One boiler or two?

[Idealhater]

I currently have two Ideal HE24 boilers each with unvented tanks supplying two wings of my house. The wings were originally separate houses and when I bought the second part a completely separate system was the simplest solution. One wing is all radiators, the other mainly underfloor heating.
Needless to say the HE24s have been a total pain ( LF errors, leaks, dead pcbs etc ) and although only about 10 and 6 years old respectively I've had enough. I'm looking to replace them, probably with Worcester Bosch.
My question is would I be better replacing them with one higher output boiler? I'm thinking mainly in terms of running costs as I'm currently paying around £2500 pa for gas. I realise you can only answer in general terms without knowing the layout of the systems but that would still be helpful.

 

[Roger Welton]

As you say impossible to say with any certainty without seeing the system(s), but I would be thinking 2 smaller boilers on a low loss header, the possible 3 circuits off the header (hw, rads and ufh) reasons being, added redundancy, both boilers can do anything and a better turn down rate than a single larger boiler. Downsides being increased install and maintenance costs and space required.

I would also avoid worcester but that is a personal thing, you can do better for the money.

 

[R1CKY]

It is completely up to you. The advantage of having two lower output boilers instead of one high output is that you'll always have half of your requirement should the one fail. The disadvantage are cost of installing, servicing and maintaining.

The Worcester CDI system boiler is a very reliable unit in my experience, the outputs seem to not match your requirements, the largest is 42KW and the smallest is 30KW, so you'll be oversized if you choose two and undersized if you have one.

 

[gassafe]

The Worcester CDI system boiler is a very reliable unit in my experience, the outputs seem to not match your requirements, the largest is 42KW and the smallest is 30KW, so you'll be oversized if you choose two and undersized if you have one.[/QUOTE]

worcester also do 12, 15, 18 & 24 in a system, tho its not the cdi version! Which I also prefer! Lol.

it will probs be cheaper to have two seperate boilers, there would be alot of change to pipework to combine as 1 system I imagine, especially if its two seperate houses!

 

[plumber0808]

Get someone in to have a look and advise you, get a price for the two options.

 

[Idealhater]

Thanks everyone for the replies. I have got someone coming in to look at the options but I was hoping to get a broader view of the pros and cons in case he too says "its up to you".

The cheapest option short term is no doubt to replace the one that is particularly bad and do the other in a year or two when that goes again. But my thinking maybe was that a bigger spend now to combine the systems might be a wise investment if it significantly reduces running costs. As well as servicing I'm thinking of gas use which no one has commented on. Any views on consumption of one big boiler vs two smaller ones?

I have had occasion to be glad of having two boilers when one was down but I'd hoped not all are as bad as Ideal!

A more radical option would be to replace the one driving the underfloor heating with an air source heat pump but at the moment the costs don't seem to stack up. Any thoughts?

 

[DirksPlumbing]

If you are really thinking about joining the system and having stil 2 boilers then Viessmann might be best option as these actually can deal with literally any flow condition.

Fitting a neutralizer might be another option but for the two boilers you need then one pump each, one for the CH and one for the UFH at least. Which in return means your utility bills might not exactly decrease.

The UFH can get used on low temperatures, blasting it up with a neutralizer and then cooling it down again is another thing that is definitely not lowering you energy usage.

 

[snowhead]

A more radical option would be to replace the one driving the underfloor heating with an air source heat pump but at the moment the costs don't seem to stack up. Any thoughts?

Using an ASHP isn't a viable option when gas is available and won't be as reliable.

My vote would be for 2 boilers, 1 now and 1 later.

Maybe you need to look at the control of the heating to make sure you are only fully heating when required.

 

[TBServices]

2 boilers on LLH would be my choice.

 

[TBServices]

And you may get recommended an ideal, don't be put off due to past experience, new models are excellent.

 

[Bronze_tap]

Using an ASHP isn't a viable option when gas is available and won't be as reliable.

My vote would be for 2 boilers, 1 now and 1 later.

Maybe you need to look at the control of the heating to make sure you are only fully heating when required.

Thats quite a good option. I may also suggest adding good insulation to your's house (>2k oa for gas is a bit over the top, unless the outside walls are 1/2 brick thick and no loft insulation), and aim for maximum system flow temperature no more than 40-50 degrees . (so that you can rig UFH directly to the boiler circuit).

That may mean some expenditure on the double panel double convector rads... but usually it is worth every penny in my opinion.

Also it may make ASHP economical when the outside temp is >5 degrees and the COP will be >=4...

The bypass link can be also beneficiall, so you can hat both halves with one boiler. Also consider zoning and partitioning those open plan areas, which are huge energy/money hogs unless insulated to A class and airtight.

