New Combi or Unvented Cylinder

[ddsull]

Hi,

I’m looking for some advice on whether to get a new combi boiler or keep our current boiler and get an unvented cylinder.

We have just moved in to a new house. 1930’s dormer bungalow and a a gravity fed vented hot water tank (in the loft). The boiler is an Alpha CD18R (18kw). This is in the garage and seems to work ok (was installed in 2016) and is controlled by a Hive thermostat. The boiler is about 82% efficient and does it’s current job ok. The boiler seems to be a heat only boiler. However it may be possible to use with an unvented cylinder.

The cold water (apart from the kitchen tap) is off the cold tank in the attic and the pressure isn’t great. Toilets take about 10 mins to refill (might need some new fill valves). The HW tank is very small about 100 litres or so) not measured it so not sure if the exact volume. There are two bathrooms but the upstairs one has an electric shower (good mains pressure).

Bad points (current setup)
Hot water pressure very low in kitchen tap (this might be fixable with low pressure tap but not sure).
Not great pressure in the mixer shower.
Hot water runs out frequently (we have 2 small kids) due to small tank
Water too hot in tank sometimes (may need to adjust the boiler thermostat)
Low pressure on toilets etc.

We will at some stage look at doing work on the house which may mean the tank in the the loft is relocated. However, this might not be for a couple of years. As we might just build a single storey out the back. So TBC.

I have a number of options

Option A
Live with the system for now. Maybe try and improve kitchen hot water pressure (low pressure tap) and change the cold water to mains (might lead to issues with the shower mixer).

Option B
Swop out the gravity fed system for an unvented cylinder. The current tank is in behind the shower which has an aqua panel which could be taken out and replaced easily enough. The cold water tank and header tank would be taken away etc. As far as I’m aware a heat only boiler will work with an unvented cylinder?

Option C
Replace the current boiler with a good quality combi (looking at Worcester Bosch Greenstar 8000 30/35KW). Not sure what the house layout will be in the future but we could add an unvented cylinder to run off the boiler in the future. However, for now the combi would be more than adequate.

I reckon keeping the existing boiler and getting a unvented cylinder would be around £2K (depending on the type of cylinder). The new combi would be £4K plus.

Cylinder only is cheaper but may have to move it again in the future. New combi might be a neater solution but a lot more upfront.

I’d appreciate any thoughts or suggestions on the above.

 

[vulcancontinental]

Whats the cold main flow rate and pressure is the first question. There's no reason a combi couldn't be fitted initially then used later to supply DHW and heating plus reheat for an unvented cylinder in the future.

 

[ddsull]

Thanks for the reply.
I haven’t got an actual measurement of the pressure yet but it does seem as though it will be fine. Plenty of pressure in the kitchen and outside taps. This was option C. I will get the plumber to check it out though before making any decisions.

I guess the main question is should I keep the current boiler and add a tank (cheaper, but may be moving the tank again a few years down the line) Or bite the bullet and change the boiler?

thanks

 

[JohnD]

Pressure may not be the key point. Flow is. Fill a bucket at the cold tap, time it. How many litres per minute does it deliver?

If your existing boiler is working properly, is there a reason to change it?

 

[bernardgreen]

The boiler is about 82% efficient and does it’s current job ok.

So no need to replace it

The boiler seems to be a heat only boiler.

Which is much easier to service than a combi which has pumps, secondary heat exchangers, diverter valves etc etc all packed into the boiler casing

However it may be possible to use with an unvented cylinder.

It can be used with a cylinder ( unvented or vented ). Having a cylinder provides the option of hot water from an electric immersion heater if ( when ) the boiler is not working

 

[ddsull]

At the outside cold tap around 15 litres a minute. I’d imagine that should be ok for the combi.

The boiler is working fine. I guess if I kept it and went with thee new cylinder, I might be (not confirmed) spending a couple of grand on something which might be moved taken out when we extend. I could use it again and relocate though.

The new boiler would be more efficient and would mean I wouldn’t be doing work I would potentially be taking out.

so new cylinder in the loft or new combi? What are the thoughts

 

[cdbe]

Unvented can go anywhere - garage?

 

[bernardgreen]

The new boiler would be more efficient

The boiler may be more efficient but with a combi the over all system may be less efficient. Take into account that running the hot tap while waiting for hot water to reach the hot tap is wasteful of both water and gas.

 

[ddsull]

Unvented can go anywhere - garage?

I looked at that but thought I needed the room. However, it might be a neater solution as all in the same place.
I guess the next question is, is the current boiler (heat only boiler) ok for the unvented cylinder. Would 18kw be good enough for a 250l tank?

 

[oldbutnotdead]

15lpm is ok flow, any idea on pressure?
It's quite likely your variable tap flow rates are due to a system that's been added to or modified without any great thought.
You won't see payback on energy efficiency from a new boiler for many years (if ever given your 'old' boiler is only 6).
If you have the room for header tanks etc. then vented dhw is a very simple system with none of the complexities required for unvented (pressure relief valves, annual maintenance) and can give good performance. Plus installation is completely DIYable if you fancy a go

 

[ddsull]

So no need to replace it


Which is much easier to service than a combi which has pumps, secondary heat exchangers, diverter valves etc etc all packed into the boiler casing



It can be used with a cylinder ( unvented or vented ). Having a cylinder provides the option of hot water from an electric immersion heater if ( when ) the boiler is not working

Thanks or this. Thinking of 'making room' for an unvented cylinder in the garage.

That keeps it all in one place and hopefully gives good pressure hot water in the house. Would the 18KW be ok for 250 l tank?

 

[ddsull]

15lpm is ok flow, any idea on pressure?
It's quite likely your variable tap flow rates are due to a system that's been added to or modified without any great thought.
You won't see payback on energy efficiency from a new boiler for many years (if ever given your 'old' boiler is only 6).
If you have the room for header tanks etc. then vented dhw is a very simple system with none of the complexities required for unvented (pressure relief valves, annual maintenance) and can give good performance. Plus installation is completely DIYable if you fancy a go

I haven't tested the pressure yet. We just moved in a couple of weeks ago. We might need a goo check of what's there before jumping in. I think I'm coming around to keeping the existing boiler and getting a cylinder next to it if possible.

 

[picasso]

Nobody in their right mind would choose to install a vented cylinder from scratch, 18kw if fine with a 250ltr cylinder.

 

[Madrab]

IMO it's all about what you want and what the mains can deliver.

Firstly it is key to know the cold mains dynamic flow and pressure before anything can be considered. You need to take the flow measurement from a full flow tap as close to the mains supply as possible, your outside tap should do fine and then connect a pressure gauge to say an appliance valve and run at least one other tap at the same time. Time filling a bucket over 1 minute and post the results. Try and take the readings at a peak time if possible.

 

[ddsull]

Nobody in their right mind would choose to install a vented cylinder from scratch, 18kw if fine with a 250ltr cylinder.

It would definitely be an unvented cylinder! I want good pressure hot water throughout the house. As the boiler is so new it would be a shame to take it out.