge any quote on are they??!!

You now know what boiler you want & what you want done with you system, just get quotes from indie RGIs that have been recommended to you and hand them the specification, it's a no brainer!!

 

[kevplumb]

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

 

[Agile]

Your measurement of 15 li/min is presumably the open pipe flow rate.

That will be reduced a little to bath taps but reduced a lot at a shower as they have considerable resistance to create a powerful jet and you might only get 10 li/min from a shower.

What is really needed is the dynamic flow rate. The flow rate which will leave a residual prressure in the pipework of say 1 Bar.

In many ways a smaller power boiler may be all that is justified by your flow rate. It all depends on your choice of baths or showers.

Tony

 

[bluey831]

agile - the 15l per minute is the flow out of the bath taps. I have had four around and they all gave slightly different readings i.e. 15, 16, 18, 19 l per minute. I am taking the lowest figure. We live on a road in semi rural so not on an estate or other situation where the flow rate would alternate as such.

We have an outside tap but it is way down at the bottom of the garden so they all say it would not give an accurate reading.

I have also done a room by room heat loss calculation and have come up with a 13.5kw for the whole house.

kevplump - not sure what you mean by your comment 'says it all' - are you having a dig at the poster of the comment or are you in agreement with him??

 

[D_Hailsham]

agile - the 15l per minute is the flow out of the bath taps.

The correct place to check is the cold water tap at the sink in the kitchen. All hot water taps will be gravity fed from the cold water tank in the loft, via the HW cylinder. The other cold water taps could also be gravity fed from the cold water tank. You can easily check by running the kitchen tap full on and then opening another cold tap. If the kitchen flow reduces, the other cold tap is off the mains.

When you have a combi, all taps are fed directly from the mains - there are no tanks or cylinders. So opening two at a time could mean reduced flow to both.

Check the flow at the kitchen cold water tap using a bucket with litre marks and a watch. If other cold taps come off the mains, also do the test with one of these (bath is best) open fully. The smaller the difference between the flow rates at the kitchen tap the better.

If your other cold taps come of the CW tank, do the tests with the garden tap closed and then with it fully open. Again, the smaller the flow difference the better. You need a minimum flow of at least 10 litre/min for a combi boiler.

You must do a heat loss calculation for the house, using the link I gave earlier, to make sure the CH output of the boiler is correct.

 

[bluey831]

D_Halis - ah - had me worried for a second there. Nobody had mentioned about the cold water being gravity fed and most of the flow rate checks were done in bathroom (downstairs bathroom - to be fair though they all may have checked without mentioning it). Done as you suggested and sure enough the cold taps are all mains fed. When I put kitchen tap on full and then turn on bath cold tap full there is a noticable decrease in both taps. Turning one off brings the other back to normal pressure. That test does also demonstrate the shower problem if someone else is turning on taps.

I did the heat loss from the link you suggested and done it from another calculator that allowed me to do it room by room. That is probably more accurate - especially in our case as we have an odd shaped house. Double fronted - singled backed - semi detached but part single storey extension - old house such that this and next door was once just one house a bungalow - then a two storey - then split into two houses in 1922.

 

[Agile]

As all cold are off the mains then run bath and kitchen together and add the two flow rates together.

Open pipe flow rate is about twice what the dynamic flow rate is but that depends on the static supply pressure.

You should also measure the hot flow rate from the bath tap. That will indicate how a combi output would compare with your existing.

I always go through all these things with customers before I can recommend what power boiler should be provided.

Tony

 

[bluey831]

agile - have done the tests. I get 12l min out of kitchen tap on its own. The kitchen tap is a mixer and an old one so I'm not sure if is opening fully. Bath tap is giving me 20l min on its own i.e. 30 seconds to fill 10l bucket. When both on together the kitchen tap goes down to 7l min and the bath tap to 15l min. The two together gives 22l min.

The bath hot tap is a real power thing at 30l min. It is gravity fed and downstairs bath so it is whooshing it out. I know I aint gonna get anything near that on a combi but we dont use the bath that much and the shower is the big thing.

