What size combi boiler do I need

[Ahmiccc]

Hi all,

 

just bought a 1965 house has a cavity wall insulation and a 30 year old boiler, it needs a full refurb the only thing it don’t need is to be bulldozed down and re built that’s how big the project is, anyway it’s our first home and wanting to make it a home so we want to install underfloor heating system on both floors.

 

Upstairs and downstairs are both suspended floors on.

downstairs I was planning on putting 100mm insulation this is the most we can put and that’s us having to pull the joists out and put bigger ones in then have UFH fitted on that

upstairs I was going to have 75mm insulation in joists and electrics below the insulation and UFH Above of course. The attic is fully insulated aswel I believe with 75mm insulation but this isn’t a problem too much if needs more

 

Downstairs the plan is to have a two port(zone)  valve, pump, 5 port manifold, and electrics and upstairs is another two port (zone valve), pump, 4 port manifold and electrics reason for this is mainly peace of mind and more control the zone valves will be teed of coming from the boiler!

 

The house is 42square meter per floor totalling 84 square meter maximum of UFH Ofcourse showers and other fixtures like that will reduce amount of square meter available but let’s work to the 84 square meter, we’ll be running 16mm pipe at 200 pipe centres 2 in between each joist. And I’m pretty happy this is all possible and fitted but something thrown a spanner in the works that might scrap it, as we need a new boiler plan is to get a combi, what size combi boiler do I need for the 84 square meter ? The one I was looking at was the Worcester Bosch 8000 life but not sure what the kW as the 35 law and higher all output 35kW of heat would this be enough or am I going to have to go into fitting a different type of boiler ?

I understand the downside of a combi boiler when someone is showering and stuff the heating is turned off but I think since the rooms will be well insulated this will be ok.

 

so going back to the question is a 35kW heating output combi boiler enough for 84 square meter UFH ??

[PeterW]

Firstly ... 200mm spacing will not heat a 1965 house unless you are significantly upgrading the wall insulation and windows etc. Maximum 150mm spacing with that sort of build. 
 

You could also just use a single manifold. No point with one per floor on a house that size, but you will need to find somewhere to put a buffer tank as a large combi will not modulate down far enough to provide low levels of heat when you are reaching the temperature set point. 
 

I can’t see how you need 9 zones across that area, even with 150mm spacing you’re only just over 500m of pipe so at best I would be looking at a single 6 port manifold on the first floor, and use actuators to control the zones not zone valves to each manifold. That will at least halve your spend on the manifolds etc and there is no issue running pipes down to the lowest loops from an upstairs manifold. 
 

 

[Ahmiccc]

Yeah won’t be upgrading the wall insulation and the windows are staying double glazed maybe newer windows but that’s it, the 150mm spacing I can do though.

 

Obviously due to the small house the boiler will be downstairs in utility room don’t think a 9 port valve firing up stairs will be sufficient enough that’s why I wanted the two manifold with pumps and stuff and want each room separately controlled which is why there will be 9 zones downstairs will be, hallway, living room, dining room, kitchen, utility room and small bathroom (toilet and sink really) and upstairs is 3 bedrooms and a bathroom = 4

 

I don’t have space anywhere for a buffer tank and I don’t think the attic has the head room or the best place to have it ? Also I don’t know how to fit a buffer tank into a system to be honest, never done it, Ive read an automatic bypass valve could help this but again not too educated in this either ?

[PeterW]

Buffer in the attic is fine. It’s only 100 litres so not massive. Can you not also get the boiler up there and free up some utility space ..??
 

In terms of zones I wouldn’t use individual controls for hallways etc. as you won’t get any benefit and you’ll end up tripling your costs with controls that are pointless. 
 

Even a 9 port manifold will easily cope with that pipe volume. Once you get up to 1000m of pipe on a manifold then it’s usually better to split to a second pump. 

[Ahmiccc]

With current plan doesn't really free up some space, as it leaves a dead hole to fit something else in, so yes it opens up space but nothing of use but can do that too, but then come across the problem of how to fit the buffer tank what’s needed really how to wire in what’s needed and how to pipe it in is the buffer tank just for heating or ? Which is why I wanted to stick with the combi boiler only if there is anyway possible.

 

zones i can put the hallway in line with the utility room and toilet not a problem but only brings me down to 8 zones and then where would I fit it as current plan is to have it downstairs is that good enough or is it best to put it up in attic, where there’s a chance as pipes are going in between walls or whatever they could get hit with screws in many years from now.

