Oversized boiler and downrating it

[Alex100]

I previously posted a thread "New CH system underpowered/ wrong layout?" - and many thanks to everyone who contributed. Issue is now about boiler size.

The installer hasn't once been back to inspect the system since I complained it doesn't heat the house enough, but now he says he just wants to sort this out and put in bigger radiators like I (not he) suggested. He asked me what size ones I want? I suggested he comes and looks in case something else is wrong, especially if it is an oversized boiler, but he just wants to be told what size radiators I want instead (I'm not prepared to take that responsibility).

This installer put a WB Greenstar 38CDi Classic [FONT=arial, sans-serif]combi in two months ago and a whole new system. He put in 9 radiators on two floors with total BTU of about 35,000. However, the boiler does about 38kW or 102,000 BTU. He says he chose the big boiler because I wanted good hot water flow (suggesting it's my fault the boiler that he chose is too big). A heating engineer I met, plus several contributors on this forum, told me that it is essential to have the boiler downrated by the installer to something more appropriate on the heating side (around 10-12kW) - and that it does have the facility to do this without downrating the water heating side.
[/FONT]
We told the installer the boiler keeps cutting out and he said we are getting confused listening to people on plumbing forums, and that the boiler cuts out because the water in the system has reached temperature and everything is okay.[FONT=arial, sans-serif]

Questions:
[/FONT]1. I spoke to Worcester-Bosch today and they said there is no need to downrate the boiler and that it will automatically self modulate down to the 35,000 BTU which is being drawn. Does this sound right to any WB engineers out there?

[FONT=arial, sans-serif]2. [/FONT]Also, downstairs rads "rush" loudly, which makes me think either the pump on default setting 3 is too high (but installer says that's normal combi setting) or else could it be an improperly balanced system causing rushing far from the boiler (which is upstairs and other side of building). Many rads are long long branches from flow & return (up to 8 metres in one case) in 22mm reducing to 15mm - could that cause rushing in rad?

 

[dancinplumba]

1, this is true.

2, sounds like air in rads/system

the installer needs to calculate size of rads.

get a better installer in 🙂

 

[Heating-eng]

The boiler is a great choice, your installer has chosen you a good boiler.
the boiler is not oversized as it will adjust to the load required.
your rooms needs to be sized correctly and then the right size rads can be fitted.

 

[Heating-eng]

Your pump should be inside the boiler on a combi, I do not think you can adjust this on the combi.

 

[Whyme]

Dont the pumps on those modulate ? (good boiler like heating-eng said)

 

[Heating-eng]

I fitted a 38 CDI Combi this week, one of the best boilers on the market.

 

[Heating-eng]

Dont the pumps on those modulate ? (good boiler like heating-eng said)

Yes I think the pumps do.

 

[AWheating]

the pump on this boiler modulates, it can be set up to run at different speeds/parameters but its not normally required tbh.

the noise at the rads may indicate that lockshield valve is closed down, might not hurt to go around your rads and open up the lockshield fully and see how this effects the heating.

setting/range rating the boiler on the heating side can be an advantage but the boiler will modulate on its own to suit also.

 

[Silent Dancer]

The CH of the boiler is from 9.4 to 30kw. Your installer has said you need 10.2kw presumably at -3℃.

When the boiler gets a heating demand it will supply 30kw at first and it will never get the chance to
modulate down to the minimum. Matters may be made worse by software on the boiler that prevents short cycling, the boiler firing to soon.

Setting the CH max to a more sensible level of 15kw should help the problems you describe and ensure the boiler runs at it's most efficient. If you're installer won't help then contact Worcester technical department for assistance.

Regarding the pump the boiler has a modulating pump so there no need to change the speed. The noise is either plumbing related or part of the above story that the pump is working flat out to start off with and never gets the chance to modulate down.

 

[doitmyself]

I previously posted a thread "New CH system underpowered/ wrong layout?"

In that topic I suggested you use the Whole House Boiler Size Calculator to estimate the heating requirement of your house. You would then be able to compare this with the 11kW (37,600BTU) of your radiators to see if they are giving off enough heat. (It won't tell you if each rad is correct; only the total.)

If you want to do a room by room calculation, use this Radiator Calculator. It's more reliable than any other I have found - apart from Stelrad Stars, which is too technical for most people.

 

[Heating-eng]

Just guess them, that's what most people do, I use an app on my ipad ,works really well

 

[snoopy8888]

In that topic I suggested you use the Whole House Boiler Size Calculator to estimate the heating requirement of your house. You would then be able to compare this with the 11kW (37,600BTU) of your radiators to see if they are giving off enough heat. (It won't tell you if each rad is correct; only the total.)

If you want to do a room by room calculation, use this Radiator Calculator. It's more reliable than any other I have found - apart from Stelrad Stars, which is too technical for most people.

I have a question about whole house boiler calculation. Let say i done a calculation for a house and the boiler required is a 24kw. A 24kw combi only give a 9-10 litre hot water flow rate.
Let say the customer wants a high flow rate, to get a F/R of 15ltr, you need a 35kw boiler. But according to the house boiler calculation, this is inefficient. So who do you listen to your customer or part L ?

 

[Heating-eng]

I have a question about whole house boiler calculation. Let say i done a calculation for a house and the boiler required is a 24kw. A 24kw combi only give a 9-10 litre hot water flow rate.
Let say the customer wants a high flow rate, to get a F/R of 15ltr, you need a 35kw boiler. But according to the house boiler calculation, this is inefficient. So who do you listen to your customer or part L ?

you would then do the rest of the tests to find the right boiler, ie check the water pressure and flow, this is what is normally used to size a combi boiler.
the whole house size is not even relervent ,if the size of the house is over 24kw then a combi boiler is not normally the right boiler.

