How many radiators can the Valiant 428kw run?

sapphirenick1 · 15 Jan 2015

Hello everyone

We are having a loft conversion carried out soon and we have a Vaillant 428kw boiler.

Currently we have 14 radiators (incl 2 towel rails) of different sizes and we are looking at adding another 3 radiators (perhaps 4). Can the Vaillant 428kw heat this many? I remember when installing it, I was told it could heat a lot, but I wonder if there is a way to find out?

All the best
Nick

Mark Br · 15 Jan 2015

Impossible to answer that without knowing the size (btu's) of each radiator. But I doubt the existing 14 come to much mire than 20kw so in theory ok.

D_Hailsham · 15 Jan 2015

The Vaillant 428 is a 28kW boiler. So it can heat up to 28kW or rads. This could be 28 one kW rads or 14 two kW rads or any combination of different sized rads which add up to 28kW.

To find out what your existing rads add up to, use the info on page 44 of the Stelrad Catalogue. (I am assuming they are standard rads, not "designer".)

sapphirenick1 · 15 Jan 2015

Ah okay, thanks! I will go and check the Btus for the existing rads.
The boiler is 28kw, so assuming that the radiator BTU's are less than 28kw (when converted to BTU) it will be okay?

Is there anything else to consider? Will I need to increase the size of the expansion vessel? The expansion vessel is separate to the boiler and is in the cellar.

D_Hailsham · 15 Jan 2015

sapphirenick1 said:
Ah okay, thanks! I will go and check the Btus for the existing rads.
The boiler is 28kw, so assuming that the radiator BTU's are less than 28kw (when converted to BTU) it will be okay?

Rad outputs are now measured in kW. BTUs are old - like Pounds, Shillings and Pence.

As well as the rads, you may need to take into account the Hot water cylinder. If it's an old one, it will only use about 2kW. But if it's a modern rapid recovery, it can use 20kW. However that does not mean you have to add 20kW to the rad total to get the required boiler size. All you do is set the hot water times so they occur when heating is not required, e.g overnight or when everybody is at work.

Is there anything else to consider? Will I need to increase the size of the expansion vessel? The expansion vessel is separate to the boiler and is in the cellar.

Your installer is best placed to answer this question.

sapphirenick1 · 15 Jan 2015

As well as the rads, you may need to take into account the Hot water cylinder. If it's an old one, it will only use about 2kW. But if it's a modern rapid recovery, it can use 20kW. However that does not mean you have to add 20kW to the rad total to get the required boiler size. All you do is set the hot water times so they occur when heating is not required, e.g overnight or when everybody is at work.

Thanks - good point! I have two Hot Water Cylinders - they are Megaflow 145l each. What do I need to look for on those?

I am sure that they heat water while our heating is on and it seems okay at present.

sapphirenick1 · 15 Jan 2015

Okay all is good rad wise - with the new radiators for the new loft conversion, we are still within 19k.

The pipe runs on the system are huge, though, so I am sure that 10k will get sapped up by the long pipe runs between the boiler and the cylinders. The boiler is in the kitchen and cylinders are in the cellar. Perhaps 15 metres distance of 22mm pipe between them. Not sure if that makes a difference?

Also these are the specs for the cylinders - They seem to be 18.7kw at the coil - I have 2 of them - does that not mean that the boiler is under-specced for the hot water? 18.7 x2 is 37.4kw - the boiler has been fine for 3+ years now, but on paper doesn't this suggest that I should have the 438kw Valiant?

Specs for the Megaflo's:

Capacity: 145 Litres
Coil rating: 18.7 kW
Element Rating @ 240V: 1 x 3kW
Recovery time: 17 minutes
Heat loss: 1.32 kW/h
Max flow rate at 3 bar: 70 ltr/min
Max flow rate at 1 bar: 40 ltr/min

D_Hailsham · 16 Jan 2015

sapphirenick1 said:
Okay all is good rad wise - with the new radiators for the new loft conversion, we are still within 19k.

Thought that would be the case.

The pipe runs on the system are huge, though, so I am sure that 10k will get sapped up by the long pipe runs between the boiler and the cylinders. The boiler is in the kitchen and cylinders are in the cellar. Perhaps 15 metres distance of 22mm pipe between them. Not sure if that makes a difference?

You may loose a bit, but nothing like 10kW; probably less than 1kW. In any case it's not really lost as it will be heating the house.

Also these are the specs for the cylinders - They seem to be 18.7kw at the coil - I have 2 of them - does that not mean that the boiler is under-specced for the hot water? 18.7 x2 is 37.4kw - the boiler has been fine for 3+ years now, but on paper doesn't this suggest that I should have the 438kw Valiant?

