New boiler and control system questions... Vitodens 200W

Bit of an in depth one here, so bear with me please...

I'm doing what amounts to a whole house refurb at the moment, which includes fitting wet underfloor heating to the whole of the downstairs, in 3 rooms. I also have a new water connection in 25mm MDPE coming into the house which will connect to 22mm copper via the stopcock. I still have radiators upstairs and in the attic, and am looking at getting a new boiler. Current boiler is a Wickes unit, about 10 years old i guess, and has a scorch mark on the front of the casing and is pretty noisy, although otherwise has proven reliable!

So, to the meat... In terms of control i was envisaging having 2 zone valves and pumps from the boiler CH supply, one to the rads and 1 to a blending valve and circulating pump at the ufh manifold. The rads would be controlled by a programmable thermostat upstairs, and the ufh by setback stats in each room controlling zone valves, and a timeclock. The property is 3 beds, 1 bathroom, and there are 2 of us living here, although really it should be sized for 4 adults i think. Using online calculators, the heat requirement of the house seems to be 6-8kW. Although the washing machine is cold fill, the dishwasher is not, and the bathroom has a bath and a shower. Given the on-demand nature of a combi, i guess the question is really can one fill a bath/have a shower whilst the dishwasher is filling and someone is runing the hot tap in the kitchen?

When looking into boilers i started to read about weather compensators, and to cut a long story short, i have nigh on settled on a Veissmann 200W combi with the H01A WC controller and the submounted extra VT pump. Main VT to supply the rads, and sub VT to supply ufh.

I have 3 main questions really (though others may arise as the discussion continues!).

• 1) Will say a 26kW combi be sufficient?
  2) Can i do away with seperate valves and pumps for my 2 heating circuits and have the main VT and sub VT handle cirulation for rads and ufh system?
  3) How does the WC affect my proposed control system?

I've seen people say that having a WC does away with the need for other controls, but i fail to understand how the boiler can work out when the house is at the correct temperature. I envisaged keeping the timeclocks and stats that i already planned for and using the WC to adjust the flow temp according to outside temperature, and hence maximise the condensing period for the boiler. (I have assumed that the 2 separate VT circuits can have independent temps/slopes - is this correct?)

Thanks for reading

Thats the problem with Viess controls as they dont monitor inside temps so I would suggest you keep a room stat. The controller includes timing options.

The boiler does not support two different zones and has only one setting of slopes etc. I am not sure why you want anything different.

UFH needs lower flow temps but has a 1-2 hour time constant so WC is not usually used with WC except with an intelligent controller.

I dont recommend DIY heating system design!

Tony

dodgem101 said:

1) Will say a 26kW combi be sufficient?

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No

dodgem101 said:

2) Can i do away with seperate valves and pumps for my 2 heating circuits and have the main VT and sub VT handle cirulation for rads and ufh system?

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No

dodgem101 said:

3) How does the WC affect my proposed control system?[/list]

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Not

dodgem101 said:

Using online calculators, the heat requirement of the house seems to be 6-8kW.

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Sounds about right, but it's probably better to up it a bit to allow for the colder winters we seem to be having. 10kW would be OK down to -6.5C.

Most installers think that the heating requirement is not relevant when you have a combi as the boiler size is determined by the cold water flow rate and hot water demand, so it is always much bigger than the heating demand. However the output of a boiler is not constant, it varies with the weather. You may need 6-8kW when it is cold outside, but 90-95% of the time you will need much less than this. If the boiler can, it will modulate down to provide the correct amount of heat. But when it reaches the lower limit of modulation, all it can do is turn on and off, which is much less efficient.

We now come to your proposed Viessmann 200W combi boiler. The heating output is 6.5 to 26kW, so it's already working at the bottom end, even in the cold weather. So most of the time it will be running in on-off mode. This does not give weather compensation much to work with.

Viessmann realize this can be a problem and say:

To reduce burner cycling in low energy houses with a correspondingly low heat demand, we recommend the use of a low loss header or a Vitodens 300-W with 3.8 to 13 kW.

However, the 300W does not appear to be available in the UK. So why give the above advice in their UK Technical Guide?

One solution would be the Vitodens 200-W 19kW system boiler and an unvented HW cylinder. The 19kW boiler can modulate down to 4.8kW.

Although the washing machine is cold fill, the dishwasher is not, and the bathroom has a bath and a shower. Given the on-demand nature of a combi, i guess the question is really can one fill a bath/have a shower whilst the dishwasher is filling and someone is runing the hot tap in the kitchen?

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A 26kW combi will be able to supply about 10-12 litre/minute. You really need about 20lmp if you want to run all those at the same time.

Don't forget that, if you have a combi, all water, hot and cold is supplied from the incoming mains. So, flushing the toilet, washing the car and watering the garden can all effect someone in the bath/shower or ho quickly the washing machine/dishwasher fills.

I've seen people say that having a WC does away with the need for other controls, but i fail to understand how the boiler can work out when the house is at the correct temperature.

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It's done by trial and error!

The installer will set the WC "curve" to an average position and see what happens. If the house gets too cold/hot he will adjust the curve. This may take some time, depending on the weather. The system has to be adjusted so the coldest room in the house gets correctly heated; all other rooms are then controlled by TRVs.

D_Hailsham said:

dodgem101 said:

Using online calculators, the heat requirement of the house seems to be 6-8kW.