PS: The plummer which may suggest up sizing rads will loose the top position if sorted by lowest quote price first...
But will jump to the top when ROI is concerned (Return On Investement).

 

[vern]

2 x WB 30 CDi system boilers ( can be down rated via internal software to suit system and gas meter load)
WB LLH
Fullmodulating Pumps, zoned and sized to suit
Controls updated
TRV's fitted where needed
A sprinkle of my magic

 

[Idealhater]

Thanks to everyone for your suggestions.

The consensus is clearly to stick with two boilers. This surprises me because I had assumed one boiler would be much cheaper to run than two of the same total output. That's good to know given that replacing one at a time is the cheaper initial option.

So it looks like the way forward is to replace the one boiler and get the system checked over. The money saved on the more radical options can go to better insulation. This is tricky as it's a listed building - I have plenty of loft insulation but walls are solid stone 2 - 3 ft thick and no chance of double glazing. Also surprisingly big windows for such an old house, on 2 or even 3 sides of most rooms. I think a programme of window repair and secondary glazing is called for.

BTW - the two current systems are on separate controls so the part we use mostly during the day goes off in the evening and vice versa. And we do have TRVs set to minimum in unused rooms. And in addition to the £2500 oa gas we spend about £600 pa on logs. No, we're not nudists - we wear jumpers!

 

[DirksPlumbing]

Thanks to everyone for your suggestions.The consensus is clearly to stick with two boilers. This surprises me because I had assumed one boiler would be much cheaper to run than two of the same total output. That's good to know given that replacing one at a time is the cheaper initial option.So it looks like the way forward is to replace the one boiler and get the system checked over. The money saved on the more radical options can go to better insulation. This is tricky as it's a listed building - I have plenty of loft insulation but walls are solid stone 2 - 3 ft thick and no chance of double glazing. Also surprisingly big windows for such an old house, on 2 or even 3 sides of most rooms. I think a programme of window repair and secondary glazing is called for.BTW - the two current systems are on separate controls so the part we use mostly during the day goes off in the evening and vice versa. And we do have TRVs set to minimum in unused rooms. And in addition to the £2500 oa gas we spend about £600 pa on logs. No, we're not nudists - we wear jumpers!

You definitely would need some energy advice. Seems your setup is not very good. 2-3ft walls are relatively good insulators anyway.

And do not get put off from double glazing. Of course you can have it. I live in a listed building myself and we got double glazing.It might not be the cheapest option but some good joiners/glazers can make replicas of your windows just with double glazing.Sometimes even double glazing can get fitted into the existing frames.

As for the insulation they have replaced the plasterboard by foam back plasterboard. Makes quite a difference. As much as for draught proofing as well as for making the rooms heating up quickly.

With your gas usage I would first look at the UFH. As plenty of them have been fitted as UTFH (under the floor heating). Been replacing a few of them now already with little or no insulation under the pipes/screed.

In one scenario we fitted once trench heating round the walls to create a hot air curtain as a shield against cold draughts from the walls combined with UFH in the rest of the floor area. Walls were meant to be left coarse looking with no insulation inside or outside.

The other thing is trying to switch things off to much can sometimes be a costly mistake as you turn your inside walls into outside walls and enlarge the surface in addition. The radiators in the remaining rooms then have to fight against heat losses they have not been specified for.

If it needs be then make sure that unused rooms are adjoining and temperatures increase towards used rooms.Further it can sometimes (lets say quite often) safe to keep a constant background heat at all times to prevent cold draught effects from walls which require you to heat higher than normal to still be comfortable.

Rule of thumb is: If the wall temperature differs more than 3 degC from the air temperature in the room you do get this "wind chill effect". Most people describe it as the walls radiating cold.

Problem really is, without enough information it is guessing. And even with enough information it is sometimes a bit playing around with monitoring to find a better setup.

 

[DirksPlumbing]

By the way, a good thing is to place radiators underneath windows to prevent streams of cold air falling into the room. Even more so if you are still on single glazing.

 

[dancinplumba]

Only read first bit so far and it's misinformation, Solid walls BLEED heat

 

[dancinplumba]

And contradictory, ie As for the insulation they have replaced the plasterboard by foam back plasterboard



.???

 

[Idealhater]

Thanks Dirksplumbing. I'm afraid double glazing really is a non starter - most of the windows are leaded and with very old glass so there is no way they could be replicated. There are a couple with oak frames and float glass where it might be acceptable to insert double glazed panels but for the most part it would have to be secondary glazing.

Most of the outside walls are lime plastered rather than plasterboarded - this too is covered by the listing.

