Weather Compensation

[RandomTask23]

Hi,
I've recently moved house into an extended 4 bed semi built in the 1950s. The house currently has an ancient Ideal Mexico boiler with 11 old "radiating" radiators (rather than newer "convecting" radiators). I have been looking into changing the boiler and have obtained a few quotes to do so (currently I'm not planning to change the rads).

I have narrowed this down to a couple of options and both have recommended a Viessmann 100W boiler. However, they have offered differing advice regarding weather compensation and I am a little unsure about who to believe!

One has recommended installing weather compensation quoting the expected savings in line with the Viessmann literature, whereas the other has suggested that, whilst the boiler is compatible with the system, my radiators are not as they need to be sized correctly for the system to work properly, meaning it would be a waste of money.

My concern over the latter statement is that he didn't measure any of my radiators and he only visually looked at 3 (hall, lounge & kitchen) therefore I'm not certain how accurate his claims are.

Does anyone have experience/advice about using this system with older rads? Or is there any calculations I can do to work out if I have the right size rads?

Thanks in advance.

 

[ajg68]

Im not sure why your radiator size would affect the functioning of weather compensation. I had a Viessmann 200-w fitted a couple of weeks ago & the weather comp tries to keep the radiators at a pre-determined temperature (according to the set-point, external temperature, heat loss graph etc). This means that the rads normally run slightly warm rather than hot.

What he might have meant is that your radiators are too small to encourage condensing.

Im sure that someone who knows what they're talking about will be along shortly.....

 

[Agile]

You can compare your rad sizes with similar current ones to see what the heat output is.

Then use the whole house method to calculate the total heat loss.

Weather comp will work fine with rads even if they are a lit undersized.

If the rads are undersized then the only problem will be an inability to reach the set room temperature when its very cold outside.

Your old boiler was designed for a flow temp of 80° whereas condensing boilers are designed for 70­° ( although most will give up to 75° ).

So the rad output will in theory be rather low.

In practice rads used to be significantly oversized and were calculated on the basis of single glazing and little loft insulation. So with double glazing and good loft insulation most old rads are adequately sized for condensing boilers.

Tony

 

[mickyg]

you can have WC, but the more undersized the rads are the lower the savings will be. Bigger the rads the better. It will still work though regardless.
We can't however tell you if your rads are undersized or not.

 

[D_Hailsham]

You can compare your rad sizes with similar current ones to see what the heat output is.

Assuming the current rads are pressed steel, but without the fins, then you can use Stelrad Elite Catalogue to estimate the output of your rads.

Guide to Radiator outputs is useful if you have the old cast iron rads.

Then use the whole house method to calculate the total heat loss.

Use the Boiler Size Calculator

It does not matter if the rad output is greater than the required boiler size. In fact, it is better for the rads to be larger, as it then means the boiler can run at a lower temperature, which saves fuel.

 

[Gasman77]

It will still work, the outdoor sensor is pretty cheap so its worth putting in at this stage. Oversized rads would be benificial , to take advantage of the lower flow temp the the weather comp gives, but , it will still work fine until you wish to replace your rads - then they can be slightly oversized.

You can simply unplug the weather comp on a Viessmann 100 range, meaning the system can be installed , and if they don't get hot enough, unplug it at the boiler until you can afford/want to upgrade the rads! With it unplugged it will work like any other condensing boiler.

 

[mehran]

put it this way... when its warm out you dont have your heating on...and when it is bloody freezing outside you have your heating full whack.


what Weather Compensation does is fill the gap in between when it is only a little bit cold. it was designed for people that don't have over-sized rads to gain more efficiency. so I would go for wc.

it should save at least the extra installation cost. every thing past that is a bonus

 

[electronicsuk]

Worth noting that if you have a HW cylinder heated by the boiler on a Y or S plan system, the pipework will have to be altered to work with a system boiler with weather compensation controls. Otherwise, on warm days, the flow temp is unlikely to be high enough to sufficiently heat the water in the tank.

 

[Gasman77]

Do you have gravity hw op? You can always go for Viessmann 4 pipe system if the boiler is going back roughly the same place.

 

[RandomTask23]

Thanks for the great replies. It's good to know that I was thinking along the right lines. Although the current rads are old they do kick out plenty of heat (and that's with them half sludged up!) so I'd guess that they might be oversized if anything...I'll use the links to the calculations to measure up properly later this evening. I do like the idea of the extra control that the weather compensation offers and it does sound like there is potential to at least recover the cost of installation and possibly a bit more.

What about TRVs? Some of the rads have them and others don't...would I need to have them all the same to get the weather comp working properly?

Currently I do have a gravity hw system. I had been leaning towards one quote that suggested changing to a system boiler with hot water priority (but maintaining low pressure water to work with my power shower) and weather comp. However, that last quote which has suggested keeping a more traditional layout (but without weather comp) has come in substantially cheaper so that has thrown a bit of a spanner in the works!

I've done some internet searches regarding the differences between the two types and it seems to be that in general a system boiler just contains some more components within the boiler itself. But I think I need to do a bit more research into the hot water priority system to check it's right for me.

 

[D_Hailsham]

it does sound like there is potential to at least recover the cost of installation and possibly a bit more.

The days of boilers which would last 25 years or more are long gone, so the saving in fuels costs is unlikely to be enough for you to recover the installation costs of any boiler over its life.

What about TRVs? Some of the rads have them and others don't...would I need to have them all the same to get the weather comp working properly?