You would only need a 38kW boiler if both cylinders were being heated from cold simultaneously and you need them to reach temperature in 17 minutes. The fact that they have been satisfactory for 3+ years shows that this scenario has never occurred. Presumably each cylinder is dedicated to different outlets, so they get used at different times and consequently get reheated at different times.

You say that the cylinders get reheated when the CH is on. This could cause problems if it occurs when the house is first heated up in the morning. That would depend on how cold the house is allowed to get overnight. You could minimise this by timing the hot water to come on an hour before the CH, or even to stay on for an hour after the CH stops at night. It all depends on when you use the most water. We heat the cylinder from 9pm to 12:30am (covering shower time) and find that the cylinder is still very hot next morning and only needs "topping up" for an hour around lunch time.

picasso · 16 Jan 2015

The 428 is an open vented boiler, just wondering why you have expansion vessels ?

Dan Robinson · 16 Jan 2015

Be more concerned with the pump size personally

muggles · 16 Jan 2015

picasso said:
The 428 is an open vented boiler, just wondering why you have expansion vessels ?

It's a Heat Only boiler, it can be run on both open and sealed systems

JohnD · 16 Jan 2015

you mentioned a long hot pipe run. Be sure to insulate this thoroughly with Climaflex or similar. If you have room, fit the "Water Regulations" grade which is much thicker than budget pipe lagging. It is especially important in unheated spaces such as lofts, outbuildings and under ground floors.

In the winter lost heat will contribute to heating your cellar, which you may want to spend money on, but in summer it will be entirely wasted.

sapphirenick1 · 16 Jan 2015

Also these are the specs for the cylinders - They seem to be 18.7kw at the coil - I have 2 of them - does that not mean that the boiler is under-specced for the hot water? 18.7 x2 is 37.4kw - the boiler has been fine for 3+ years now, but on paper doesn't this suggest that I should have the 438kw Valiant?

You would only need a 38kW boiler if both cylinders were being heated from cold simultaneously and you need them to reach temperature in 17 minutes. The fact that they have been satisfactory for 3+ years shows that this scenario has never occurred. Presumably each cylinder is dedicated to different outlets, so they get used at different times and consequently get reheated at different times.

Both cylinders are actually heated at the same time - they are connected together to make a total of 290l hot water, they don't supply a different zone, but they each have their own motorised valve.

My boiler does enter anti-cycling mode when heating the hot water, I wonder if that is why. I did ask the Valiant engineer when he serviced the boiler and he said that the anti-cycling was a normal feature.

You say that the cylinders get reheated when the CH is on. This could cause problems if it occurs when the house is first heated up in the morning. That would depend on how cold the house is allowed to get overnight. You could minimise this by timing the hot water to come on an hour before the CH, or even to stay on for an hour after the CH stops at night. It all depends on when you use the most water. We heat the cylinder from 9pm to 12:30am (covering shower time) and find that the cylinder is still very hot next morning and only needs "topping up" for an hour around lunch time.

Thanks so much for the tip - I will definitely do this. I take it your heating is still on when you heat up the water at 9pm to 12:30am?

sapphirenick1 · 16 Jan 2015

JohnD said:
you mentioned a long hot pipe run. Be sure to insulate this thoroughly with Climaflex or similar. If you have room, fit the "Water Regulations" grade which is much thicker than budget pipe lagging. It is especially important in unheated spaces such as lofts, outbuildings and under ground floors.

In the winter lost heat will contribute to heating your cellar, which you may want to spend money on, but in summer it will be entirely wasted.

I do notice that the cellar is warm - the fitters definitely lagged the pipes but I am sure it is not good stuff! What thickness is the Climaflex Water Regulations grade?

I see various thicknesses: 9, 13, 19 & 25mm - should I be using 25mm in the cellar?

Unfortunately, they definitely didn't lag the pipes in the floor voids/ behind walls and there is nothing I can do about that now, but I can do something about the cellar. Thanks for the tip.

I also notice that on the pipe bends coming out of the Megaflo's there is no lagging, same around the thermostatic valves, but then you can't do much about that!

sapphirenick1 · 16 Jan 2015

Dan_Robinson said:
Be more concerned with the pump size personally

The pump is a Grundfos Red Type UPS 15-60 130 p/n: 96281478 and it is set to level 3, maximum.

Does that sound like an appropriate pump? It has to pump approximately 8 metres up from the cellar to the top room.

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How many radiators can the Valiant 428kw run?

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