Boiler: What temp drop should I expect between out and in?

[cwienands]

Hey guys,

I recently had a Knight Lochinvar condensing boiler put into our house, replacing an age-old monster. Now that the outdoor temperatures have finally dropped I made an observation that does not look right to me.

When the boiler runs on the 1st floor heating loop (haven't tried with 2nd floor yet), I only see a temparature drop between output and input of 3 degrees Farenheit, from 131 to 128. Therefore, the boiler modulates down to 20%. The loop has approx. 80 feet of copper/aluminum fin heating elements. Outdoor temperatures are around 20F, indoor temperatures are around 65F. Part of the loop is running through a super poorly insulated part of our house.

I have a hard time believing that there is only a 3 degree temperature drop. One explanation would be that the pump is circulating the water so quickly that the water (when measuring one roundtrip) has little time to give off its heat energy. Explanation: I assume that the performance of the boiler can be measured by energy rise per amount of water per time interval. Therefore, if I increase the throughput of water, the energy rise would sink while the performance stays the same.
The other explanation would be that there is a shortcut between input and output. Indeed there is a pump in between the two ( guess it could be used to continue cycling existing heated water after the boiler shuts off). If there was a shortcut, less energy would reach the heating elements and I would likely waste energy.

Before I call the installer, I wanted to arm myself and check whether anybody would know what kind of temperature drop I should expect, or how this could be calculated.

Thanks a lot, Christoph

 

[Paul Barker]

an autobypass kicking in?

 

[cwienands]

Hello Paul,

thanks for your reply. What would this auto bypass look like and how would I determine whether it is turned on?

Thanks, Christoph

 

[kevplumb]

you should be looking for a 20 deg diff

 

[cwienands]

Hey Kev,

Thanks for confirming my expectations.

After that I took a closer look at the piping and I think I discovered a shortcut between input and output (the way it is piped), so that water from the boiler pump it not forced to enter any heating loop and conversely, any water returning from the heating loop is not forced to enter the boiler loop. Heat exchange seems to happen in that shortcut section where boiler loop water and heating loop water mix for about 6 inches. Now I'm not the expert and there might be some hidden magic behind this but I called the plumber to stop by and take a look at it.

I'll post the findings later.