Thanks to Ed Sirett who questioned my boiler sizing! Somehow, despite having done detailed spreadsheet totalling approx 20Kw, I had drifted into 28/29Kw boiler territory! The house is 1970s originally 3 bed detached, having 3 single room extensions: 1 kitchen, 1 back room and 1 bedroom over garage. Original boiler is Glow Worm Space saver 50. Both for age and capacity this is struggling.
From other posts I see that Worcester Bosch is well regarded, but the boiler that comes up on a 4-5 bedroom house from their site is 28Kw HE unit! Is this just their nearest suitable animal?
So the questions are:
Is this massive overkill? Will it modulate anyway so the only money lost is the initial outlay? Is there a more appropriate smaller system unit from other supplier? (I was advised to go system in previous thread Worcester Boilers - slight confusion). Should I go for the larger unit to give headroom/flexibility?
I had one of those. In its day it was OK, but is not well made and very inefficient by today's standards.
Yes. Keep in mind that the term "4-5 bedroom house" has little meaning in this context. The heat requirement can vary enormously based on size of rooms, construction and location.
I don't think so.
Yes, and with a higher rating, it will heat the radiators and hence the house more quickly in the winter mornings.
I would (did) but from a different manufacturer (MAN Heiztechnik).
You could perhaps look at 24kW products, but I don't think that 28kW is unreasonable in the context of a 20kW requirement.
Yep. You say 20kW minimum. If it is above and modulates then no problem. It will heat the rads very quickly though, a bonus. If the cheapest boilers are around 25kW then go for that, as long as it modulates. .
Go for the best value 20kW or above, modulating system boiler.
If 28kW isn't enough for you, you should be considering fixing your insulation, not increasing your boiler capacity. Indeed, if any 3 bedroom house is coming out at 20kW, some serious look at some Kingspan catalogues is long overdue.
As stated in the original post, the house started as 3 bed detached but has had 3 significant extensions. I take the point though Christian, additional insulation generally wouldn't go amiss. Not sure with these modulating boilers that it would affect the final choice greatly though.
I'm unfamiliar with the current WB range but if there is a comparable 24kW I'd certainly go for it. Modern boilers are much more tolerant of being incorrectly over sized but not entirely so.
The existing unit which is 15kW is clearly struggling to meet the 20kW load. This confirms that 20kW is about right.
Can you explain this statement please Ed? In the context of the load I am considering ie 20Kw from cold in the depths of winter down to 3.5 Kw hot water heating only in the summer. What is my range?
Also, somewhat away from the initial story line, is there a "standard" install height such that a boiler can be fitted into a wall cupboard. I would like to replace the boiler before deciding on final kitchen layout - is this a recipe for disaster?
I'll throw something in on this one, although I'm sure Ed will add to it later.
Modulation range is simply the ratio of max to min output, so a 20kW boiler with 3:1 range would have a minimum of just over 6kW.
If you had a minimum load of 3.5kW as you suggest, the boiler will run at minimum output, and cycle on and off to keep average heat output at
3.5kW. Above the minimum modulation output, it runs continuously.
Did you get the 3.5kW output from one of the heatloss programs? This is the typical figure for an old basic cylinder connected by gravity circulation. You might be able to get a bit more heat into it than that when the contents are cold, and thus be above the minimum modulation point sometimes. However, it's not that satisfactory.
If you can, it's best to use a fast recovery cylinder, replacing the old one. These have larger coil area through one means or another and can take a much higher rate of heat from the boiler - 20kW+ is typical. The advantages of this are that you can reheat the cylinder faster and either have more hot water available over a given amount of time or possibly install a smaller cylinder. Moreover, the boiler will run continuously for most of the reheat cycle and hence more efficiently. Since the typical control system is HW priority, the boiler is normally switched to drive the cylinder only when there is HW demand. This means no heat to the radiators during that time. Therefore, ideally, you don't want HW cycles to last too long.
