Gas boiler vs ASHP & short cycling in low heat loss property

[ruggers]

Just looking at high efficiency low modulation gas boilers vs modern ASHP for a self build with around 4kw heat loss with MVHR installed running weather comp.

 

Two in mind would be a Viessmann 200-W system boiler vs Vaillant Arotherm plus 7Kw.

Before ASHP's became popular in the UK, how did we manage/reduce short cycling of gas boilers in low heat loss homes?

 

A Viessmann will modulate down to 1900W.

This means that the gas boiler will be at its minimum modulation & starting to short cycle for anything over 9°C outdoors while maintaining a 21°C indoor temperature.

 

How big of an issue is short cycling really? Are ASHP's better at minimising short cycling issues or more suited than gas boilers?

 

Taking away green credentials, ASHP doesn't save anything. Todays prices the heat pump is £125 per year cheaper to run for me, but the annual service is up to 2 to 3x more than a gas boiler. The unit costs more possibly even after a grant.

[Chablais]

Hi I have a Viessman 200-W system boiler installed with weather comp, no internal stats, building heatloss is 3.7kw and have MVHR installed. You are correct, even though the boiler modulates to 1900w, it does in fact, in the words of the Germans, micro start. We have UFH at 150mm centres, in 100mm concrete screed across approx 128m2. Plus a basement with UFH same spec but 33m2. We maintain 21deg internal temp. When first installed the boiler was firing circa 100 times a day!! I now time the heating periods, 7.30am-10.30am and then 2.30pm-9.30pm. I have had to adjust the set back temp to 4deg to prevent the bolier firing all night. Our floor just about coasts through the night. So with a fair bit of faffing we are down to about 20 fires in a 24hr period on the above heating times. HW is timed once a day for 1/2hr, we have 300lt UVC. I am toying with the idea of a buffer to try and smooth out the feed to the UFH. Today with outside temp at 12deg the flow temp into the UFh is 26deg. The UFH is one zone for the whole house, plus one zone for the basement, they are both mixed circuits controlled by the boiler. So we have no actuators and all circuits are run fully open. It is a great system, but does micro fire in the shoulder seasons. I had words with Viessman in the UK and Germany as they said what I am experiencing is normal and not a problem! the boilers heating rating has been turned down to 25%. Hope this helps.

[JohnMo]

28 minutes ago, ruggers said:

how did we manage/reduce short cycling of gas boilers in low heat loss homes?

A, there weren't many low energy homes.

B, gas was cheap and no one knew any better.  

C, People that knew better installed a buffer, I assume as they are not new.

 

 

29 minutes ago, ruggers said:

modulate down to 1900W.

This means that the gas boiler will be at its minimum modulation & starting to short cycle for anything over 9°C outdoors while maintaining a 21°C indoor temperature.

May even start short cycling before that at very low flow temps.  You either need a big capacity system or a buffer.  Most heat pump manufacturers mandate you have a buffer, or a minimum volume, and have a min flow rate all the time.

 

33 minutes ago, ruggers said:

How big of an issue is short cycling really

I can still get it with a gas boiler and 160L buffer at very low flow temps.  Have just increased buffer temp to compensate.

 

36 minutes ago, ruggers said:

Are ASHP's better at minimising short cycling issues or more suited than gas boilers?

Think when all heat pumps were non modulating the issue was seen and installs had to have a buffer.  With inverter drives, there is more modulation but not as much as a gas boiler.  Both will short cycle at low flow temps and low heat loading, unless you design around it.

[ruggers]

@Chablais

 

All good feedback thanks. Mine will be 88m2 of UFH ground floor at 100 centres and 55mm screed and upstairs 125m2 8 radiators including the towel rail. Looking to fit 2 mixed units to control a curve for each floor.
 

Heat loss is 6Kw without MVHR, around 4kw with, but need to fine tune this figure & update a few things. 100 fires sounds a lot, how long does the building retain the heat before it needs to fire up once its reached 21C with such a low heat loss? 

 

Have you tried lowering the curve, I thought the idea of weather comp was to have it on all the time but with a night set set back 3 degrees less.
Do you think the heat loss is too low for the boiler then? I'm not sure if a heat pumps any better in these situations or equally as bad? My knowledge of ASHP's is very basic.

 

What your outdoor design temp in Cumbria?

[ruggers]

Oops, I've posted the same post twice.

 

@JohnMo

 

Whats your house heat loss again, I remember it being low? My understanding is, no short cycling is best, a little bit isn't too big of an issue, but lots of it repeating all day is going to be a problem.

 

I've heard heat pumps dont modulate as well as gas boilers but don't understand enough about them to know if they are affected as badly as gas or worse due to them needing to fire up and build pressure?

 

I'm going to need to try to calculate the flow rate and volume of the system for an idea. I can get all of the pipe lengths for 90% accuracy i think and diameters, can size the radiators but not sure how you know the volume they hold.

[JohnMo]

Heat loss on a -5 day is about 3kW.

 

The issue with short cycling, 

having to heat up the metal and any water that has cooled while the boiler is off.  That all takes energy.  In the heat pump most that metal is outside, on boiler I think a lot of heat gets wasted out the exhaust.  By getting on top of short cycling and boiler control I have halved my gas consumption.

 

Lots of cycles also mean additional wear and component life reduction.

 

[ReedRichards]

9 hours ago, ruggers said:

I've heard heat pumps dont modulate as well as gas boilers but don't understand enough about them to know if they are affected as badly as gas or worse due to them needing to fire up and build pressure?

 

According to my installer, heat pumps don't generally have the modulation range of a gas boiler (i.e their minimum output is a larger percentage of their maximum output}.  But it should be much easier to get a heat pump with a small maximum output matched to a well-insulated house than it is to get a gas boiler with a small maximum output.

 

It seems to me that short cycling might be worse for the life a boiler that works via combustion compared to one that does not (i.e. a heat pump).

 

I got my heat pump with a third party controller.  Originally there wasn't the option to tell it that it was controlling a heat pump so I was advised to choose "oil boiler" which limited the number of cycles to 3/4 per hour.  Now there is an option to select "heat pump" but it does the same thing.