[PeterW]

You’ve got to get pipes to the manifolds on each floor so it’s no more of a risk than putting loops in a wall - you can protect the pipework if needed or box it in somewhere like the back of a wardrobe. 
 

You don’t have to fit a buffer but a combi will short cycle and will not be as efficient and it will shorten its life. 
 

A buffer just has 2 or 4 connections, 2 to the boiler and 2 to the manifold, can even just add it as a tee off to the flow and return to the manifold. It has a thermostat on it and it’s just a case of that controlling the signal to the boiler to fire. 
 

Are you planning on installing this all yourself .?

[Ahmiccc]

Right I’m completely on track now with the buffer!! This whole time I was thinking the buffer was going to be like the system boiler concept but it’s not right okay this is very simple then!

so boiler to buffer (the boiler pump is enough I’m guessing ?) then from the buffer into a zone valve (not needed?) into a pump into the mixer valve and then manifold and so on from there ?

so when it comes to the wiring electrics I was thinking the 12v 8 zone heat miser, this way easier to get the controller into the bathroom In regards to keeping with standards (wiring regs) so this will work without having to wire in the contact to tell the boiler to switch on and pump ? As all that will be wired for in is the pump to the manifold to kick on

 

yeah most of this will be doing my self and dad who’s a jack of all trades I’m an engineer with 18th edition find my self competent in all this from all research done just the buffer tank put a spanner in the works just cause I thought it was something it wasn’t. So this in my eyes isn’t anything technical at all just an addition and little more piping

[PeterW]

Heatmiser are expensive for what they are and there are plenty on here who have had issues. You’ve got the rest pretty much sorted - don’t need zone valves

[Ahmiccc]

Okay great Your a star thanks for your help!!

Yeah pretty new to this I like the way heat miser as they do everything actuators and everything and don’t really know of any other brands either, since I’m new feel to this think it’s the safest way without issues

[PeterW]

2 hours ago, Ahmiccc said:

Okay great Your a star thanks for your help!!

Yeah pretty new to this I like the way heat miser as they do everything actuators and everything and don’t really know of any other brands either, since I’m new feel to this think it’s the safest way without issues

Its expensive and crap tbh. 
 

Look at WundaTrade - all the same stuff, 1/3rd of the price. Buy the self adjusting actuators and fit and forget. 

[Ahmiccc]

5 minutes ago, PeterW said:

Its expensive and crap tbh. 
 

Look at WundaTrade - all the same stuff, 1/3rd of the price. Buy the self adjusting actuators and fit and forget. 

Self adjusting ? Those the electric ones I tend to call them ? 

[PeterW]

1 hour ago, Ahmiccc said:

Self adjusting ? Those the electric ones I tend to call them ? 

They are all electric however these are self balancing to a delta 7°C on flow and return. 

https://www.wundatrade.co.uk/shop/home/quick-shop/wundatherm-quick-shop/controls-quickshop/actuator-auto-balancing/

 

[Ahmiccc]

Just a question is an expansion vessel needed ? I wouldn’t say so as it’s a closed loop, if it was for taps then yes ? Hope I’m right ?

[PeterW]

Boiler should have an expansion vessel built in, if not you need one. 

[Ahmiccc]

Last one from me I think, Is the bigger buffer tank the better so less use of boiler or ? Doesn't matter too much ?

Thinking bigger boiler more hot water so more hot water to send round and less of an impact of the cooler water coming back is affecting ? Think it’s more of an efficiency question really is a bigger buffer tank more efficient ? And better ?

[Ahmiccc]

I was thinking maybe a 150liter Worcester Bosch tank something like link attached 

https://www.worcester-bosch.co.uk/products/cylinders/directory/green-storage-wb-range-cylinders

 

is this what is required or something else

[PeterW]

Just a plain 90 litre direct (ie no coil) water tank will do. 

[Ahmiccc]

2 minutes ago, PeterW said:

Just a plain 90 litre direct (ie no coil) water tank will do. 

Can’t seem to find a range or a source to buy from or brand or anything spent past few hours, don’t suppose you can help. Is a bigger tank better or ?

[PeterW]

https://www.ebay.co.uk/itm/Stainless-Steel-Buffer-Tank-50litre-bought-new-never-used-fittings-included-/265121216524?_trksid=p2349624.m46890.l49286
 

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