 

[Heating-eng]

you would then do the rest of the tests to find the right boiler, ie check the water pressure and flow, this is what is normally used to size a combi boiler.
the whole house size is not even relervent ,if the size of the house is over 24kw then a combi boiler is not normally the right boiler.

Most combis heat output for the heating is lower than for the hot water, if you get a 35kw boiler for hot water the heating might only be 24kw.

 

[doitmyself]

I have a question about whole house boiler calculation. Let say i done a calculation for a house and the boiler required is a 24kw. A 24kw combi only give a 9-10 litre hot water flow rate.
Let say the customer wants a high flow rate, to get a F/R of 15ltr, you need a 35kw boiler. But according to the house boiler calculation, this is inefficient. So who do you listen to your customer or part L ?

There seems to be some misunderstanding about the relevance of the whole house boiler calculation when a combi boiler is involved.

The WHBC gives the heating requirement. (You should really deduct 2kW from the result as this is the allowance for heating a cylinder).

Now this is the maximum heating output; most of the time the boiler will be producing less than this. So you need to ensure that the boiler can modulate as far below this as possible.

The hot water requirement is calculated from the actual flow rate. Just putting in a 35kW boiler will not give you a 15 litre hot water flow rate if the incoming cold water is flowing at a slower rate. You have to match the boiler to the actual flow rate.

Let's say the hot water requirement is 35kW but the 12kW requirement is only heating. So you need a boiler with a 35kW hot water output which, on the heating side, can modulate as far below 12kW as possible.

 

[eaton]

Nearly every combi is way over sized on heating bits that's because their sized for hw flow rate, most don't need touching as they will modulate and some ramp up from low to high instead of go flat out and ramp down, very rarely have I had to range rate the heating but it has been done.

 

[lame plumber]

Nearly every combi is way over sized on heating bits that's because their sized for hw flow rate, most don't need touching as they will modulate and some ramp up from low to high instead of go flat out and ramp down, very rarely have I had to range rate the heating but it has been done.

the best answer yet

 

[doitmyself]

Nearly every combi is way over sized on heating bits that's because their sized for hw flow rate, most don't need touching as they will modulate and some ramp up from low to high instead of go flat out and ramp down, very rarely have I had to range rate the heating but it has been done.

You need 35kW for hot water but only 8kW for heating. You install a 35kW combi boiler which, for heating, it can only modulate down to 12kW. Result: the boiler spends all its time, when on heating, in on-off mode. If you're lucky and the boiler always starts at minimum output, that's not so bad.

But if, on the other hand, you install a boiler which always starts at max output and then modulates down, then you are in trouble. It will light and go to max output; the flow and return temperatures will rise rapidly; the boiler will modulate down in an attempt to maintain the correct flow temp; when it reaches minimum modulation and the flow temp still rises it will turn off and wait for the anti-cycling delay time; during this time the flow temperature will drop, possibly as much as 20 or 30 degrees. The boiler then starts again and the cycle repeats. Result: the mean flow temperature will be about 45C; the radiators never reach the correct working temperature; the house takes hours to heat up.

 

[SafeGasInstall]

i am just signing my self to follow this post for later read

 

[Heating-eng]

You need 35kW for hot water but only 8kW for heating. You install a 35kW combi boiler which, for heating, it can only modulate down to 12kW. Result: the boiler spends all its time, when on heating, in on-off mode. If you're lucky and the boiler always starts at minimum output, that's not so bad.

But if, on the other hand, you install a boiler which always starts at max output and then modulates down, then you are in trouble. It will light and go to max output; the flow and return temperatures will rise rapidly; the boiler will modulate down in an attempt to maintain the correct flow temp; when it reaches minimum modulation and the flow temp still rises it will turn off and wait for the anti-cycling delay time; during this time the flow temperature will drop, possibly as much as 20 or 30 degrees. The boiler then starts again and the cycle repeats. Result: the mean flow temperature will be about 45C; the radiators never reach the correct working temperature; the house takes hours to heat up.

You are talking total rubbish. For one it would need to be a very small house to only need 8kw for heating, and if it was so small then it would not need 35kw for hot water as the house would only have one bathroom tops.

 

[Heating-eng]

Example. Ideal logic 35 kw combi boiler . Heating min is about 7kw.
so would work well in nearly any house.

 

[zzzjim]

the house takes hours to heat up.

Could poor pipe layout /sizing contribute to boiler ability stay within operating limits

(Every install has its own personality )

 

[Heating-eng]

Could poor pipe layout /sizing contribute to boiler ability stay within operating limits

(Every install has its own personality )

yes maybe his pipework is wrong, will not be the boiler.

 

[doitmyself]

You are talking total rubbish. For one it would need to be a very small house to only need 8kw for heating, and if it was so small then it would not need 35kw for hot water as the house would only have one bathroom tops.

8kW may be a bit low, but you need to take a look at new build houses. They are so well insulated that a four bed, two bathroom house will have a 12kW system boiler and a large unvented cylinder. If a combi boiler was installed you would get exactly the situation I described.

 

[Heating-eng]

8kW may be a bit low, but you need to take a look at new build houses. They are so well insulated that a four bed, two bathroom house will have a 12kW system boiler and a large unvented cylinder. If a combi boiler was installed you would get exactly the situation I described.

no you wouldn't , your totally wrong.
a 24kw combi will work from about 4 kw for heating.

not sure you would want a combi in that type of house.