Boiler and UVC sizing

[ELLIOTTJC]

Grateful for advice on the following. I'm doing a self build in the Stirling area. Its a five bed property (180m2 in total) with ufh downstairs (120m2) including three bedrooms downstairs and two bedrooms upstairs with rads. One bathroom and three en-suites in total.  I was hoping for some advice on boiler and UVC sizing? We will be required to install PVs so I'm also wondering what issues to consider  in terms of integrating it with the heating system/hot water? Looking for the most cost effective option. I did consider ASHP but have decided to opt for LPG and PVs. Any pointers much appreciated.

[AliG]

Hi,

 

I am guessing it is 5 bedrooms plus an open plan living area as otherwise it seems a lot to fit into 180m2.

 

It could be that your hot water needs are the most important, especially if well insulated. Do you foresee multiple showers being used simultaneously as that will make a difference.

 

A 10m shower is around 100l of water and a bath around 100-150l depending on how big it is. Assuming you are storing water at 60C and using it at 40C then the amount of hot water required for a couple of showers or a shower and a bath is maybe 120l. Thus the usual size of UVC in this size of house would probably be 200-250l. If you thought 3 showers/baths could be used at once then I might up it to 300l. 

 

This has some quite useful information on UVCs.

 

http://www.thermsaver.co.uk/wp-content/uploads/2015/07/Thermsaver-Excelsior-Brochure-2015.pdf

 

If you have a bath and a shower a 250l UVC is going to need about 25 minutes to get back up to temperature using 20kW of heat input. This is why you need a larger UVC so that the water isn't totally cooled down by the cold input as you use it. If you expect to use more water than this in one go you need a larger UVC.

 

So I would say you are probably looking at 250l and possibly 300l if you plan to use a lot of water at one time.

 

This will have a 20kW coil so I would expect you to need a 30kW gas boiler so that you can heat the house and hot water at the same time. Gas boilers are a lot less sensitive to being sized correctly than ASHPs.

 

If you aren't going to be a heavy water user I would consider an ASHP, I am not sure how the costs are with LPG.

 

As to PV you may limited by the amount of south facing roof you have, I would perhaps figure that out first. You will be able to generate around 200W per square metre of panel. They take up a lot of room. The standard used to be to go for 4kW, most people probably end up between 2.5 and 5kW depending on space, costs and whether or not it pays for itself in electricity savings.

 

 

Edited May 22, 2020 by AliG

[scottishjohn]

I had lpg boiler and swopped it for an ASHP ,no other alterations to heating system--running  costs are less than 50% of lpg boiler+RHI grant as well

[Alexphd1]

First off you need to work out your heat loss for your project as its impossible to guess a boiler size accurately on the info you have supplied. 

 

Your hot water demand sounds like it could be high so I personally would be looking at a 300l UVC. 

 

Get as much PV as possible and excess electric produced by PV is pretty easily diverted to the hot water via solar diverter (bought or DIY unit) but has no benefit to your heating side (with a standard boiler UVC set up).

 

Do your sums and dont rite of ashp just yet but I would concentrate on getting your energy usage down on the building first. A few extra pounds just now spent on the fabric of the building will be saving you money way down the line.

 

Edited May 22, 2020 by Alexphd1