Going green(er) with a Viessmann 200W (& heating efficiency home improvements)

Screwdriva

BUFF said:

bear in mind that it has been a very mild winter - my gas consumption dropped not only compared to last year (which, for me, was above average) but noticeably below my 12 year average.

This has been raised before. I compared months with nearly identical average monthly temperatures from 2022/2021 when comparing efficiency measures and any reductions achieved. As an example, the average London temperatures in Jan '22 and Feb '21 were similar. 

michaels

Based on Heating degree days this winter (Oct - Apr) was 13% less cold here than last year, we have been a bit more energy conscious and used 17.5% less energy.  No boiler upgrade needed.

Heating & Cooling Degree Days – Free Worldwide Data Calculation

Reed_Richards

michaels said:

Based on Heating degree days this winter ...

Using the default base temperature of 15.5 C?  If so, why?  

michaels

Reed_Richards said:

michaels said:

Based on Heating degree days this winter ...

Using the default base temperature of 15.5 C?  If so, why?  

Because I didn't know how to adjust it so assumed the default was a good starting point and suspected that any change in total from using a different value would be approximately proportionate across both winters.

What should I have done?

QrizB

Reed_Richards said:

michaels said:

Based on Heating degree days this winter ...

Using the default base temperature of 15.5 C?  If so, why?  

There's a short article on this here:
https://www.energylens.com/articles/degree-days#base-temperature-problem

You need to choose a base temperature at which your particular building balances out heat gain vs. loss and needs no heating to maintain your desired internal temperature. Very few people know what this temperature is.

The degree-days website includes an option "include base temperatures nearby" which then produces a table with degree-day data for base temps up to 3C higher and lower than the base you choose, in 0.5C steps.

I haven't heated my house this month. Looking at degree-day data from that website for RAF Odiham (one of my neareer weather stations):

• The default 15.5C base gives a total of 74.5 degree-days for the month to date;
• A 17C base gives 108.2 degree-days;
• A 14C base gives 46 degree-days; and
  
• A 12.5C base gives 24.6 degree-days.

None of these are "correct" for my house - I haven't heated it at all - but it sugggests to me that my own household base temperature should be lower than 15.5C.

michaels

QrizB said:

Reed_Richards said:

michaels said:

Based on Heating degree days this winter ...

Using the default base temperature of 15.5 C?  If so, why?  

There's a short article on this here:
https://www.energylens.com/articles/degree-days#base-temperature-problem

You need to choose a base temperature at which your particular building balances out heat gain vs. loss and needs no heating to maintain your desired internal temperature. Very few people know what this temperature is.

The degree-days website includes an option "include base temperatures nearby" which then produces a table with degree-day data for base temps up to 3C higher and lower than the base you choose, in 0.5C steps.

I haven't heated my house this month. Looking at degree-day data from that website for RAF Odiham (one of my neareer weather stations):

• The default 15.5C base gives a total of 74.5 degree-days for the month to date;
• A 17C base gives 108.2 degree-days;
• A 14C base gives 46 degree-days; and
  
• A 12.5C base gives 24.6 degree-days.

None of these are "correct" for my house - I haven't heated it at all - but it sugggests to me that my own household base temperature should be lower than 15.5C.

It is tricky, we normally just leave the weather comp controls set but if does mean after a chilly spring night we get some heating and then probably have the doors and windows open later as solar gain kicks in.  Plus the fabric of the building means that if we get one chilly day (like yesterday) we don't really need to heat whereas if we had a run of chilly days then either we are cold or we need to heat.

Thus in spring/autumn where warm and cold days are interspersed it may be true that we can set a lower base but for sustained cold months the higher values might be more realistic.

Reed_Richards

Thank you @QrizB, that's a really helpful response.  I know that when I was away two weeks ago my house did not maintain its desired internal temperature.  In my case the default 15.5 C might not be too far wrong; it was certainly colder than that most of the time I was away.

michaels

Q1. If we were to go for a bi-valent system is there a way from the heat days data to work out what proportion of total heating requirement is when temp is below 5 degrees (gas energy) as oppsed to 5 and above (heat pump).

[Note 5 is just my best guess as to when the economics of gas might become better than HP due to the falling COP)

Q2. Our gas boiler has weather comp, the output modulates to the level where the flow temp required to match the heat loss is maintained.  With a HP that has a fixed temp output setting is the output controlled via a room stat that switches the pump on and off as the room temp rises above or falls below the desired level?

QrizB

michaels said:

Q1. If we were to go for a bi-valent system is there a way from the heat days data to work out what proportion of total heating requirement is when temp is below 5 degrees (gas energy) as oppsed to 5 and above (heat pump).

I would guess so; get two sets of degree-day data, one at your lower temp and one at your higher one, and look at the difference. It would give you a feel for things, at least.

michaels

QrizB said:

michaels said:

Q1. If we were to go for a bi-valent system is there a way from the heat days data to work out what proportion of total heating requirement is when temp is below 5 degrees (gas energy) as oppsed to 5 and above (heat pump).

I would guess so; get two sets of degree-day data, one at your lower temp and one at your higher one, and look at the difference. It would give you a feel for things, at least.

Wouldn't the lower temp degree days only include the degrees to get up to 5 though so I would need to add on 10 degrees times the number of days/hours which I wouldn't know.