Gas Boiler sizing - is 18kw enough for a 170 sqm house?

thewiseowl12

Grateful for any advice.

Looking at ordering a gas boiler and wasn't impressed with the RGI who came out so wasn't sure how reliable his opinion was - he's recommending the install of a 24kw gas boiler system. I'm very wary of oversizing the system - I'm intending to zone the house using smart tado trv's so as a result it is extremely unlikely the heat is going to be blasting in the house non stop, more likely to specific rooms at specific times, which means a boiler that modulates lower would be better I guess?

The house is old, draughty with solid masonry walls which I am intending to renovate in the next year (mostly external insulation), currently replacing the old single glazing windows to triple glazed as it is.

Based on a rough rule of thumb estimate from the heat geek website attached below - if I take the worst scenario, i.e. an uninsulated draughty house, its saying the heat loss would be 170sqm*110w/M2 = 18kw at -3C. Say 3kw for hot water potentially, which would lead to a 21KW system?

However, house will have 13 radiators, which includes 3 bathroom heated towel rads - I've sized them and it appears they'll come to 15kw if I'm generous with the sizing and so an 18kw is appropriate

If I do go with the 24KW system and it is oversized - has it the potential to modulate down to accommodate the lower heating demand?

Just doing the research so grateful for anyone pointing out my interpretation is completely wrong in the above if so

https://www.heatgeek.com/how-to-size-my-heat-pump-or-boiler-heat-loss-cheat-sheet/

Ush1

It depends on the minimum modulation of the boiler really. I have an Ideal 24kw and it's not great, think minimum is 5kw, I have read Viessman have a modulation ratio of 17 so are much better.

wersal gummage

Hi,

Not sure if any help but some of your details are similar to here.

We have 170sq Mt, externally insulated BER B, with 14 rads.

32kw boiler here, only put in a few years ago and that was recommended by builder and his plumber

Gusser09

5 Bed house with 11 Rads. 140sqm.

We have the Grant Vortex 90-120 condenser. It's a 35kw. Savage yoke. Has the Rad jumping off the wall after 5 minutes. I have also noticed a considerable drop in what we are are using on oil.

Ush1

https://www.boards.ie/discussion/comment/119930650#Comment_119930650

Jumping off the wall isn't good for efficiency. Needs to be a low flow temp.

Gusser09

https://www.boards.ie/discussion/comment/119934013#Comment_119934013

It isn't on for as long. Of course it's good if they are heating up quicker.

kirving

https://www.boards.ie/discussion/comment/119934725#Comment_119934725

In general, the higher the output, the less efficient a boiler will be.

Running a 35kW (output) boiler for 1 hour, will use more fuel than running a 17.5kW for two hours. But if the house isn't well insulated, or you need it to warm up quickly after being off all day, then it's good to have the higher output of course.

Wearb

"Running a 35kW (output) boiler for 1 hour, will use more fuel than running a 17.5kW for two hours."

On the face of it that paragraph isn't true. Eg Both boilers could be running at, let's say 90%efficiency.

In condensing boilers its the return hw temperature that has the greater influence on efficiency.

Ush1

https://www.boards.ie/discussion/comment/119934725#Comment_119934725

It might be good for reaction times but not gas usage, the boiler won't ever be condensing.

Ush1

https://www.boards.ie/discussion/comment/119934915#Comment_119934915

Correct. Dew point of around 55c so return needs to be lower than that.

Wearb

While condensing mode is desirable, there are substantial benefits (from a condensing boiler) even when not running in condensing mode. This can be seen in the difference in flue gas temperature between a condensing (high efficiency boiler) and a standard efficiency (non condensing) boiler.

thewiseowl12

In the end I went with the 18kwh boiler which means that I will have to do a good job on the insulation now. The system wouldn't be as responsive as a larger boiler in a 3 zone system but I'm hoping that by having individual room thermostats I can compensate for that.