If you look on the manufacturer's web site and download the install manual for the boiler it will tell you. Normally for the inside there are clearance dimensions around the boiler mainly for maintenance. These may not include additional clearance to fit in the flue so that has to be allowed for.
You then have the exterior location rules for the flue, which are mainly dictated by British Standard etc. Obviously with flue types/components you can position the flue terminal some way from the boiler. I am not sure if your proposed supplier has them, but some manufacturers do a flue system and adaptors where high temperature plastic waste pipe can be used for the flue and run over many metres.
Many boilers are designed to fit into a line of kitchen wall cupboards, but do look out for possible compartment ventilation requirements (install instructions will say this if there are any) and overall height. It would be tearful if you mounted the boiler unnecessarily high or low, for example.
It makes sense if you can to fit the boiler before doing the kitchen refit because you have more space to work, but I would plan the two together because you may find that you can so something you want with the kitchen layout or cupboard choice by having the boiler a few cm to the right or left of nominal position.
Thanks (once again) for such a thorough explanation.
This came from a fairly recent Plumbing and Heating installation book (the exact title escapes me). As you say, it is taken from advised heat loss figure of 1Kw for every 50l of water. The cylinder is I believe a bog standard indirect cylinder with pump rather than gravity feed. Is this likely to present a higher load than the 3.5kW assumed? What price a fast recovery cylinder? Our current capacity I think is 160l and we rarely run out of water. But both kids are reaching teenage! Surely most people will have a boiler replaced without replacing the indirect cylinder.
Yes I have seen the install dimensions, but was trying to determine a height at which I could (almost) guarantee it would fit within a wall cupboard in a standard kitchen layout.
Have you had problems with knowledge theft/plajorism (sp)?
Quick recovery cylinders are cheap to run, lighter on boiler controls, as less cycling, and save space. You can have a 140 litre cylinder and still get more hot water than your 160 litre. The system should be a DHW priority system.
Around the £300 mark, probably less if you shop around or perhaps buy boiler and cylinder together.
Have a look at Albion Superduty and Range Ultracal, although Albion seem more readily available.
Then I wouldn't (from experience) reduce storage capacity.
Depends. If the cylinder and boiler were in the house for some time, perhaps new, then the cylinder may well have been a cheap and crappy one (risk of eventual leak) or scaled.
I took the view that if I'm going to have the upheaval of the boiler replacement, I might as well do the cylinder at the same time. I also took the opportunity to clean up the pipework layouts to neaten the job.
IIRC, kitchen wall cupboards come in standard heights of something like 720mm and 920mm. Then, typically a cornice is put on the top and there is a gap to the ceiling of around 100mm or so. This assumes total wall height around 2400mm of course.
There are tricks that you can do depending on the shape of the room and other stuff. My boiler is in the utility room in one corner and I fabricated a cupboard and used it with a taller door. Even though that comes lower down to the worktop in that part of the room, it all looks in keeping.
It does look rather naff if you have the bottom part of the boiler sticking out of the bottom of the cupboard.
There are not many boilers which modulate down to 3.5kw afaik. If you are still looking around at different makes I'd suggest you get more information on the Atag boilers. There are two models: 'Blue Angel' and 'Premier'. The quality of components is on par with Viessmann stuff. They also run extremely quiet, unlike other boilers I could mention.
Of course, you might not find your local plumber will be interested in them as they are bit more involved than just wiring in to a fused spur and bringing back a switched live to the boiler.
The combi versions have a plate HE for the Premier and a small storage vessel for the Blue Angel.
You should note that your cylinder probably has a bigger coil than 3.5kW, but the heatloss programme just put on 3.5kW to ensure there was always some capacity to heat it, even in cold weather. When it's just the boiler and the cylinder, it probably sinks a lot more power.
If he is satisfied with 160 litres, then 140 will do. If he intends to increase his usage that is another matter.
Yep. being dynamic I would have.
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