Done each test a few times just to make sure. Pretty much same each time.

Not entirely sure what you mean by Open pipe flow rate versus dynamic flow rate. My understanding is open pipe flow rate is the rate it is coming into the house while dynamic flow rate is the rate it is coming out of the taps. Would this be right???

 

[Agile]

Thats pretty good!

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

Thats with properly sized pipework of course.

Tony

 

[bluey831]

Yes - the 937 would indeed do that. I dont know how much flow rate you can get through a shower head and also the rate of it discharge down the waste pipe. We sometimes get a slow discharge on our shower waste - presumably through temporary blockage. Incidentally, what is the best way of keeping the waste pipe clean?

We have a two bar pump to power the shower currently and if we can keep that equivalent pressure then we would be happy. If we can make it slightly better then great. For this reason, from what I can gather from my readings etc the 837 or WB37 will give us that.

 

[D_Hailsham]

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

According to the first post of the OP's other topic Oversized Boilers, the heat loss of the house is about 13kW. I did suggest doing a "whole house" calculation but that seems to have been ignored; so we have to rely on the 13kW estimate.

The 837 and 937 both have heating outputs of 12kw to 28KW which, I think you will agree, means they would be oversized for heating.

 

[bluey831]

D_Hali - I did do as you suggested and used the link. However, I mentioned in another thread that the house is unusual in that it is double fronted, single backed, was part of one house now split in two etc. There is therefore an unusal layout etc. For that reason I done a room by room Heatloss calculation on the basis that it would be more accurate.

The BG guy came around and calculated the 13kw. I since done my own room by room calculation and got 13.5Kw. I am presuming a room by room calculation would be more accurate than the whole house calculation? Or is there something I am missing here?

Apologies if I gave the impression your suggestion was ignored - not the case.

I do understand about the range for heating being between 12kw and 28kw and assuming there will be times when our heating loss will be below 12kw. This then brings in the boiler cycling - turning off and on. What I have not been able to clearly decipher is the length of time this might take place. My understanding is that when it goes below 12kw it will be getting close to the point where the room heat will be reaching the level set by the thermostat and boiler will then be knocked off in any case. In addition, I'm told the Vaillant will knock off for 5 minutes rather than continue to recycle. The Vaillant 831 has a lower setting of 7.7 if memory serves me correctly and the WB31 and 37 are in that range also.

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.

 

[D_Hailsham]

D_Hali - I did do as you suggested and used the link. However, I mentioned in another thread that the house is unusual in that it is double fronted, single backed, was part of one house now split in two etc. There is therefore an unusal layout etc. For that reason I done a room by room Heatloss calculation on the basis that it would be more accurate.

Sorry, I must have overlooked that post.

The BG guy came around and calculated the 13kw. I since done my own room by room calculation and got 13.5Kw. I am presuming a room by room calculation would be more accurate than the whole house calculation? Or is there something I am missing here?

A room by room calculation is obviously much better - provided your data is correct. I think 13-14kW is as good as you will get.

What I have not been able to clearly decipher is the length of time this might take place. My understanding is that when it goes below 12kw it will be getting close to the point where the room heat will be reaching the level set by the thermostat and boiler will then be knocked off in any case.

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]

There is data available which showing how the outside temperature varies throughout the year. But, speaking generally, the lower the temperature, the fewer the days. This means that, most of the year the boiler will be producing less than the design output - 14kW in the example.

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.

The Vaillant 831 has a lower setting of 7.7 if memory serves me correctly and the WB31 and 37 are in that range also.

The 831 is 8.7kW, WB37CDi is 9.4kW and the WB30CDi is 7.7kW However the hot water flow rate would not be adequate if the WB30CDi is used.

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.

 

[ChrisR]

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

Combis quote the flow typically for a 35 degree rise. In the winter it comes in the house at about 5, which gives you 40, which is OK for a shower but not a bath (45), so you'd have to slow it down somewhat.
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.

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.