In our case I think the UFH is OK - it sits on 125 mm Celotex with screed and wooden floor. It's comfortable at 17c whereas the main house needs to be set at 20c for comfort. Partly that's because sitting around in the evening you need more heat but it also reflects the lossy nature of that area.

Just checked this quarters gas bill - £1265!

Sorry - didn't mean to turn this into a double glazing forum.

 

[DirksPlumbing]

Only read first bit so far and it's misinformation, Solid walls BLEED heat

I take you can demonstrate that at a small thermal resistance calculation based on a 3ft solid wall with a small air gap and traditional plaster lathe on one side and around 25mm roughcast outside?

 

[DirksPlumbing]

And contradictory, ie As for the insulation they have replaced the plasterboard by foam back plasterboard



.???

Can you elaborate?

 

[DirksPlumbing]

Thanks Dirksplumbing. I'm afraid double glazing really is a non starter - most of the windows are leaded and with very old glass so there is no way they could be replicated. There are a couple with oak frames and float glass where it might be acceptable to insert double glazed panels but for the most part it would have to be secondary glazing.

Most of the outside walls are lime plastered rather than plasterboarded - this too is covered by the listing.

In our case I think the UFH is OK - it sits on 125 mm Celotex with screed and wooden floor. It's comfortable at 17c whereas the main house needs to be set at 20c for comfort. Partly that's because sitting around in the evening you need more heat but it also reflects the lossy nature of that area.

Just checked this quarters gas bill - £1265!

Sorry - didn't mean to turn this into a double glazing forum.

Well that obviously is answering a few questions.
Lime render is not too bad either.

Having little heatloss in noticeable floor areas and loft insulation leaves at assumed avg. 2.5 ft walls rather radiation losses and to much air exchange.

 

[DirksPlumbing]

Can you roughly tell how many windows and an average size?

Edit: Not for glazing advice, just for heatloss guestimate for the window area.

 

[Idealhater]

I think it's 22 but size varies enormously - smallest 2 x 1.5 ft to biggest 8 x 5ft.

Take the sitting room which is approx 25 x 18ft. It has 1 window 5 x 3, 1 at 6 x 4 and 2 stone mullioned windows each with 4 sections approx 6 x 1.

Every room is different, and in some rooms each window is different not only in size but construction. So nice thought but I don't think you'll get very far. However it's probably fair to say that I'll get a better return from tackling windows than from trying to combine heating systems.

I may not have been clear - the lime plaster is on the internal surface of the outside walls. The exterior is bare Cotswold limestone.

 

[Idealhater]

There was obviously a difference of opinion over heat loss through solid stone walls. I've never been very clear about that - you hear contradictory views. I think it also gets mixed up with the heat store effect of the sheer mass of stone.

I've just found an interesting SPAB ( Society for the Protection of Ancient Buildings ) report from 2011 looking at the U-Values for a variety of traditional wall types. They compared the actual measured value and the calculated value using a "standard tool" ( BuildDesk V3.4 whatever that is ). They found a near linear relationship between thickness and U-Value and also that the calculated U-Value was in most cases much higher than the actual ( 50% or more ). The tool is more accurate with modern standardised construction.

This suggests that walls like mine should be comparable with a modern but uninsulated cavity wall - probably in the 1.2 to 1.6 range. Not great but not as dreadful as you might think.

The windows however are another story.

 

[DirksPlumbing]

There was obviously a difference of opinion over heat loss through solid stone walls. I've never been very clear about that - you hear contradictory views. I think it also gets mixed up with the heat store effect of the sheer mass of stone.

I've just found an interesting SPAB ( Society for the Protection of Ancient Buildings ) report from 2011 looking at the U-Values for a variety of traditional wall types. They compared the actual measured value and the calculated value using a "standard tool" ( BuildDesk V3.4 whatever that is ). They found a near linear relationship between thickness and U-Value and also that the calculated U-Value was in most cases much higher than the actual ( 50% or more ). The tool is more accurate with modern standardised construction.

This suggests that walls like mine should be comparable with a modern but uninsulated cavity wall - probably in the 1.2 to 1.6 range. Not great but not as dreadful as you might think.

The windows however are another story.

I think you are on the right path, just this knowledge is available at least since the 80s. But indeed the high thermal mass suggests to only introduce a slight ripple in the temperatures like on the UFH.
Reason one is the time it takes to lift the temperature in the room including wall surface. Reason two is the fact that a warmer wall gets dryer and therefore increases its thermal resistance.

As I said already, it is important to make sure to place radiators underneath windows where feasible to lift the bottom temperature of the room. Otherwise make sure to be always near the ceiling. Which only works out if your room heights are pretty low.