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Sounds about right, but it's probably better to up it a bit to allow for the colder winters we seem to be having. 10kW would be OK down to -6.5C.

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Another totally irrelevant answer from an amateur.

The heatloss, real or imagined, is of no importance whatsoever here.

Agile said:

Thats the problem with Viess controls as they dont monitor inside temps so I would suggest you keep a room stat. The controller includes timing options.

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Never heard of the Vitotrol range?

The boiler does not support two different zones

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The Vitotronic 200, type HO1A can control of one heating circuit without mixer and two heating circuits with mixer.

The boiler ... has only one setting of slopes etc.

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Not sure what you mean by that. Are you thinking of the 100W, which has built in simplified WC?

I don't recommend DIY heating system design!

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From what I read on this and other forums, I have my doubts if the average "professional" does it correctly.

D_Hailsham said:

I don't recommend DIY heating system design!

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From what I read on this and other forums, I have my doubts if the average "professional" does it correctly.

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One thing is for sure: a google here who does not understand that heatloss is irrelevant in 99% of combi choices is certain to do it wrong.

I can understand that the heatloss is not an important consideration when choosing a combi boiler... The boiler needs to be sized according to hot water usage, which will be massively higher than the CH requirements. This would appear to mean that the boiler will have to cycle when doing CH duties since it won't be able to modulate that low. The WC would still be of use though would it not? It will fire the boiler intermittently to maintain a flow temperature of say 60C. Whilst this isn't as efficient as maintaining a low flame to keep the flow temp at 60C it is still allowing the boiler to operate in condensing mode. Is this correct or have i got it all wrong?

However, I am able to go the route of a system boiler with DHWS which may suit me better. Need to have a think on that one...

Agile said:

The boiler does not support two different zones and has only one setting of slopes etc. I am not sure why you want anything different.

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As D_Hailsham pointed out, the H01A controller does support these functions. And i would want them because the rads and the ufh need to operate at different temps, and the construction of the building will affect the required WC slope...

Agile said:

UFH needs lower flow temps but has a 1-2 hour time constant so WC is not usually used with WC except with an intelligent controller.

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The room temperature in the ufh zones is controlled by room stats and actuators on the manifold. The temperature of water that the system has to work with is controlled by the WC, and a lower temp will result in long valve open times rather than more rapid on-off cycles. An intelligent ufh controller is designed to take into account the 1-2 hours required to bring the room from, eg. 16C to 21C. It learns the time taken with respect to outside temp, and adjusts switch on time accordingly. If the WC changes the water temp according to outside temps, then the ufh controller will simply see that there is less variance in the heatup time than if the supply temp was constant. Taking this line of thought to its limit, i would say that if the slope on the ufh WC is spot on, the room heatup time from 16C to 21C would be constant whatever the outside temp, and an intelligent ufh controller is redundant (so long as you remember to set the ufh to come on 1 hour before the actual time that you want it to be ready!)

Please don't take that as teaching my gran to suck eggs... It was more of a working through the logic for my own understanding!!

Thanks for the replies so far, some more detailed than others!

I've found a technical guide for the Veissmann boilers so i'm going to have a good read of that now, and phone them to see whether the H01A can set different slopes for each circuit, or if you can only choose one slope which each circuit uses...

I appreciate that heating system design is not necessarily for the DIYer, but not everybody is the same are they? I am confident that with a little help i can design something that i am happy with and is safe.

Well, Veissmann technical were very helpful....

The H01A controller is capable of completely separate WC slopes and positions for each heating circuit - fantastic.

The internal pump is capable of circulating to the rads and to the hot water cylinder if i go that route. I would need to use a circulating pump on the ufh manifold which i had suspected would be the case.

The radiator zone can be controlled using an extra temp sensor and work in setback mode (in fact better than standard stat setback of 4C or 5C, i can choose 2 arbitrary temps for on and setback!)

Not read the technical manual yet, but about to do so and i will come back with my proposal and hopefully get some good advice on what needs changing...

Cheers!

bengasman said:

The heatloss, real or imagined, is of no importance whatsoever here.

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So you would happily install a Vaillant 837 or 937 (12-28kW CH output) in a house with only only 6kW heatloss, if that size boiler was required for the hot water demand?

Can you explain then why, in every 3 or 4 bed new build house I have visited recently, they have, without exception, installed a small system boiler (12-15kW) and a large unvented cylinder, which is probably a more expensive solution.

If the heat loss is not important, why do Viessmann recommend a low loss header or the 300-W for houses with a low heat demand? It's because the efficiency of a boiler running in on/off mode can reduce to 50%.

When you take into account the fact that a boiler will only be running at the calculated output for something like 5% of the time and could be running in on/off mode for over 50% of the time, the reduction in efficiency from 90% to 50% is significant, resulting in increased gas consumption and, consequently, higher bills.

If you do put in a 200W, and why not, it is a heavyweight product - I suspect you will be the only one to understand the controls. Hope you are not married.

simond said:

If you do put in a 200W, and why not, it is a heavyweight product - I suspect you will be the only one to understand the controls. Hope you are not married.

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Haha That made my girlfriend laugh!

I guess it does sound kind of full on, but i'm hoping that once it's set, we won't have to touch the controls year round.

mysteryman wrote

In this case the best way is to use the 'sub mounting kit with mixer' which does what it says

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How much does this kit cost and what all does it include?

Anyone know the cost of this item?

Would have thought it was your field mysteryman?
Have you ever fitted one?