Best to have TRVs on all rads, except the area where the wall thermostat is fitted.

Currently I do have a gravity hw system. I had been leaning towards one quote that suggested changing to a system boiler with hot water priority (but maintaining low pressure water to work with my power shower) and weather comp. However, that last quote which has suggested keeping a more traditional layout (but without weather comp) has come in substantially cheaper so that has thrown a bit of a spanner in the works!

If you are installing a new boiler, you must convert the system to fully pumped - Building Regulations.

I've done some internet searches regarding the differences between the two types and it seems to be that in general a system boiler just contains some more components within the boiler itself. But I think I need to do a bit more research into the hot water priority system to check it's right for me.

The system boiler is used with a sealed system - no feed and expansion tank in the loft. It also has the pump built in.

As for hot water priority, the traditional system, with one or two motorized valves to control heating and hot water, needs the boiler temperature to always be well above 60C, so the water in the cylinder can be heated up to 60C. If you install weather compensation, the water temperature in the boiler can frequently be below 60C, which is fine for the central heating but no use for the hot water.

The way to get round this is hot water priority. When the cylinder calls for heat a motorized valve will switch the water to circulate through the cylinder only and at the same time the boiler output rises to max so the water temperature is above 60C. For a short time the heating is off while the cylinder is topped up. It's very similar to the way a combi boiler works.

Which boilers have been recommended by your different installers?

 

[RandomTask23]

Sorry for not being clear. I actually meant that it sounds like I might be able to recover the cost of installing the weather comp not the whole boiler system.

Interesting comment about not having a TRV on the rad by the room stat. Currently the rad in the hall does have a TRV but it doesn't work and it's wide open all the time. I was actually going to replace it with another one but I might just get a standard connection instead. (these things are all new to me as this is only my second house and the first the central heating as I had Economy 7 before).

The proposed system boiler is a Viessmann Vitodens 100 26KW Condensing System Boiler installed as a hot water priority 4 pipe system & weather comp.

The other proposed boiler is a Viessmann Vitodens Compact 19KW Gas fired condensing boiler (WB1B) with a new three port valve cylinder stat and pump.

Both are approved Viessmann installers so they would come with the 5 year warranty.

 

[D_Hailsham]

Sorry for not being clear. I actually meant that it sounds like I might be able to recover the cost of installing the weather comp not the whole boiler system.

That should be possible.

Interesting comment about not having a TRV on the rad by the room stat. Currently the rad in the hall does have a TRV but it doesn't work and it's wide open all the time. I was actually going to replace it with another one but I might just get a standard connection instead.

That would be sensible.

The problem with having a TRV in the same area as a wall stat is that one will negate the other. If the wall stat is set to 20C and the TRV to 22C, the TRV will never operate; and, if the TRV is set to 20C and the wall sat to 22C, the wall sat will never turn the boiler off.

The proposed system boiler is a Viessmann Vitodens 100 26KW ... The other proposed boiler is a Viessmann Vitodens Compact 19KW

The size difference is a worry. How was the boiler size determined? If the installer just plucked a number out of the air, you should use the Boiler Size Calculator to find out what size boiler you really need.

What insulation is there in the house? If not much, you should get this done first as it will mean that you can install smaller (cheaper) boiler and save money on fuel. There are many schemes for free/low cost insulation and you can find most of these at Energy Savings Trust Grant Search

 

[ajg68]

Interesting comment about not having a TRV on the rad by the room stat.

Im not sure that you'll have a room stat, my 200 doesnt. You set a 'set-point' on the boiler control & it tries to match this by changing the flow temperature, measuring the outside temp & looking at the heatloss curve.

Ive found that the system works best (for my situation) by leaving all the TRVs at max. Weather comp is more about long periods of low flow temperature not short bursts of high temperatures, clicking room thermostats & opening & closing TRVs.

 

[ianniann]

Im not sure that you'll have a room stat, my 200 doesnt. You set a 'set-point' on the boiler control & it tries to match this by changing the flow temperature, measuring the outside temp & looking at the heatloss curve.

Ive found that the system works best (for my situation) by leaving all the TRVs at max. Weather comp is more about long periods of low flow temperature not short bursts of high temperatures, clicking room thermostats & opening & closing TRVs.

I'm struggling to find any part of this that isn't horribly and completely wrong.

Just because there isn't a dial on the wall that clicks when you turn it up and down doesn't mean you have no thermostat. Your weather compensation controller includes a thermostat or it couldn't operate. Weather compensation is a refinement of flow temperature modulation that allows a more or less aggressive heating curve based on the outside temperature, and hence upon an estimation of heat loss at the current time. The flow temperature is adjusted based on many factors, but basically on how far the current room temperature is below the desired temperature (or at least the temperature being recorded at the location of the controller).

TRVs are used to set and maintain different temperatures in different rooms. This cannot be achieved by a single central controller no matter how intelligent it is. With the TRVs fully open, the temperature of each room will be controlled by the size of the radiator, any restriction from the lockshield valve, and the heat loss (or gain from warmer rooms) from the room. This may or may not be an appropriate temperature, hence the mandatory inclusion of TRVs on all new installations.

You still shouldn't have a TRV in the room where the weather compensation controller is housed for the same reasons that a simpler thermostat should be in a room with a TRV-less radiator. If the TRV were to turn off before the thermostat setpoint then the heating would stay on indefinitely as it tries in vain to warm up that room. If the TRV is set to be open at the setpoint temperature, this is not so harmful but of course the TRV is completely redundant.