Plumber specced 30KW system boiler, online calc says 35KW

DrunkOnLust · 27 May 2010

Hi,

I've recently done a whole house calculation using the energy saving trust's site and some fairly accurate measuring of rooms. The total comes to 33.216 kW, which I believe needs 2kW added for water heating, making approx 35 kW required. My plumber did no measurements or (visible) calculations and specced a 30KW ariston clas HE system boiler. Prior to this I had 2 quotes, both of which recommended a Worcester Greenstar 40CDI boiler.

I also did my own calculations using Stelrad's STARS v3 program which gave a total BTU requirement of 117898 BTU (34.554 kW) (-1degrees outside, MTW set at 65, average room temp 21degrees).

I'm wondering whether I should go with my own calculations and insist on a bigger boiler, or go with what my plumber's specified. I feel like a bit of an armchair expert but I'm getting the feeling my calculations might be better.

The house which needs the new CH system is a detached, 2 storey, solid brick, Victorian property with single glazing at the front, double at the back, high ceilings, 4 Bedrooms, 3 Reception rooms (2 with big bay windows), large hall (with 6m^2 of glazing), 2 bathrooms, 1 ensuite, largish kitchen/diner with french doors and skylights and a utility room. Total floorspace for both floors is around 211m^2.

Insulation wise the house will be draughtproofed, the ground floor will have insulation added between the floor joists, the basement will be insulated, the kitchen is having a solid insulated floor put in.

So, do the energy saving trust's and stelrad calcs sound about right or am I oversizing too much?

Thanks.

DP · 27 May 2010

All the rooms need not be 21 degreeC. Halls are have design temp of 18, bathroom 25 and bedrooms 16. But these are theoretical figures.

Also bear in mind, south facing rooms will heat up quicker than northern aspect ones.

Also, I would be considering a stainless steel HE boiler. Possibly two small boilers and spilt the house so if one goes belly up, other keeps on chugging.

If you have a modern factory lagged cylinder, it will need a lot more than 2kw to heat the water in 30 minutes.

Agile · 27 May 2010

Its a sad fact that many owners can quickly become more clever at calculating the heat loses than some "plumbers".

Contrary to popular practice, there is no advantage in oversizing. In fact because the boiler is forced to cycle for most of the year that reduces the efficiency.

Many of the on line calcs already add the 2 kW for water heating. Water heating is timed to start before CH so the correct figure is still 2 kW.

A salient factor is the power of the existing boiler. What is that? Dont forget most old boiler were range rated and to see what power yours is actually consuming you need to look at FAQs and measure gas at the meter. The power you get should then be reduced by about 30% in respect of the efficiency of the old boiler to reach the current output power.

My view is that the 30 kW is fine. You did not say anything about the loft insulation. Thats an easy place to cheaply reduce heat loss by increasing to 300-400mm.

Your house is going to be very expensive to heat. What you can do with TRVs is to reduce the temperature of areas like halls and unused bedrooms to a minimum and close curtains in cold weather.

Tony

DrunkOnLust · 27 May 2010

Thanks for the replies.

DP, I think we're pretty much set on having a cylinder and system boiler, we've made a reinforced space for one in the loft which took some doing (had to widen an opening) so that's been decided now.

Tony, the current boiler is a 28KW gloworm combi boiler (I don't think it's condensing, it's quite an old one). It's never really heated the house up satisfactorily, but that could be because the radiators were never sized properly - they're all 600 x 1400 single panels, 1 in each room.

Loft insulation is currently around 300mm, we will likely have that topped up some more.

Maybe I'm just overthinking all of this and I should let my plumber get on with it, lol. But I did read that the whole house boiler sizing calc was accurate and so too is the Stars program.

Agile · 27 May 2010

Using a higher power boiler is NOT going to improve the situation at first sight.

But before doing anything at all you need to find out why its not heating it adequately at present.

Certainly the size of your existing rads is probably the problem. You should calcalute the out put power at current values based on your existing rads.

Then see what heat output each room needs and see how under sized your rads are and what you can do to upgrade this.

Modern rads have fins which increase output by about 30% for a single panel.

Fitting double panel rads with fins will about double an existing single panel with no fins.

Tony

D_Hailsham · 27 May 2010

DrunkOnLust said:
they're all 600 x 1400 single panels, 1 in each room.

A 600 x 1400 single rad without fins produces 850W and 1400W if it has fins.

Calculate the total output of all your rads and compare this to the whole house figure. If your rads add up to less than the boiler, you will have to change some of the rads for larger ones. Putting in a larger boiler will not produce more heat.

Loft insulation is currently around 300mm, we will likely have that topped up some more.

That's already more than is required by current building regulations.

Do you have cavity insulation? If not, get that done. It will mean a smaller boiler and lower fuel bills. Check out Energy Savings Trust Grant Search

But I did read that the whole house boiler sizing calc was accurate and so too is the Stars program.

Certainly more accurate than most plumbers/heating engineers who seem to base their choice on "experience" and factors such as the perceived wealth of the customer.

The whole house method already includes 2kW for hot water, so there's no need to add it in.

ianniann · 27 May 2010

35kW sounds a lot! Scary high. Still, you have the full information, and the guys quoting for you probably just stuck a finger in the air and vaguely remembered a mate who had a similar job in the next street. They also have little to lose by over-sizing and much to lose by under-sizing. Solid walls, single glazing, and it sounds like some single story extensions all add up. Still sounds high though.

I agree with the others (I'm guessing Tony meant "isn't" going to improve things?), don't jump for the new boiler until you get to the bottom of your current problems. The dozen (?) or so small radiators won't come close to being able to push out 30kW into the rooms. You can get a bit more heat by running the water at high temperature (do you know what your existing boiler is set to?), but it is a particularly bad plan with a condensing boiler since they work best at lower water temperatures. Also, do all the radiators get equally warm? No cold tops or bottoms?

I also agree that topping up 300mm of loft insulation is an exercise in diminishing returns. With that thickness, 90% of the heat loss is through any tiny gaps, round the hatch, or through boarded areas with limited insulation. You could top the main layers up right to the slates and it wouldn't make a blind bit of difference.

1CANHELP · 27 May 2010

drunk on lust,

You and customers like you make me despair!

Look this whole house boiler sizing method was introduced when the PartL regulations came into place way back in April 2005.

Us installers went and done the energy efficiency certficate and were introduced to this most easy way to calculate boiler output.

Let me tell you this, this method came about when deskbound boffins commssioned by the government got involved and like most of these non practical types made an ar3e of it.

The whole house method would be fine in new builds or complete renovations however, in existing older properties TIMESERVED PROFESSIONAL TRADESMEN use the old way along with many years experience.

All in all, let the chap get on with it AND SPEC THE SYSTEM, after all he is the PROFESSIONAL not you.

Or? like many customers now with the internet at there fingertips.....get on and do the job yourself as you are all experts in everything now.

I often when coming accross customers like you just don't bother with the job and explain why, i'll let some other poor sod deal with you.

Sheeeesh some people.

JohnD · 27 May 2010

35kW is a lot. How huge is your house, and what is unusual about it that makes the heat loss so high?

just to check, you already have 300mm loft insulation in the entire roof? that's a foot thick

No point in topping that up.

do you have cavity wall insulation?

powell30 · 27 May 2010

How odd......if you're unhappy take one of the other quotes with a 40kw boiler........or is it they are more expensive.

Customers!

Razor900 · 27 May 2010

Personally I'd be more concerned that he's recommended an Ariston

ianniann · 27 May 2010

One quote is for 40kW and another for 30kW, and the customer is an idiot to look for clarification? And how about when it turns out the existing radiators can't even handle probably below 20kW from the old boiler while running at 80C? Still an idiot? Or a genius to come here and get abused?

Personally, I think the Energy Savings Trust calculator is a bit pessimistic. It says I need about double what I actually needed in the coldest winter for 30 years, but then I don't heat the entire house to 21C

A second heating source like a fire can also let you have a boiler that isn't over-sized for most of the time but might not quite cope in exceptional conditions.

powell30 · 27 May 2010

Read the OP's post again...he had had TWO quotes at 40kw. But he is arguing over the 30kw quote after doing calcs showing he needs 35.......

It's money.....trust me. And i didn't insult him......i questioned his motives.

Boilerman2 · 27 May 2010

Just to be sure why not fit a Hoval Ultragas 250... 250Kw should do it!!

Happyplumber · 28 May 2010

The house which needs the new CH system is a detached, 2 storey, solid brick, Victorian property with single glazing at the front, double at the back, high ceilings, 4 Bedrooms, 3 Reception rooms (2 with big bay windows), large hall (with 6m^2 of glazing), 2 bathrooms, 1 ensuite, largish kitchen/diner with french doors and skylights and a utility room. Total floorspace for both floors is around 211m^2.

Solid brick is the key word there for those who dont understand building terms there is no cavity.

30kw will be fine but you really need to check the rad sizes andare they adequate to heat up the rooms,if not then oversize them slightly and fit modern rads which are alot more efficient ,

The rads are just as important as the boiler itself for they are there to get rid of the heat as quick and efficiently as possible,those all important flow n return temperatures to make that boiler condense properly

Speak to your installer i am sure he will put your mind at ease and explain where he came to this conclusion .This just a DIY plumbing forum and unfortunetly there are alot of armchair experts here .YOu need the man at the mine head!

diynot

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Plumber specced 30KW system boiler, online calc says 35KW

1CANHELP

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